Minimize Hubble

HST (Hubble Space Telescope) Mission

Sensor Complement   HST Imagery    References 

The HST (Hubble Space Telescope) of NASA is named in honor of the American astronomer Edwin Hubble (1889-1953), Dr. Hubble confirmed an "expanding" universe, which provided the foundation for the big-bang theory. Hubble, the observatory, is the first major optical telescope to be placed in space, the ultimate mountaintop. Above the distortion of the atmosphere, far far above rain clouds and light pollution, Hubble has an unobstructed view of the universe. Scientists have used Hubble to observe the most distant stars and galaxies as well as the planets in our solar system. 1)

The planning for HST started in the early 1970s. The HST was launched into LEO (Low Earth Orbit) on April 24, 1990 on STS-31 (12:33:51 UTC, on Shuttle Discovery). Hubble is operational as of 2019, in its 30th year on orbit, and is one of NASA's Great Observatories. Hubble's launch and deployment in April 1990 marked the most significant advance in astronomy since Galileo's telescope. Thanks to five servicing missions and more than 25 years of operation, our view of the universe and our place within it has never been the same.


• Deployment of Hubble: April 25, 1990

• First Image: May 20, 1990: Star cluster NGC 3532

• Servicing Mission 1 (STS-61): December 1993

• Servicing Mission 2 (STS-82): February 1997

• Servicing Mission 3A (STS-103): December 1999

• Servicing Mission 3B (STS-109): February 2002

• Servicing Mission 4 (STS-125): May 2009

In 2020, the Hubble Space Telescope achieved its 30th year in orbit. Hubble's unique design, allowing it to be repaired and upgraded with advanced technology by astronauts, has made it one of NASA's longest-living and most valuable space-based observatories, beaming transformational astronomical images to Earth for decades.

Figure 1: Hubble has fundamentally changed our understanding of the cosmos, and its story — filled with challenges overcome by innovation, determination, and the human spirit — inspires us (video credit: NASA's Goddard Space Flight Center, Paul R. Morris (USRA): Lead Producer) 2)

Spacecraft: The spacecraft has a length of 13.2 m, a mass at launch of 10,886 kg, post SM (Servicing Mission) 4 of 12,247 kg, and a maximum diameter of 4.2 m.

Orbit: LEO with an altitude of 547 km an inclination of 28.5º, and a period of 95 minutes.

The HST (Hubble Space Telescope) of NASA features a ULE TM(Ultra-Low Expansion) primary mirror of 2.4 m diameter (f/24 Ritchey-Chretien) and a 0.3 m Zerodur secondary mirror. The HST primary mirror was a lightweighted monolithic design (824 kg) by Perkin-Elmer (now Goodrich Inc.), Danbury, CN, using a lightweight, thick egg crate core sandwiched between two plates and fused together.

The HST is the most precisely pointed instrument in spaceborne astronomy. The pointing requirements call for a continuous 24 hour target lock maintenance of 0.007 arcseconds (2 millionth degree).


Figure 2: IMAX Cargo Bay Camera view of the Hubble Space Telescope at the moment of release, mission STS-31 in April 1990 (image credit: NASA)

Some background:

The telescope's original equipment package included the Wide Field/Planetary Camera (WF/PC), Goddard High Resolution Spectograph (GHRS), Faint Object Camera (FOC), Faint Object Spectograph (FOS), and High Speed Photometer (HSP). 3) 4)

After a few weeks of operation, scientists noticed that images being sent back from Hubble were slightly blurred. While this distortion still allowed scientists to study the cosmos and make significant discoveries, it resulted in less spectacular images, and some of the original mission could not be fulfilled. An investigation finally revealed a spherical aberration in the primary mirror, due to a miscalibrated measuring instrument that caused the edges of the mirror to be ground slightly too flat. Engineers rushed to come up with a fix to the problem in time for Hubble's first scheduled servicing mission in 1993. The system designed to correct the error was designated COSTAR (Corrective Optics Space Telescope Axial Replacement). COSTAR was a set of optics that compensated for the aberration and would allow all of Hubble's instruments to function normally.

In December, 1993, the crew of STS-61 embarked on a service mission to replace a number of Hubble's parts. Following intensive training on the use of new tools never used before in space, two teams of astronauts completed repairs during a record five back-to-back spacewalks. During the EVAs, COSTAR was installed and the Wide Field/Planetary Camera was replaced with the Wide Field/Planetary Camera 2, which was designed to compensate for the mirror problem. The team also performed basic maintenance on the craft, installed new solar arrays, and replaced four of Hubble's gyroscopes.

Shortly after the crew returned to Earth and the Hubble Space Telescope began returning sharp and spectacular images, NASA deemed the servicing mission a success. Astronomers could now take advantage of a fully functional space telescope, and the public was treated to breathtaking photos of stars, galaxies, nebulae, and other deep-space objects. Subsequent servicing missions improved Hubble's capabilities and performed routine repairs.

In February, 1997, the crew of STS-82 installed the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) and the Space Telescope Imaging Spectograph (STIS) to detect infrared light from deep-space objects and take detailed photos of celestial objects. Servicing mission 3A in December, 1999 replaced all six of the telescope's aging gyroscopes, which accurately point the telescope at its target. STS-103 astronauts also replaced one of the telescope's three fine guidance sensors and installed a new computer, all in time to redeploy Hubble into orbit on Christmas Day. The most recent servicing mission to the spacecraft, servicing mission 3B, came aboard STS-109 in March, 2002. Columbia crewmembers installed the new Advanced Camera for Surveys (ACS), which had sharper vision, a wider field of view, and quicker data gathering than the Wide Field/Planetary Camera 2. Astronauts also replaced Hubble's solar panels with a more efficient array and conducted repairs on the NICMOS.

STS109-E-5700 (9 March 2002) — The Hubble Space Telescope, sporting new solar arrays and other important but less visible new hardware, begins its separation from the Space Shuttle Columbia. The STS-109 crew deployed the giant telescope at 4:04 a.m. CST (1004 GMT), March 9, 2002. Afterward, the seven crew members began to focus their attention to the trip home, scheduled for March 12. The STS-109 astronauts conducted five space walks to service and upgrade Hubble. 5)

The power for the NASA/ESA Hubble Space Telescope's scientific discoveries comes from solar cells. Designing and constructing Hubble's first two sets of solar cell arrays, and the accompanying Solar Array Drive Mechanism (SADM) and Solar Array Drive Electronics (SADE), constituted a huge technological achievement for the European Space Agency (ESA) and European industry. After an in-orbit life of more than 10 years, the ESA-built solar arrays were replaced by new, more powerful arrays. However, ESA's SADM and SADE, which control the telescope's current solar arrays, are still on board and under ESA purview. They are among the telescope's oldest subsystems. 6)

In December 2019, the accumulated slew angles of the SADM had reached 1,000,000 degrees of travel. This travel began accumulating on this day 18 years ago, 5 March 2002, when ESA's solar arrays were replaced during the Space Shuttle Servicing Mission 3B.

"This milestone is a special occasion to recognize that after all of these years of operation, the SADM and SADE are still functioning perfectly without any sign of degradation. It's a fantastic achievement," said Lothar Gerlach, former ESA project manager for the European hardware onboard the Hubble Space Telescope. "The SADM and SADE have greatly exceeded their design life, and we are very proud they are still a key part of Hubble scientific operations".

Notes: The Hubble Space Telescope is a project of international co-operation between ESA and NASA. The ESA Hubble Space Telescope solar arrays have been provided to the European Space Agency by Astrium (UK/Germany — formerly British Aerospace, United Kingdom, AEG/Telefunken and Dornier — now Airbus, Germany), and Oerlikon Contraves Space (Switzerland).

Europe & Hubble 7)

ESA's contribution to the Hubble Project guarantees European scientists access to 15% of Hubble observing time. Hubble time is allocated on scientific merit by an international panel that includes European experts. Over Hubble's lifetime, European astronomers have, in open competition, been allocated more than the guaranteed 15%, and in some years the proportion has been closer to 25%.

Europe also provided one of the scientific instruments Hubble was launched with, designed Hubble's solar panels, and has provided astronauts to participate in servicing missions.

Scientists from most ESA Member States have had an opportunity to observe with Hubble. To date, almost 800 observing programs with European principal investigators (lead scientists) have been carried out or are scheduled to be in the next observing round, with many others involved as co-investigators.


Figure 3: An overview of the fraction of observing time that has been rewarded to ESA proposals. It is measured in two different ways: In number of proposals and in time (here measured in units of Hubble orbits, i.e. 96 minutes). By both metrics, European scientists have won comfortably more than 15% of observing time in the majority of years since launch (image credit: ESA)

The success of a scientific mission can be measured by the number and quality of scientific papers that are published in the specialized press. The number of papers based on Hubble observations published each year has been increasing continuously since the telescope's launch. There is at least one European author or co-author on about 30% of these papers, indicating the importance of Hubble to European astronomy.


Figure 4: This photograph of NASA's Hubble Space Telescope was taken on the fifth servicing mission to the observatory in May 2009 (image credit: NASA)


Figure 5: Artist's view of the HST in space along with the designation of the key element locations (image credit: NASA)

The Hubble Space Telescope is an international collaboration among NASA and ESA (European Space Agency). NASA has overall responsibility for the Hubble mission and operations. ESA provided the original FOC (Faint Object Camera) and solar panels, and provides science operations support.


Figure 6: Photo of the Hubble mission operations team at NASA's Goddard Space Flight Center in Greenbelt, Maryland, as of Hubble's 25th anniversary of flight in April 2015. Since Hubble's official start in 1977, thousand of people from the United States and Europe have supported the mission through building and testing hardware and software, operating the vehicle, and performing science operations. More than 30 astronauts have flown to Hubble to deploy, upgrade and repair the observatory with the support of a human spaceflight and space shuttle staff. Thousands of astronomers from dozens of countries have used Hubble and analyzed its data to produce more than 15,000 peer reviewed papers to date (image credit: NASA/GSFC, Bill Hrybyk) 8)


Note: At this stage of the mission (2018), no attempt is being made to recover all facets of Hubble regarding the spacecraft, instrumentation and the past history (it would have required a constant accompaniment of the mission with all updates over its lifetime). Instead, some fairly recent images of the mission and the operational status of the mission are presented.

The Hubble Servicing Missions are shortly described in a separate chapter of this file.



HST sensor complement: (ACS, WFC3, STIS, COS, FGS, NICMOS)

The Hubble Space Telescope has three types of instruments that analyze light from the universe: cameras, spectrographs and interferometers. 9)


Figure 7: Hubble's scientific instruments analyze different types of light ranging from ultraviolet (UV) to infrared (IR). This graphic shows which wavelengths each instrument studies (image credit: NASA)



Hubble has two primary camera systems to capture images of the cosmos. Called the Advanced Camera for Surveys (ACS) and the Wide Field Camera 3 (WFC3), these two systems work together to provide superb wide-field imaging over a broad range of wavelengths.

ACS (Advanced Camera for Surveys)

Installed on Hubble in 2002, ACS was designed primarily for wide-field imagery in visible wavelengths, although it can also detect ultraviolet and near-infrared light. ACS has three cameras, called channels, that capture different types of images. An electronics failure in January 2007 rendered the two most-used science channels inoperable. In 2009, astronauts were able to repair one of the channels and restored ACS's capacity to capture high-resolution, wide-field views.

WFC3 (Wide Field Camera 3)

Installed in 2009, WFC3 provides wide-field imagery in ultraviolet, visible and infrared light. WFC3 was designed to complement ACS and expand the imaging capabilities of Hubble in general. While ACS is primarily used for visible-light imaging, WFC3 probes deeper into infrared and ultraviolet wavelengths, providing a more complete view of the cosmos.


Figure 8: Astronaut Andrew Feustel prepares to install WFC3 (Wide Field Camera 3) on Hubble during Servicing Mission 4 in 2009 (image credit: NASA)



Spectrographs practice spectroscopy, the science of breaking light down to its component parts, similar to how a prism splits white light into a rainbow. Any object that absorbs or emits light can be studied with a spectrograph to determine characteristics such as temperature, density, chemical composition and velocity.

Hubble currently utilizes two spectrographs: COS (Cosmic Origins Spectrograph) and the STIS (Space Telescope Imaging Spectrograph). COS and STIS are complementary instruments that provide scientists with detailed spectral data for a variety of celestial objects. While STIS is a versatile, "all purpose" spectrograph that handles bright objects well, COS measures exceedingly faint levels of ultraviolet light emanating from distant cosmic sources, such as quasars in remote galaxies. Working together, the two spectrographs provide a full set of spectroscopic tools for astrophysical research.


Figure 9: Hubble's STIS captured a spectrum (right) of material ejected by a pair of massive stars called Eta Carinae, while the Wide Field and Planetary Camera 2 took an image of the billowing clouds of gas enveloping the stellar pair (left). The spectrum reveals that one of the lobes contains the elements helium (He), argon (Ar), iron (Fe) and nickel (Ni), image credit: NASA, ESA and the Hubble SM4 ERO Team


Figure 10: Hubble's 2.4 m diameter primary mirror collects light from its astronomical target and reflex it to a 0.3 m diameter secondary mirror located in the optical tube. This secondary mirror then reflects the light through a hole in the primary mirror to form an image at the telescope's focal plane. There it is intercepted by pick-off mirrors that pass it into the scientific instruments (image credit: Hubblesite) 10)



Hubble's interferometers serve a dual purpose — they help the telescope maintain a steady aim and also serve as a scientific instrument. The three interferometers aboard Hubble are called the FGS (Fine Guidance Sensors). The Fine Guidance Sensors measure the relative positions and brightnesses of stars.

When Hubble is pointing at a target, two of the three Fine Guidance Sensors are used to lock the telescope onto the target. For certain observations, the third Fine Guidance Sensor can be used to gather scientific information about a target, such as a celestial object's angular diameter or star positions that are ten times more accurate than those obtained by ground-based telescopes.

The Fine Guidance Sensors are very sensitive instruments. They seek out stable point sources of light (known as "guide stars") and then lock onto them to keep the telescope pointing steadily. When a light in the sky is not a point source, the Fine Guidance Sensor cannot lock on and so it rejects the guide star. Often, a rejected guide star is actually a faraway galaxy or a double-star system. Since Hubble was launched in 1990, the Fine Guidance Sensors have detected hundreds of double-star systems that were previously thought to be single stars.


Past Instruments

Only one of the instruments remaining on Hubble — the third Fine Guidance Sensor — was launched with the observatory in 1990. The rest of the instruments were installed during Hubble's five servicing missions. In addition to installing new instruments, astronauts also repaired two instruments (ACS and STIS) while visiting Hubble on Servicing Mission 4 in 2009. The NICMOS (Near-Infrared Camera and Multi-Object Spectrometer) on Hubble is in hibernation following a cryocooler anomaly, but most of its infrared duties have since been taken over by WFC3.

Hubble's past instruments include:

• High Speed Photometer

• Faint Object Camera (FOC), provided by ESA

• Faint Object Spectrograph

• Goddard High Resolution Spectrograph

• Wide Field and Planetary Camera

• Wide Field and Planetary Camera 2

• Fine Guidance Sensors (three).


Current Instruments

ACS (Advanced Camera for Surveys) - ACS is a third-generation imaging camera. This camera is optimized to perform surveys or broad imaging campaigns. ACS replaced Hubble's Faint Object Camera (FOC) during Servicing Mission 3B. Its wavelength range extends from the ultraviolet, through the visible and out to the near-infrared (115-1050 nm). ACS has increased Hubble's potential for new discoveries by a factor of ten.

COS (Cosmic Origins Spectrograph) - COS focuses exclusively on ultraviolet (UV) light and is the most sensitive ultraviolet spectrograph ever, increasing the sensitivity at least 10 times in the UV spectrum and up to 70 times when looking at extremely faint objects. It is best at observing points of light, like stars and quasars. COS was installed during during Servicing Mission 4 in May 2009.

STIS (Space Telescope Imaging Spectrograph) - STIS is a second-generation imager/spectrograph. STIS is used to obtain high resolution spectra of resolved objects. STIS has the special ability to simultaneously obtain spectra from many different points along a target. The STIS instrument has a mass of 318 kg and a wavelength range of 115-1000 nm.

STIS spreads out the light gathered by a telescope so that it can be analyzed to determine such properties of celestial objects as chemical composition and abundances, temperature, radial velocity, rotational velocity, and magnetic fields. Its spectrograph can be switched between two different modes of usage:

C So-called "long slit spectroscopy" where spectra of many different points across an object are obtained simultaneously.

1) So-called "echelle spectroscopy" where the spectrum of one object is spread over the detector giving better wavelength resolution in a single exposure.

STIS also has a so-called coronagraph which can block light from bright objects, and in this way enables investigations of nearby fainter objects.

WFC3 (Wide Field Camera 3) - Wide Field Camera 3 is the main imager on the telescope. It has a camera that records visible and ultraviolet (UVIS, 200-1000 nm) wavelengths of light and is 35 times more sensitive in the UV wavelengths than its predecessor. A second camera that is built to view infrared (NIR, 850-1700 nm) light increases Hubble's IR resolution from 65,000 to 1 million pixels. Its combination of field-of-view, sensitivity, and low detector noise results in a 15-20 time improvement over Hubble's previous IR camera. WFC3 was jointly developed at GSFC, STScI (Space Telescope Science Institute) in Baltimore and Ball Aerospace & Technologies Corporation in Boulder, CO. 11)

FGS (Fine Guidance Sensor) – The FGS provides pointing information for the spacecraft by locking onto guide stars. The FGS can also function as a scientific instrument by precisely measuring the relative positions of stars, detecting rapid changes in a star's brightness, and resolving double-star systems that appear as point sources even to Hubble's cameras. Hubble has three FGSs onboard the observatory.

NICMOS (Near Infrared Camera and Multi-Object Spectrometer) – NICMOS has the ability to obtain images and spectroscopic observations of astronomical targets at near-infrared wavelengths. Although NICMOS is currently inactive, most of its functionality is replaced by Hubble's other science instruments.


Note: As of 25 April 2020, the previously large Hubble file has been split into three files, to make the file handling manageable for all parties concerned, in particular for the user community.

This article covers the Hubble mission and its imagery in the period 2021, in addition to some of the mission milestones.

Hubble status and imagery in the period 2020

Hubble status and imagery in the period 2019

Hubble status and imagery in the period 2018-2015 as well as the Hubble Servicing Missions & Ground Segment



HST (Hubble Space Telescope) - Status and some observation imagery in the period 2021

• September 10, 2021: This star-studded image from the NASA/ESA Hubble Space Telescope depicts NGC 6717, which lies more than 20,000 light-years from Earth in the constellation Sagittarius. NGC 6717 is a globular cluster, a roughly spherical collection of stars tightly bound together by gravity. Globular clusters contain more stars in their centers than their outer fringes, as this image aptly demonstrates; the sparsely populated edges of NGC 6717 are in stark contrast to the sparkling collection of stars at its center. 12)

- The center of the image also contains some interlopers from closer to home. Bright foreground stars close to Earth are surrounded by criss-cross diffraction spikes formed by starlight interacting with the structures supporting Hubble's secondary mirror.


Figure 11: The area of the night sky which contains the constellation Sagittarius also contains the center of the Milky Way, which is filled with light-absorbing gas and dust. This absorption of light — which astronomers refer to as extinction — makes studying globular clusters near the Galactic center a challenging endeavor. To determine the properties of NGC 6717, astronomers relied on a combination of Hubble's Wide Field Camera 3 and the Advanced Camera for Surveys (image credit: ESA/Hubble and NASA, A. Sarajedini; CC BY 4.0)

• September 6, 2021: Could dying stars hold the secret to looking younger? New evidence from NASA's Hubble Space Telescope suggests that white dwarf stars could continue to burn hydrogen in the final stages of their lives, causing them to appear more youthful than they actually are. This discovery could have consequences for how astronomers measure the ages of star clusters, which contain the oldest known stars in the universe. 13)

- These results challenge the prevalent view of white dwarfs as inert, slowly cooling burned-out stars where nuclear fusion has stopped. Now, an international group of astronomers has discovered the first evidence that white dwarfs can slow down their rate of aging by burning hydrogen on their surfaces.

- "We have found the first observational evidence that white dwarfs can still undergo stable thermonuclear activity," explained Jianxing Chen of the Alma Mater Studiorum Università di Bologna and the Italian National Institute for Astrophysics, who led this research. "This was quite a surprise, as it is at odds with what is commonly believed."


Figure 12: To investigate the physics underpinning white dwarf evolution, astronomers compared cooling white dwarfs in two massive collections of stars: the globular clusters M13 and M3. These two clusters share many physical properties such as age and metallicity, but the populations of stars which will eventually give rise to white dwarfs are different. This makes M13 and M3 together a perfect natural laboratory in which to test how different populations of white dwarfs cool (image credits: ESA/Hubble, NASA, and G. Piotto et al.)

- White dwarfs have cast off their outer layers during the last stages of their lives. They are common objects in the cosmos; roughly 98% of all the stars in the universe will ultimately end up as white dwarfs, including our own Sun. Studying these cooling stages helps astronomers understand not only white dwarfs, but also their earlier stages as well.

- To investigate the physics underpinning white dwarf evolution, astronomers compared cooling white dwarfs in two massive collections of stars: the globular clusters M3 and M13. These two clusters share many physical properties such as age and metallicity (the abundance of heaver elements), but the populations of stars which will eventually give rise to white dwarfs are different. This makes M3 and M13 together a perfect natural laboratory in which to test how different populations of white dwarfs cool.

- "The superb quality of our Hubble observations provided us with a full view of the stellar populations of the two globular clusters," continued Chen. "This allowed us to really contrast how stars evolve in M3 and M13."

- Using Hubble's Wide Field Camera 3 the team observed M3 and M13 at near-ultraviolet wavelengths, which is ideal for finding blue and faint stellar objects in the crowded globular clusters. This allowed the researchers to compare more than 700 white dwarfs in the two clusters. They found that M3 contains standard white dwarfs, which are simply cooling stellar cores. M13, on the other hand, contains two populations of white dwarfs: standard white dwarfs and those which have managed to hold on to an outer envelope of hydrogen, allowing them to burn for longer and hence cool more slowly.

- Comparing their results with computer simulations of stellar evolution in M13, the researchers were able to show that roughly 70% of the white dwarfs in M13 are burning hydrogen on their surfaces, slowing down the rate at which they are cooling. The hydrogen would have been supplied by the star's outer shells leaking into space.

- This discovery could have consequences for how astronomers measure the ages of stars in the Milky Way galaxy. The evolution of white dwarfs has previously been modeled as a predictable cooling process. This relatively straightforward relationship between age and temperature has led astronomers to use the white dwarf cooling rate as a natural clock to determine the ages of star clusters, particularly globular and open clusters. However, white dwarfs burning hydrogen could cause these age estimates to be inaccurate by as much as one billion years.

- "Our discovery challenges the definition of white dwarfs as we consider a new perspective on the way in which stars get old," added Francesco Ferraro of the Alma Mater Studiorum Università di Bologna and the Italian National Institute for Astrophysics, who coordinated the study. "We are now investigating other clusters similar to M13 to further constrain the conditions which drive stars to maintain the thin hydrogen envelope which allows them to age slowly."

- The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

• August 27, 2021: This Picture of the Week shows an open cluster known as NGC 2164, which was first discovered in 1826 by a Scottish astronomer named James Dunlop. NGC 2164 is located within one of the Milky Way galaxy's closest neighbors — the satellite galaxy known as the Large Magellanic Cloud. The Large Magellanic cloud is a relatively small galaxy that lies about 160 000 light-years from Earth. It is considered a satellite galaxy because it is gravitationally bound to the Milky Way. In fact, the Large Magellanic cloud is on a very slow collision course with the Milky Way — it's predicted that they will collide 2.4 billion years from now. 14)


Figure 13: The Large Magellanic Cloud only contains about one hundredth as much mass as the Milky Way, but it still contains billions of stars. The open cluster NGC 2164 is in good company in the Large Magellanic Cloud — the satellite galaxy is home to roughly 700 open clusters, alongside about 60 globular clusters. This image of NGC 2164 was taken by the NASA/ESA Hubble Space Telescope's Wide Field Camera 3 (WFC3), which has previously imaged many other open clusters, including NGC 330 and Messier 11 (image credit: ESA/Hubble & NASA, J. Kalirai, A. Milone; CC BY 4.0)

• August 20, 2021: This jewel-bright image from the NASA/ESA Hubble Space Telescope shows NGC 1385, a spiral galaxy 68 million light-years away from Earth, which lies in the constellation Fornax. The image was taken with Hubble's Wide Field Camera 3 (WFC3), which is often referred to as Hubble's workhorse camera, thanks to its reliability and versatility. It was installed in 2009 when astronauts last visited Hubble, and 12 years later it remains remarkably productive. 15)


Figure 14: NGC 1385's home — the Fornax constellation — is not named after an animal or an ancient God, as are many of the other constellations. Fornax is simply the Latin word for a furnace. The constellation was named Fornax by Nicolas-Louis de Lacaille, a French astronomer who was born in 1713. Lacaiile named 14 of the 88 constellations that are still recognized today. He seems to have had a penchant for naming constellations after scientific instruments, including Atlia (the air pump), Norma (the ruler, or set square) and Telescopium (the telescope), image credit: ESA/Hubble & NASA, J. Lee and the PHANGS-HST Team; CC BY 4.0

• August 19, 2021: It's suspected that about 5,000 years ago a comet swept within 23 million miles of the Sun, closer than the innermost planet Mercury. The comet might have been a spectacular sight to civilizations across Eurasia and North Africa at the end of the Stone Age. 16)

- However, this nameless space visitor is not recorded in any known historical account. So how do astronomers know that there was such an interplanetary intruder? — Enter comet ATLAS (C/2019 Y4), which first appeared near the beginning of 2020.

- Comet ATLAS, first detected by the Asteroid Terrestrial-impact Last Alert System (ATLAS), operated by the University of Hawaii, quickly met an untimely death in mid-2020 when it disintegrated into a cascade of small icy pieces.

- In a new study using observations from NASA's Hubble Space Telescope, astronomer Quanzhi Ye of the University of Maryland (UMD) in College Park, reports that ATLAS is a broken-off piece of that ancient visitor from 5,000 years ago. Why? Because ATLAS follows the same orbital "railroad track" as that of a comet seen in 1844. This means the two comets are probably siblings from a parent comet that broke apart many centuries earlier. The link between the two comets was first noted by amateur astronomer Maik Meyer.


Figure 15: This pair of Hubble Space Telescope images of comet C/2019 Y4 (ATLAS), taken on April 20 and April 23, 2020, reveal the breakup of the solid nucleus of the comet. Hubble photos identify as many as 30 separate fragments. The comet was approximately 91 million miles from Earth when the images were taken. The comet may be a broken off piece of a larger comet that swung by the Sun 5,000 years ago. The comet has been artificially colored in this view to enhance details for analysis (credits: Science: NASA, ESA, Quanzhi Ye (UMD), Image Processing: Alyssa Pagan (STScI)

- Such comet families are common. The most dramatic visual example was in 1994 when the doomed comet Shoemaker-Levy 9 (SL9) was pulled into a string of pieces by Jupiter's gravitational pull. This "comet train" was short-lived. It fell piece by piece into Jupiter in July 1994.

- But comet ATLAS is just "weird," says Ye, who observed it with Hubble about the time of the breakup. Unlike its hypothesized parent comet, ATLAS disintegrated while it was farther from the Sun than Earth, at a distance of over 100 million miles. This was much farther than the distance where its parent passed the Sun. "This emphasizes its strangeness," said Ye.

- "If it broke up this far from the Sun, how did it survive the last passage around the Sun 5,000 years ago? This is the big question," said Ye. "It's very unusual because we wouldn't expect it. This is the first time a long-period comet family member was seen breaking up before passing closer to the Sun."

- Observing the breakup of the fragments offers clues to how the parent comet was put together. The conventional wisdom is that comets are fragile agglomerations of dust and ice. And, they may be lumpy, like raisin pudding.

- In a new paper published in the Astronomical Journal, after one year of analysis Ye and co-investigators report that one fragment of ATLAS disintegrated in a matter of days, while another piece lasted for weeks. "This tells us that part of the nucleus was stronger than the other part," he said. 17)

- One possibility is that streamers of ejected material may have spun up the comet so fast that centrifugal forces tore it apart. An alternative explanation is that it has so-called super-volatile ices that just blew the piece apart like an exploding aerial firework. "It is complicated because we start to see these hierarchies and evolution of comet fragmentation. Comet ATLAS's behavior is interesting but hard to explain."

- Comet ATLAS's surviving sibling won't return until the 50th century.

• August 13, 2021: Nestled among the vast clouds of star-forming regions like this one lie potential clues about the formation of our own solar system. 18)

- Stars are born in dusty environments and although this dust makes for spectacular images, it can prevent astronomers from seeing stars embedded in it. Hubble's Wide Field Camera 3 (WFC3) instrument is designed to capture detailed images in both visible and infrared light, meaning that the young stars hidden in vast star-forming regions like AFGL 5180 can be seen much more clearly.


Figure 16: This image from the NASA/ESA Hubble Space Telescope features AFGL 5180, a beautiful stellar nursery located in the constellation of Gemini (the Twins). At the center of the image, a massive star is forming and blasting cavities through the clouds with a pair of powerful jets, extending to the top right and bottom left of the image. Light from this star is mostly escaping and reaching us by illuminating these cavities, like a lighthouse piercing through the storm clouds [Text credit: European Space Agency (ESA), image credit: ESA/Hubble & NASA, J. C. Tan (Chalmers University & University of Virginia), R. Fedriani (Chalmers University); Acknowledgment: Judy Schmidt]

• August 13, 2021: Clustered at the center of this image are six luminous spots of light, four of them forming a circle around a central pair. Appearances can be deceiving, however, as this formation is not composed of six individual galaxies, but only three: to be precise, a pair of galaxies and one distant quasar. Hubble data also indicates that there is a seventh spot of light in the very center, which is a rare fifth image of the distant quasar. This rare phenomenon is caused by the presence of two galaxies in the foreground that act as a lens. 19)

- The central pair of galaxies in this image of Figure 17 are genuinely two separate galaxies. The four bright points circling them, and the fainter one in the very center, are actually five separate images of a single quasar (known as 2M1310-1714), an extremely luminous but distant object. The reason behind this "seeing quintuple" effect is a phenomenon known as gravitational lensing. Gravitational lensing occurs when a celestial object with an enormous amount of mass — such as a pair of galaxies — causes the fabric of space to warp such that the light travelling through that space from a distant object is bent and magnified sufficiently that humans here on Earth can observe multiple magnified images of the far-away source. The quasar in this image actually lies further away from Earth than the pair of galaxies. The light from the quasar has been bent around the galaxy pair because of their enormous mass, giving the incredible appearance that the galaxy pair are surrounded by four quasars — whereas in reality, a single quasar lies far beyond them!


Figure 17: These galaxies were imaged in spectacular detail by Hubble's Wide Field Camera 3 (WFC3), which was installed on Hubble in 2009 during Hubble Servicing Mission 4, Hubble's final servicing mission. The WFC3 was intended to operate until 2014, but 12 years after it was installed it continues to provide both top-quality data and fantastic images, such as this one (image credit: ESA/Hubble & NASA, T. Treu; CC BY 4.0 Acknowledgment: J. Schmidt)

• August 6, 2021: The spiral galaxy IC 1954 takes centre stage in this image from the NASA/ESA Hubble Space Telescope. The galaxy, which lies approximately 45 million light-years from Earth in the constellation Horologium (The Clock), boasts a bright central bar and lazily winding spiral arms threaded with dark clouds of dust. 20)

- These observations also lay the groundwork for future observations with the upcoming NASA/ESA/CSA James Webb Space Telescope, which will peer into nearby galaxies and observe the earliest phases of star formation.


Figure 18: This portrait of IC 1954 was captured with Hubble's Wide Field Camera 3, and is one of a set of observations designed to take advantage of some telescope teamwork. Hubble observed groups of young stars in nearby galaxies at ultraviolet and optical wavelengths while the ALMA (Atacama Large Millimeter/submillimeter Array) — a ground-based radio telescope — gathered data on star-forming discs and clouds of cold gas. Combining the two sets of observations allowed astronomers to join the dots and understand the connections between young stars and the clouds of cold gas which give rise to them (image credit: ESA/Hubble & NASA, J. Lee and the PHANGS-HST Team; CC BY 4.0)

• July 30, 2021: A dramatic triplet of galaxies takes center stage in this latest Picture of the Week from the NASA/ESA Hubble Space Telescope, which captures a three-way gravitational tug-of-war between interacting galaxies. This system —known as Arp 195— is featured in the Atlas of Peculiar Galaxies, a list which showcases some of the weirder and more wonderful galaxies in the universe. 21)


Figure 19: Observing time with the Hubble Space Telescope is extremely valuable, so astronomers don't want to waste a second. The schedule for Hubble observations is calculated using a computer algorithm which allows the spacecraft to occasionally gather bonus snapshots of data between longer observations. This image of the clashing triplet of galaxies in Arp 195 is one such snapshot. Extra observations such as these do more than provide spectacular images — they also help to identify promising targets to follow up with telescopes such as the upcoming NASA/ESA/CSA James Webb Space Telescope (image credit: ESA/Hubble & NASA, J. Dalcanton; CC BY 4.0)

• July 26, 2021: Astronomers have used archival datasets from the NASA/ESA Hubble Space Telescope to reveal the first evidence for water vapor in the atmosphere of Jupiter's moon Ganymede, the result of the thermal escape of water vapor from the moon's icy surface. 22) 23)

- Jupiter's moon Ganymede is the largest moon — and the ninth-largest object — in the Solar System. It may hold more water than all of Earth's oceans, but temperatures there are so cold that water on the surface freezes and the ocean lies roughly 160 kilometers below the crust. Nevertheless, where there is water there could be life as we know it. Identifying liquid water on other worlds is crucial in the search for habitable planets beyond Earth. And now, for the first time, evidence has been found for a sublimated water atmosphere on the icy moon Ganymede.

- In 1998, Hubble's Space Telescope Imaging Spectrograph (STIS) took the first ultraviolet (UV) pictures of Ganymede, which revealed a particular pattern in the observed emissions from the moon's atmosphere. The moon displays auroral bands that are somewhat similar to the auroral ovals observed on Earth and other planets with magnetic fields. These images were therefore illustrative evidence that Ganymede has a permanent magnetic field. The similarities between the two ultraviolet observations were explained by the presence of molecular oxygen, O2. The differences were explained at the time by the presence of atomic oxygen, O, which produces a signal that affects one UV color more than the other.

- As part of a large observing programme to support NASA's Juno mission in 2018, Lorenz Roth, of the KTH Royal Institute of Technology in Stockholm, Sweden, led a team that set out to capture UV spectra of Ganymede with Hubble's Cosmic Origins Spectrograph (COS) instrument to measure the amount of atomic oxygen. They carried out a combined analysis of new spectra taken in 2018 with the COS and archival images from the STIS instrument from 1998 and 2010. To their surprise, and in contrast to the original interpretations of the data from 1998, they discovered there was hardly any atomic oxygen in Ganymede's atmosphere. This means there must be another explanation for the apparent differences between the UV aurora images.

- The explanation was then uncovered by Roth and his team in the relative distribution of the aurorae in the two images. Ganymede's surface temperature varies strongly throughout the day, and around noon near the equator it may become sufficiently warm that the icy surface releases some small amounts of water molecules. In fact, the perceived differences between the UV images are directly correlated with where water would be expected in the moon's atmosphere.

- "Initially only the O2 had been observed," explained Roth. "This is produced when charged particles erode the ice surface. The water vapor that we have now measured originates from ice sublimation caused by the thermal escape of H2O vapor from warm icy regions."

- This finding adds anticipation to ESA's upcoming JUpiter ICy moons Explorer (JUICE) mission — the first large-class mission in ESA's Cosmic Vision 2015–2025 programme. Planned for launch in 2022 and arrival at Jupiter in 2029, it will spend at least three years making detailed observations of Jupiter and three of its largest moons, with particular emphasis on Ganymede as a planetary body and potential habitable world. Ganymede was identified for detailed investigation because it provides a natural laboratory for the analysis of the nature, evolution and potential habitability of icy worlds in general and the role it plays within the system of Galilean satellites, and its unique magnetic and plasma interactions with Jupiter and its environment (known as the Jovian system).


Figure 20: This image presents Jupiter's moon Ganymede as seen by the NASA/ESA Hubble Space Telescope in 1996. Located over 600 million kilometers away, Hubble can follow changes on the moon and reveal other characteristics at ultraviolet and near-infrared wavelengths. - Astronomers have now used archival datasets from Hubble to reveal the first evidence for water vapor in the atmosphere of Jupiter's moon Ganymede, the result of the thermal escape of water vapor from the moon's icy surface (image credit: ESA/Hubble, NASA, J. Spencer)

- "Our results can provide the JUICE instrument teams with valuable information that may be used to refine their observation plans to optimize the use of the spacecraft," added Roth.

- Understanding the Jovian system and unravelling its history, from its origin to the possible emergence of habitable environments, will provide us with a better understanding of how gas giant planets and their satellites form and evolve. In addition, new insights will hopefully be found into the potential for the emergence of life in Jupiter-like exoplanetary systems.

- A paper is published in the journal Nature Astronomy. 24)

Figure 21: Astronomers have used new and archival datasets from NASA's Hubble Space Telescope to uncover evidence of water vapor in the atmosphere of Jupiter's moon Ganymede. The vapor is present due to the thermal excitation of water molecules from the moon's icy surface. Previous research has offered circumstantial evidence for the moon containing more water than all of Earth's oceans. However, temperatures there are so cold that water on the surface freezes and the ocean lies roughly 100 miles below the crust (video credit: NASA's Goddard Space Flight Center) 25)

• July 23, 2021: The center of this image from the NASA/ESA Hubble Space Telescope is framed by the tell-tale arcs that result from strong gravitational lensing, a striking astronomical phenomenon which can warp, magnify, or even duplicate the appearance of distant galaxies. 26)

- Gravitational lensing occurs when light from a distant galaxy is subtly distorted by the gravitational pull of an intervening astronomical object. In this case, the relatively nearby galaxy cluster MACSJ0138.0-2155 has lensed a significantly more distant quiescent galaxy — a slumbering giant known as MRG-M0138 which has run out of the gas required to form new stars and is located 10 billion light years away. Astronomers can use gravitational lensing as a natural magnifying glass, allowing them to inspect objects like distant quiescent galaxies which would usually be too difficult for even Hubble to resolve.


Figure 22: This image was made using observations from eight different infrared filters spread across two of Hubble's most advanced astronomical instruments: the Advanced Camera for Surveys and the Wide Field Camera 3. These instruments were installed by astronauts during the final two servicing missions to Hubble, and provide astronomers with superbly detailed observations across a large area of sky and a wide range of wavelengths (image credit: ESA/Hubble & NASA, A. Newman, M. Akhshik, K. Whitaker; CC BY 4.0)

• July 21, 2021: According to the latest cosmological models, large spiral galaxies such as the Milky Way grew by absorbing smaller galaxies, by a sort of galactic cannibalism. Evidence for this is given by very large structures, the tidal stellar streams, which are observed around them, which are the remains of these satellite galaxies. But the full histories of the majority of these cases are hard to study, because these flows of stars are very faint, and only the remains of the most recent mergers have been detected. 27)


Figure 23: Hubble image of the Sombrero galaxy (M104), image credit: Manuel Jiménez/Giuseppe Donatiello

- A study led by the Instituto de Astrofísica de Andalucía (IAA-CSIC), with the participation of the Instituto de Astrofísica de Canarias (IAC), has made detailed observations of a large tidal flow around the Sombrero galaxy, whose strange morphology has still not been definitively explained. The results are published today in the journal Monthly Notices of the Royal Astronomical Society (MNRAS). 28)

- The Sombrero galaxy (Messier 104) is a galaxy some thirty million light years away, which is part of the Local Supercluster (a group of galaxies which includes the Virgo cluster and the Local Group containing the Milky Way). It has roughly one third of the diameter of the Milky Way, and shows characteristics of both of the dominant types of galaxies in the Universe, the spirals and the ellipticals. It has spiral arms, and a very large bright central bulge, which makes it look like a hybrid of the two types.

- "Our motive for obtaining these very deep images of the Sombrero galaxy (Messier 104) was to look for the remains of its merger with a very massive galaxy. This possible collision was recently suggested on the basis of studies of the stellar population of its strange halo obtained with the Hubble Space Telescope", says David Martínez-Delgado, a researcher at the IAA-CSIC and first author of the paper reporting the work.

- The observations with the Hubble, in 2020, showed that the halo, an extensive and faint region surrounding the Sombrero galaxy, shows many stars rich in metals, elements heavier than hydrogen and helium. This is a feature to typical of new generations of stars, which are normally found in the discs of galaxies, and are quite unusual in galactic halos, which are populated by old stars. To explain their presence astronomers suggested what is known as "a wet merger", a scenario in which a large elliptical galaxy is rejuvenated by large quantities of gas and dust from another massive galaxy, which went into the formation of the disc which we now observe.

- "In our images we have not found any evidence to support this hypothesis, although we cannot rule out that it could have happened several thousand million years ago, and the debris is completely dissipated by now -explains David Martínez-Delgado-. In our search we have in fact been able to trace for the first time the complete tidal stream which surrounds the disc of this galaxy, and our theoretical simulations have let us reconstruct its formation in the last three thousand million years, by cannibalism of a satellite dwarf galaxy".

- "Observational techniques in present day Astrophysics need advanced image processing. Our modelling of the bright stars around the Sombrero galaxy, and at the same time of the halo light of the galaxy itself has enabled us to unveil the nature of this tidal stream. It is remarkable that thanks to these advanced photometric techniques we have been able to do front line science with a Messier object using only an 18 cm (diameter) telescope", explains Javier Román, a postdoctoral researcher at the IAC and a co-author of the study.

- The research team rejects the idea that the large stellar tidal stream, known for more than three decades, could be related to the event which produced the strange morphology of the Sombrero galaxy which, if it was caused by a wet merger, would need the interaction of two galaxies with large masses.

- The work has been possible thanks to the collaboration between professional and amateur astronomers. "We have collaborated with the Spanish astrophotographer Manuel Jiménez, who took the images with a robotic telescope of 18 cm diameter, and the well-known Australian astrophotographer David Malin, who discovered this tidal stream on photographic plates taken in the 90's of the last century. This collaboration shows the potential of amateur telescopes to take deep images of nearby galaxies which give important clues about the process of their assembly which is continuing until the present epoch", concludes Martínez-Delgado.

Figure 24: Simulation of the tidal stream in the Sombrero Galaxy (M104). An image of the galaxy is shown on the left and a simulation movie that matches the current location of the flow is shown on the right [video credit: Denis Erkal (University of Surrey, UK), David Martínez-Delgado (IAA-CSIC)]


Figure 25: Artist's conception of the tidal stream of the Sombrero galaxy (M104), image credit: Jon Lomberg for the Stellar Tidal Stream Survey.

• July 19, 2021: NASA's Hubble Space Telescope is back in business, exploring the universe near and far. The science instruments have returned to full operation, following recovery from a computer anomaly that suspended the telescope's observations for more than a month. 29)


Figure 26: These images, from a program led by Julianne Dalcanton of the University of Washington in Seattle, demonstrate Hubble's return to full science operations. Left: ARP-MADORE2115-273 is a rarely observed example of a pair of interacting galaxies in the southern hemisphere. Right: ARP-MADORE0002-503 is a large spiral galaxy with unusual, extended spiral arms. While most disk galaxies have an even number of spiral arms, this one has three [image credits: Science: NASA, ESA, STScI, Julianne Dalcanton (UW) Image processing: Alyssa Pagan (STScI)]

- "I'm thrilled to see that Hubble has its eye back on the universe, once again capturing the kind of images that have intrigued and inspired us for decades," said NASA Administrator Bill Nelson. "This is a moment to celebrate the success of a team truly dedicated to the mission. Through their efforts, Hubble will continue its 32nd year of discovery, and we will continue to learn from the observatory's transformational vision."

- Hubble's payload computer, which controls and coordinates the observatory's onboard science instruments, halted suddenly on June 13. When the main computer failed to receive a signal from the payload computer, it automatically placed Hubble's science instruments into safe mode. That meant the telescope would no longer be doing science while mission specialists analyzed the situation.

- The Hubble team moved quickly to investigate what ailed the observatory, which orbits about 340 miles (547 kilometers) above Earth. Working from mission control at NASA's Goddard Space Flight Center in Greenbelt, Maryland, as well as remotely due to COVID-19 restrictions, engineers collaborated to figure out the cause of the problem.

- Complicating matters, Hubble was launched in 1990 and has been observing the universe for over 31 years. To fix a telescope built in the 1980s, the team had to draw on the knowledge of staff from across its lengthy history.

- Hubble alumni returned to support the current team in the recovery effort, lending decades of mission expertise. Retired staff who helped build the telescope, for example, knew the ins and outs of the Science Instrument and Command & Data Handling unit, where the payload computer resides – critical expertise for determining next steps for recovery. Other former team members lent a hand by scouring Hubble's original paperwork, surfacing 30- to 40-year-old documents that would help the team chart a path forward.

Figure 27: On June 13, 2021, the Hubble Space Telescope's payload computer unexpectedly came to a halt. However, the Hubble team methodically identified the possible cause and how to compensate for it (video credit: NASA's Goddard Space Flight Center)

- "That's one of the benefits of a program that's been running for over 30 years: the incredible amount of experience and expertise," said Nzinga Tull, Hubble systems anomaly response manager at Goddard. "It's been humbling and inspiring to engage with both the current team and those who have moved on to other projects. There's so much dedication to their fellow Hubble teammates, the observatory, and the science Hubble is famous for."

- Together, team members new and old worked their way through the list of likely culprits, seeking to isolate the issue to ensure they have a full inventory for the future of which hardware is still working.

- The team then moved on to explore whether other hardware was at fault, including the Command Unit/Science Data Formatter and the Power Control Unit, which is designed to ensure a steady voltage supply to the payload computer's hardware. However, it would be more complicated to address either of these issues, and riskier for the telescope in general. Switching to these components' backup units would require switching several other hardware boxes as well.

- "The switch required 15 hours of spacecraft commanding from the ground. The main computer had to be turned off, and a backup safe mode computer temporarily took over the spacecraft. Several boxes also had to be powered on that were never turned on before in space, and other hardware needed their interfaces switched," said Jim Jeletic, Hubble deputy project manager at Goddard. "There was no reason to believe that all of this wouldn't work, but it's the team's job to be nervous and think of everything that could go wrong and how we might compensate for it. The team meticulously planned and tested every small step on the ground to make sure they got it right."

- The team proceeded carefully and systematically from there. Over the following two weeks, more than 50 people worked to review, update, and vet the procedures to switch to backup hardware, testing them on a high-fidelity simulator and holding a formal review of the proposed plan.

- Simultaneously, the team analyzed the data from their earlier tests, and their findings pointed to the Power Control Unit as the possible cause of the issue. On July 15, they made the planned switch to the backup side of the Science Instrument and Command & Data Handling unit, which contains the backup Power Control Unit.

- Victory came around 11:30 p.m. EDT July 15, when the team determined the switch was successful. The science instruments were then brought to operational status, and Hubble began taking scientific data once again on July 17. Most observations missed while science operations were suspended will be rescheduled.


Figure 28: Nzinga Tull, Hubble systems anomaly response manager at NASA's Goddard Space Flight Center in Greenbelt, Maryland, works in the control room July 15 to restore Hubble to full science operations (image credit: NASA/GSFC, Rebecca Roth)

- This is not the first time Hubble has had to rely on backup hardware. The team performed a similar switch in 2008, returning Hubble to normal operations after another part of the Science Instrument and Command & Data Handling (SI C&DH) unit failed. Hubble's final servicing mission in 2009 – a much-needed tune-up championed by former U.S. Senator Barbara Mikulski – then replaced the entire SI C&DH unit, greatly extending Hubble's operational lifetime.

- Since that servicing mission, Hubble has taken more than 600,000 observations, bringing its lifetime total to more than 1.5 million. Those observations continue to change our understanding of the universe.


Figure 29: Members of the Hubble operations team work in the control room July 15 to restore Hubble to science operations (image credit: NASA/GSFC, Rebecca Roth)

- "Hubble is in good hands. The Hubble team has once again shown its resiliency and prowess in addressing the inevitable anomalies that arise from operating the world's most famous telescope in the harshness of space," said Kenneth Sembach, director of the Space Telescope Science Institute (STScI) in Baltimore, Maryland, which conducts Hubble science operations. "I am impressed by the team's dedication and common purpose over the past month to return Hubble to service. Now that Hubble is once again providing unprecedented views of the universe, I fully expect it will continue to astound us with many more scientific discoveries ahead."

- Hubble has contributed to some of the most significant discoveries of our cosmos, including the accelerating expansion of the universe, the evolution of galaxies over time, and the first atmospheric studies of planets beyond our solar system. Its mission was to spend at least 15 years probing the farthest and faintest reaches of the cosmos, and it continues to far exceed this goal.

- "The sheer volume of record-breaking science Hubble has delivered is staggering," said Thomas Zurbuchen, associate administrator of NASA's Science Mission Directorate. "We have so much to learn from this next chapter of Hubble's life – on its own, and together with the capabilities of other NASA observatories. I couldn't be more excited about what the Hubble team has achieved over the past few weeks. They've met the challenges of this process head on, ensuring that Hubble's days of exploration are far from over."

July 17, 2021: NASA has returned the science instruments on the Hubble Space Telescope to operational status, and the collection of science data will now resume. This will be the first science data collected since the payload computer experienced a problem on June 13, which placed the instruments in a safe configuration and suspended science operations. 30)

- Hubble is an icon, giving us incredible insight into the cosmos over the past three decades," said NASA Administrator Bill Nelson. "I'm proud of the Hubble team, from current members to Hubble alumni who stepped in to lend their support and expertise. Thanks to their dedication and thoughtful work, Hubble will continue to build on its 31-year legacy, broadening our horizons with its view of the universe."

- The first observation is scheduled for Saturday afternoon after some instrument calibrations are completed. Most observations missed while science operations were suspended will be rescheduled for a later date.

- The Hubble team has been investigating the cause of the payload computer problem since it first occurred. On July 15, the team switched the spacecraft to backup hardware.

- NASA anticipates that Hubble will last for many more years and will continue making groundbreaking observations, working in tandem with other space observatories including the James Webb Space Telescope to further our knowledge of the cosmos.

- Launched in 1990, Hubble has been observing the universe for over 31 years. It has taken over 1.5 million observations of the universe, and over 18,000 scientific papers have been published with its data. It has contributed to some of the most significant discoveries of our cosmos, including the accelerating expansion of the universe, the evolution of galaxies over time, and the first atmospheric studies of planets beyond our solar system.

• July 16, 2021: NASA has successfully switched to backup hardware on the Hubble Space Telescope, including powering on the backup payload computer, on July 15. The switch was performed to compensate for a problem with the original payload computer that occurred on June 13 when the computer halted, suspending science data collection. 31)

- NASA has successfully switched to backup hardware on the Hubble Space Telescope, including powering on the backup payload computer, on July 15. The switch was performed to compensate for a problem with the original payload computer that occurred on June 13 when the computer halted, suspending science data collection.

- The switch included bringing online the backup Power Control Unit (PCU) and the backup Command Unit/Science Data Formatter (CU/SDF) on the other side of the Science Instrument and Command & Data Handling (SI C&DH) unit. The PCU distributes power to the SI C&DH components, and the CU/SDF sends and formats commands and data. In addition, other pieces of hardware onboard Hubble were switched to their alternate interfaces to connect to this backup side of the SI C&DH. Once these steps were completed, the backup payload computer on this same unit was turned on and loaded with flight software and brought up to normal operations mode.

- The Hubble team is now monitoring the hardware to ensure that everything is working properly. The team has also started the process for recovering the science instruments out of their safe mode configuration. This activity is expected to take more than a day as the team runs various procedures and ensures the instruments are at stable temperatures. The team will then conduct some initial calibration of the instruments before resuming normal science operations.

• July 16, 2021: This image shows the globular cluster NGC 6380, which lies around 35,000 light-years from Earth, in the constellation Scorpio (The Scorpion). The very bright star at the top of the image is HD 159073, which is only around 4000 light-years from Earth, making it a much nearer neighbor to Earth than NGC 6380.

- NGC 6380 is not a particularly exciting name, but it indicates that this cluster is catalogued in the New General Catalogue (NGC), which was originally compiled in 1888. This cluster has, however, been known by many other names. It was originally discovered by James Dunlop in 1826, and he rather immodestly named it Dun 538. Eight years later, in 1834, it was independently rediscovered by John Herschel and he (similarly immodestly) went on to name it H 3688. The cluster was re-rediscovered in 1959 in Paris by Pişmiş, who catalogued it as Tonantzintla 1 — and who, to continue the pattern, also referred to it as Pişmiş 25. In addition to its colorful history of rediscovery, up until the 1950s NGC 6380 was thought to be an open cluster. It was A. D. Thackeray who realized that it was in fact a globular cluster. Nowadays, this cluster is reliably recognized in widely available catalogues as a globular cluster, and referred to simply as NGC 6380.


Figure 30: This image of NGC6380 was taken with Hubble's Wide Field Camera 3 (WFC3), which, as its name suggests, has a wide field of view, meaning that it can image relatively large areas of the sky in enormous detail (image credit: ESA/Hubble & NASA, E. Noyola; CC BY 4.0)

• July 14, 2021: Dr. Iair Arcavi, a Tel Aviv University researcher at the Raymond and Beverly Sackler Faculty of Exact Sciences, participated in a study that discovered a new type of stellar explosion - an electron-capture supernova. While they have been theorized for 40 years, real-world examples have been elusive. Such supernovas arise from the explosions of stars 8-9 times the mass of the sun. The discovery also sheds new light on the thousand-year mystery of the supernova from A.D. 1054 that was seen by ancient astronomers, before eventually becoming the Crab Nebula, that we know today. 32)

- A supernova is the explosion of a star following a sudden imbalance between two opposing forces that shaped the star throughout its life. Gravity tries to contract every star. Our sun, for example, counter balances this force through nuclear fusion in its core, which produces pressure that opposes the gravitational pull. As long as there is enough nuclear fusion, gravity will not be able to collapse the star. However, eventually, nuclear fusion will stop, just like gas runs out in a car, and the star will collapse. For stars like the sun, the collapsed core is called a white dwarf. This material in white dwarfs is so dense that quantum forces between electrons prevent further collapse.

- For stars 10 times more massive than our sun, however, electron quantum forces are not enough to stop the gravitational pull, and the core continues to collapse until it becomes a neutron star or a black hole, accompanied by a giant explosion. In the intermediate mass range, the electrons are squeezed (or more accurately, captured) onto atomic nuclei. This removes the electron quantum forces, and causes the star to collapse and then explode.

- Historically, there have been two main supernova types. One is a thermonuclear supernova — the explosion of a white dwarf star after it gains matter in a binary star system. These white dwarfs are the dense cores of ash that remain after a low-mass star (one up to about 8 times the mass of the sun) reaches the end of its life. Another main supernova type is a core-collapse supernova where a massive star — one more than about 10 times the mass of the sun — runs out of nuclear fuel and has its core collapsed, creating a black hole or a neutron star. Theoretical work suggested that electron-capture supernovae would occur on the borderline between these two types of supernovae.

- That's the theory that was developed in the 1980's by Ken'ichi Nomoto of the University of Tokyo, and others. Over the decades, theorists have formulated predictions of what to look for in an electron-capture supernova. The stars should lose a lot of mass of particular composition before exploding, and the supernova itself should be relatively weak, have little radioactive fallout, and produce neutron-rich elements.

- The new study, published in Nature Astronomy, focuses on the supernova SN2018zd, discovered in 2018 by Japanese amateur astronomer Koihchi Itagaki. Dr. Iair Arcavi, of the astrophysics department at Tel Aviv University, also took part in the study. This supernova, located in the galaxy NGC 2146, has all of the properties expected from an electron-capture supernova, which were not seen in any other supernova. In addition, because the supernova is relatively nearby - only 31 million light years away - the researchers were able to identify the star in pre-explosion archival images taken by the Hubble Space Telescope. Indeed, the star itself also fits the predictions of the type of star that should explode as an electron-capture supernovae, and is unlike stars that were seen to explode as the other types of supernovae. 33)


Figure 31: Hubble Space Telescope color composite of the electron-capture supernova 2018zd and the host starburst galaxy NGC 2146 (image credit: NASA/STScI/J. DePasquale; Las Cumbres Observatory)

- While some supernovae discovered in the past had a few of the indicators predicted for electron-capture supernovae, only SN2018zd had all six - a progenitor star that fits within the expected mass range, strong pre-supernova mass loss, an unusual chemical composition, a weak explosion, little radioactivity, and neutron-rich material. "We started by asking 'what's this weirdo?'" said Daichi Hiramatsu of the University of California Santa Barbara and Las Cumbres Observatory, who led the study. "Then we examined every aspect of SN 2018zd and realized that all of them can be explained in the electron-capture scenario."

- The new discoveries also illuminate some mysteries of one of the most famous supernovae of the past. In A.D. 1054 a supernova happened in our own Milky Way Galaxy, and according to Chinese and Japanese records, it was so bright that it could be seen in the daytime and cast shadows at night. The resulting remnant, the Crab Nebula, has been studied in great detail, and was found to have an unusual composition. It was previously the best candidate for an electron-capture supernova, but this was uncertain partly because the explosion happened nearly a thousand years ago. The new result increases the confidence that the historic 1054 supernova was an electron-capture supernova.

- "It's amazing that we can shed light on historical events in the Universe with modern instruments," says Dr. Arcavi. "Today, with robotic telescopes that scan the sky in unprecedented efficiency, we can discover more and more rare events which are critical for understanding the laws of nature, without having to wait 1000 years between one event and the next."

• July 9, 2021: Two things capture your attention in this spectacular Picture of the Week, which was taken using Hubble's Wide Field Camera 3 (WFC3): the two enormous galaxies that flank the left and right sides of the image. The galaxy on the left is a lenticular galaxy, which rejoices in the name of 2MASX J03193743+4137580. The side-on spiral galaxy on the right is more simply named UGC 2665. Both galaxies lie approximately 350 million light-years from Earth, and they both form part of the enormous Perseus galaxy cluster. 34)


Figure 32: Perseus is an important figure in Greek mythology, renowned for slaying Medusa the Gorgon — who is herself famous for the unhappy reason that she was cursed to have living snakes for hair. Given Perseus's impressive credentials, it seems appropriate that the eponymous galaxy cluster is one of the biggest objects in the known Universe, consisting of thousands of galaxies, only a few of which are visible in this image. The wonderful detail in the image is thanks to the WFC3's powerful resolution and high sensitivity. The WFC3 is sensitive to both visible and infrared light, so those are the wavelengths that are captured in this image. The Perseus supercluster looks very different at other wavelengths. Whilst in this image the spaces between the galaxies appear dark and peaceful, when the X-ray emission is observed the Perseus cluster appears to be burning with bright intense light (image credit: ESA/Hubble & NASA, W. Harris; CC BY 4.0 Acknowledgement: L. Shatz)

• July 2, 2021: Pictures of the Week from the NASA/ESA Hubble Space Telescope show us something new about the Universe. This image, however, also contains clues about the inner workings of Hubble itself. The crisscross patterns surrounding the stars in this image — known as diffraction spikes — were created when starlight interacted with the four thin vanes supporting Hubble's secondary mirror. 35)

- As star clusters form from a single primordial cloud of gas and dust, all the stars they contain are roughly the same age. This makes them useful natural laboratories for astronomers to learn how stars form and evolve. This image uses observations from Hubble's Wide Field Camera 3, and incorporates data from two very different astronomical investigations. The first aimed to understand why stars in star clusters appear to evolve differently from stars elsewhere, a peculiarity first observed by the Hubble Space Telescope. The second aimed to determine how large stars can be before they become doomed to end their lives in cataclysmic supernova explosions.


Figure 33: This Picture of the Week depicts the open star cluster NGC 330, which lies around 180,000 light-years away inside the Small Magellanic Cloud. The cluster — which is in the constellation Tucana (The Toucan) — contains a multitude of stars, many of which are scattered across this striking image (image credit: ESA/Hubble & NASA, J. Kalirai, A. Milone; CC BY 4.0)

June 30, 2021: NASA Preparing for Procedures to Turn On Backup Hardware on the Hubble Space Telescope. 36)

- NASA is taking additional steps to investigate the Hubble Space Telescope's payload computer issue that began on June 13, suspending science observations. In parallel with the investigation, NASA is preparing and testing procedures to turn on backup hardware onboard the spacecraft. The telescope itself and science instruments remain healthy and in a safe configuration.

- The source of the computer problem lies in the Science Instrument Command and Data Handling (SI C&DH) unit, where the payload computer resides. A few hardware pieces on the SI C&DH could be the culprit(s).

- The team is currently scrutinizing the Command Unit/Science Data Formatter (CU/SDF), which sends and formats commands and data. They are also looking at a power regulator within the Power Control Unit, which is designed to ensure a steady voltage supply to the payload computer's hardware. If one of these systems is determined to be the likely cause, the team must complete a more complicated operations procedure to switch to the backup units. This procedure would be more complex and riskier than those the team executed last week, which involved switching to the backup payload computer hardware and memory modules. To switch to the backup CU/SDF or power regulator, several other hardware boxes on the spacecraft must also be switched due to the way they are connected to the SI C&DH unit.

- Over the next week or so, the team will review and update all of the operations procedures, commands and other related items necessary to perform the switch to backup hardware. They will then test their execution against a high-fidelity simulator.

- The team performed a similar switch in 2008, which allowed Hubble to continue normal science operations after a CU/SDF module failed. A servicing mission in 2009 then replaced the entire SI C&DH unit, including the faulty CU/SDF module, with the SI C&DH unit currently in use.

- Since that servicing mission, Hubble has taken over 600,000 additional observations to exceed 1.5 million during its lifetime. Those observations continue to change our understanding of the universe.

- Launched in 1990, Hubble has been observing the universe for over 31 years. It has contributed to some of the most significant discoveries of our cosmos, including the accelerating expansion of the universe, the evolution of galaxies over time, and the first atmospheric studies of planets beyond our solar system. Read more about some of Hubble's greatest scientific discoveries.

June 25, 2021 - NASA Completes Additional Tests to Diagnose Computer Problem on Hubble Space Telescope

- NASA is continuing to diagnose a problem with the payload computer on the Hubble Space Telescope after completing another set of tests on June 23 and 24. The payload computer halted on June 13 and the spacecraft stopped collecting science data. The telescope itself and its science instruments remain in good health and are currently in a safe configuration.

- The spacecraft has two payload computers, one of which serves as a backup, that are located on the Science Instrument and Command and Data Handling (SI C&DH) unit. There are various pieces of hardware which make up both payload computers, including but not limited to:

a) a Central Processing Module (CPM), which processes the commands that coordinate and control the science instruments

b) a Standard Interface (STINT), which bridges communications between the computer's CPM and other components

c) a communications bus, which contains lines that pass signals and data between hardware

d) and one active memory module, which stores operational commands to the instruments. There are three additional modules which serve as backups.

- Additional tests performed on June 23 and 24 included turning on the backup computer for the first time in space. The tests showed that numerous combinations of these hardware pieces from both the primary and backup payload computer all experienced the same error - commands to write into or read from memory were not successful.

- Since it is highly unlikely that all individual hardware elements have a problem, the team is now looking at other hardware as the possible culprit, including the Command Unit/Science Data Formatter (CU/SDF), another module on the SI C&DH. The CU formats and sends commands and data to specific destinations, including the science instruments. The SDF formats the science data from the science instruments for transmission to the ground. The team is also looking at the power regulator to see if possibly the voltages being supplied to hardware are not what they should be. A power regulator ensures a steady constant voltage supply. If the voltage is out of limits, it could cause the problems observed.

- Over the next week, the team will continue to assess hardware on the SI C&DH unit to identify if something else may be causing the problem. If the team determines the CU/SDF or the power regulator is the likely cause, they will recommend switching to the backup CU/SDF module and the backup power regulator.

- Launched in 1990, Hubble has been observing the universe for over 31 years. It has contributed to some of the most significant discoveries of our cosmos, including the accelerating expansion of the universe, the evolution of galaxies over time, and the first atmospheric studies of planets beyond our solar system.

June 22, 2021 - Testing Underway to Identify Issue and Restore Payload Computer on NASA's Hubble Space Telescope

- NASA continues to work to resolve a problem with the Hubble Space Telescope payload computer that halted on June 13. After performing tests on several of the computer's memory modules, the results indicate that a different piece of computer hardware may have caused the problem, with the memory errors being only a symptom. The operations team is investigating whether the Standard Interface (STINT) hardware, which bridges communications between the computer's Central Processing Module (CPM) and other components, or the CPM itself is responsible for the issue. The team is currently designing tests that will be run in the next few days to attempt to further isolate the problem and identify a potential solution.

- This step is important for determining what hardware is still working properly for future reference. If the problem with the payload computer can't be fixed, the operations team will be prepared to switch to the STINT and CPM hardware onboard the backup payload computer. The team has conducted ground tests and operations procedure reviews to verify all the commanding required to perform that switch on the spacecraft.

- If the backup payload computer's CPM and STINT hardware is turned on, several days will be required to assess the computer performance and restore normal science operations. The backup computer has not been powered on since its installation in 2009; however, it was thoroughly tested on the ground prior to installation on the spacecraft.

- The payload computer is a NASA Standard Spacecraft Computer-1 (NSSC-1) system built in the 1980s that is located on the Science Instrument Command and Data Handling (SI C&DH) unit. After 18 years on orbit, the original SI C&DH experienced a failure in 2008 that delayed the final servicing mission to Hubble while a replacement was prepared for flight. In May 2009, STS-125 was launched and the astronauts installed the existing unit. The replacement contains original hardware from the 1980s with four independent 64K memory modules of Complementary Metal-Oxide Semiconductor (CMOS) memory. Only one memory module is used operationally, with the other three serving as backups. All four modules can be used and accessed from either of the redundant payload computers.

• June 25, 2021: A cataclysmic cosmic collision takes centre stage in this Picture of the Week. The image features the interacting galaxy pair IC 1623, which lies around 275 million light-years away in the constellation Cetus (The Whale). The two galaxies are in the final stages of merging, and astronomers expect a powerful inflow of gas to ignite a frenzied burst of star formation in the resulting compact starburst galaxy. 37)


Figure 34: This interacting pair of galaxies is a familiar sight; Hubble captured IC 1623 in 2008 using two filters at optical and infrared wavelengths using the Advanced Camera for Surveys. This new image incorporates new data from the Wide Field Camera 3, and combines observations taken in eight filters spanning infrared to ultraviolet wavelengths to reveal the finer details of IC 1623. Future observations of the galaxy pair with the NASA/ESA/CASA James Webb Space Telescope will shed more light on the processes powering extreme star formation in environments such as IC 1623 (image credit: ESA/Hubble & NASA, R. Chandar; CC BY 4.0)

• June 19, 2021: Spacecraft controllers are continuing to work on a faulty computer memory system on NASA's Hubble Space Telescope that has stopped telescope operations for nearly a week. 38)

- A payload computer on Hubble stopped working June 13, the agency said in a June 16 statement. Engineers speculated that the computer, used to manage operations of Hubble's science instruments, malfunctioned because of a degrading memory module, putting the instruments into a safe mode.

- The agency said at the time that it would switch of a backup memory module that day and, after about a day of testing, restart the instruments and resume science observations.

- However, in a June 18 statement, NASA said those efforts to switch to a backup memory module failed because "the command to initiate the backup module failed to complete." An attempt to restore the computer with both the original memory module and the backup unit also failed.

- NASA didn't elaborate on the next steps it will take to correct the problem stating only that the operations team "will be running tests and collecting more information on the system to further isolate the problem." The instruments themselves, and the rest of the telescope, remain in good health.

- The payload computer is a 1980s-vintage system that can use any one of four memory modules, each containing 64 kilobytes of complementary metal-oxide semiconductor memory. A backup computer is also available.

- This is not the first technical glitch for Hubble, launched 31 years ago and last serviced by the space shuttle 12 years ago. In March, a problem linked to a software "enhancement" recently uploaded to the telescope put the telescope into a safe mode for several days. A faulty gyro took the telescope offline for three weeks in October 2018.

- "We do have anomalies. That happens when you have a decades-old observatory, but we have been able to resolve those anomalies," Nancy Levenson, deputy director of the Space Telescope Science Institute, said at a town hall session during the 238th Meeting of the American Astronomical Society June 8.

- She emphasized that the telescope, in general, was working well and remained in high demand among astronomers. The institute, which handles science operations of Hubble and, soon, the James Webb Space Telescope, is making plans for extended operations of Hubble.

- "We're continuing to plan for the very long term," she said. One example she gave was "COS 2030," a program to extend the life of the Cosmic Origins Spectrograph, an instrument installed on Hubble on that final servicing mission in 2009, through the end of the decade.

• June 18, 2021: This image shows the spiral galaxy NGC 3254, observed using Hubble's Wide Field Camera 3 (WFC3). WFC3 has the capacity to observe ultraviolet, visible and near-infrared light, and this image is a composite of observations taken in the visible and infrared. In this image, NGC 3254 looks like a typical spiral galaxy, viewed side-on. However, NGC 3254 has a fascinating secret that it is hiding in plain sight — it is a Seyfert galaxy, meaning that it has an extraordinarily active core, known as an active galactic nucleus, which releases as much energy as the rest of the galaxy put together. 39)


Figure 35: Seyfert galaxies are not rare — about 10% of all galaxies are thought to be Seyfert galaxies. They belong to the class of "active galaxies" — galaxies that have supermassive black holes at their centers that are actively accreting material, which releases vast amounts of radiation as it is accreted. There is a second, far more active, type of active galaxy that is known as a quasar. The active cores of Seyfert galaxies, such as NGC 3254, are brightest when observed in light outside the visible spectrum. At other wavelengths, this image would look very different, with the galaxy's core shining extremely brightly. (image credit: ESA/Hubble & NASA, A. Riess et al.; CC BY 4.0)

• June 17, 2021: What if oceanographers found the "tip" of an iceberg and nothing else? Mysteriously missing was the iceberg's immense body, which extends far below the waves. 40)

- Astronomers faced this puzzle when they aimed Hubble at the spheroidal galaxy NGC 1052-DF2, or DF2. It looks like a denizen of intergalactic space that is the closest thing there is to nothing, but is still something. It's physically larger than our Milky Way, but its loose beehive swarm of stars is so thinly scattered that Hubble sees right through it, capturing myriad background galaxies.

- The missing "bottom of the iceberg" for DF2 is the lack of dark matter. Galaxies are partly made up of visible matter—stars and gas. But the bulk of a galaxy's makeup is in dark matter, the invisible glue that keeps a lid on stars, so they don't escape from the galaxy.

- Because this innocuous galaxy challenges conventional theories of how galaxies are put together, astronomers were naturally skeptical when it was first announced that the universe harbored such a rule breaker. After all, the entire cosmos is built on the invisible scaffolding of dark matter.

- To double-check their conclusion, the researchers used a lot more Hubble exposures to better nail down the distance to the stealthy galaxy. If DF2 were closer than they thought, the dark matter mystery goes away.

- They actually found that the galaxy is a little bit farther away than first measured. The researchers say the new milepost helps them confirm that dark matter is really missing in the galactic oddball. They say it's now up to theorists to figure out why.

- When astronomers using NASA's Hubble Space Telescope uncovered an oddball galaxy that looked like it didn't have much dark matter, some thought the finding was hard to believe and looked for a simpler explanation.

- Dark matter, after all, is the invisible glue that makes up the bulk of the universe's matter. All galaxies appear to be dominated by it; in fact, galaxies are thought to form inside immense halos of dark matter.

- So, finding a galaxy lacking the invisible stuff is an extraordinary claim that challenges conventional wisdom. It would have the potential to upset theories of galaxy formation and evolution.

- To bolster their original finding, first reported in 2018 (Dark Matter Goes Missing in Oddball Galaxy (, a team of scientists led by Pieter van Dokkum of Yale University in New Haven, Connecticut, followed up their initial study with a more robust Hubble look at the galaxy, named NGC 1052-DF2. Scientists refer to it simply as "DF2."

- "We went out on a limb with our initial Hubble observations of this galaxy in 2018," van Dokkum said. "I think people were right to question it because it's such an unusual result. It would be nice if there were a simple explanation, like a wrong distance. But I think it's more fun and more interesting if it actually is a weird galaxy."

- Determining the amount of the galaxy's dark matter hinges on accurate measurements of how far away it is from Earth.

- If DF2 is as far from Earth as van Dokkum's team asserts, the galaxy's dark-matter content may only be a few percent. The team's conclusion is based on the motions of the stars within the galaxy; their velocities are influenced by the pull of gravity. The researchers found that the observed number of stars accounts for the galaxy's total mass, and there's not much room left for dark matter.

- However, if DF2 were closer to Earth, as some astronomers claim, it would be intrinsically fainter and less massive. The galaxy, therefore, would need dark matter to account for the observed effects of the total mass.


Figure 36: This Hubble Space Telescope snapshot reveals an unusual "see-through" galaxy. The giant cosmic cotton ball is so diffuse and its ancient stars so spread out that distant galaxies in the background can be seen through it. Called an ultra-diffuse galaxy, this galactic oddball is almost as wide as the Milky Way, but it contains only 1/200th the number of stars as our galaxy. The ghostly galaxy doesn't appear to have a noticeable central region, spiral arms, or a disk. Researchers calculated a more accurate distance to the galaxy, named NGC 1052-DF2, or DF2, by using Hubble to observe about 5,400 aging red giant stars. Red giant stars all reach the same peak brightness, so they are reliable yardsticks to measure distances to galaxies. - The research team estimates that DF2 is 72 million light-years from Earth. They say the distance measurement solidifies their claim that DF2 lacks dark matter, the invisible glue that makes up the bulk of the universe's contents. The galaxy contains at most 1/400th the amount of dark matter that the astronomers had expected. - The observations were taken between December 2020 and March 2021 with Hubble's Advanced Camera for Surveys[image credits: Science: NASA, ESA, STScI, Zili Shen (Yale), Pieter van Dokkum (Yale), Shany Danieli (IAS), Image Processing: Alyssa Pagan (STScI)]


Figure 37: This Hubble Space Telescope image offers a sampling of aging, red stars in the ultra-diffuse galaxy NGC 1052-DF2, or DF2. The galaxy continues to puzzle astronomers because it is lacking dark matter, an invisible form of matter that provides the gravitational glue to hold galaxies together. Precisely establishing the galaxy's distance form Earth is a step toward solving the mystery. The close-up at right reveals the many aging red giant stars on the outskirts of the galaxy that are used as intergalactic milepost markers. Researchers calculated a more accurate distance to DF2 by using Hubble to observe about 5,400 red giants. These older stars all reach the same peak brightness, so they are reliable yardsticks to measure distances to galaxies. The research team estimates that DF2 is 72 million light-years from Earth. They say the distance measurement solidifies their claim that DF2 lacks dark matter. The galaxy contains at most 1/400th the amount of dark matter that the astronomers had expected, based on theory and observations of many other galaxies. Called an ultra-diffuse galaxy, the galactic oddball is almost as wide as the Milky Way, but it contains only 1/200th the number of stars as our galaxy. The ghostly galaxy doesn't appear to have a noticeable central region, spiral arms, or a disk. The observations were taken between December 2020 and March 2021 with Hubble's Advanced Camera for Surveys [image credits: SCIENCE: NASA, ESA, STScI, Zili Shen (Yale), Pieter van Dokkum (Yale), Shany Danieli (IAS) IMAGE PROCESSING: Alyssa Pagan (STScI)] 41)

A Better Yardstick

- Team member Zili Shen, from Yale University, says that the new Hubble observations help them confirm that DF2 is not only farther from Earth than some astronomers suggest, but also slightly more distant than the team's original estimates.

- The new distance estimate is that DF2 is 72 million light-years as opposed to 42 million light-years, as reported by other independent teams. This places the galaxy farther than the original Hubble 2018 estimate of 65 light-years distance.

- The research team based its new result on long exposures with Hubble's Advanced Camera for Surveys, which provide a deeper view of the galaxy for finding a reliable yardstick to nail down the distance. They targeted aging red giant stars on the outskirts of the galaxy that all reach the same peak brightness in their evolution. Astronomers can use the stars' intrinsic brightness to calculate vast intergalactic distances. "Studying the brightest red giants is a well-established distance indicator for nearby galaxies," Shen explained.

- The more accurate Hubble measurements solidify the researchers' initial conclusion of a galaxy deficient in dark matter, team members say. So the mystery of why DF2 is missing most of its dark matter still persists.

- "For almost every galaxy we look at, we say that we can't see most of the mass because it's dark matter," van Dokkum explained. "What you see is only the tip of the iceberg with Hubble. But in this case, what you see is what you get. Hubble really shows the entire thing. That's it. It's not just the tip of the iceberg, it's the whole iceberg."

- The team's science paper has appeared in the The Astrophysical Journal Letters. 42)

A Stealthy Galaxy

- DF2 is a giant cosmic cotton ball that van Dokkum calls a "see-through galaxy," where the stars are spread out. The galactic oddball is almost as wide as the Milky Way, but it contains only 1/200th the number of stars as our galaxy.

- The ghostly galaxy doesn't appear to have a noticeable central region, spiral arms, or a disk. The team estimates that DF2 contains at most 1/400th the amount of dark matter than astronomers had expected. How the galaxy formed remains a complete mystery based on the team's latest measurements.

Figure 38: When astronomers using NASA's Hubble Space Telescope uncovered an oddball galaxy that looks like it doesn't have much dark matter, some thought the finding was hard to believe and looked for a simpler explanation. Dark matter, after all, is the invisible glue that makes up the bulk of the universe's contents. All galaxies are dominated by it; in fact, galaxies are thought to form inside immense halos of dark matter. So, finding a galaxy lacking the invisible stuff is an extraordinary claim that challenges conventional wisdom. It would have the potential to upset theories of galaxy formation and evolution (video credit: NASA's Goddard Space Flight Center)

- DF2 isn't the only galaxy devoid of dark matter. Shany Danieli of the Institute for Advanced Study in Princeton, New Jersey, used Hubble in 2020 to obtain an accurate distance to another ghostly galaxy, called NGC 1052-DF4 (or simply DF4), which apparently lacks dark matter, too. In this case, however, some scientists suggest the dark matter may have been stripped out of the galaxy due to tidal forces from another galaxy.

- The researchers think both DF2 and DF4 were members of a collection of galaxies. However, the new Hubble observations show that the two galaxies are 6.5 million light-years away from each other, farther apart than they first thought. It also appears that DF2 has drifted away from the grouping and is isolated in space.

- Both galaxies were discovered with the Dragonfly Telephoto Array at the New Mexico Skies observatory.

- "Both of them probably were in the same group and formed at the same time," Danieli said. "So maybe there was something special in the environment where they were formed."

- The researchers are hunting for more of these oddball galaxies. Other teams of astronomers are searching, too. In 2020, a group of researchers uncovered 19 unusual dwarf galaxies they say are deficient in dark matter (Off the Baryonic Tully–Fisher Relation: A Population of Baryon-dominated Ultra-diffuse Galaxies – IOPscience). However, it will take uncovering many more dark matter-less galaxies to resolve the mystery.

- Nevertheless, van Dokkum thinks finding a galaxy lacking dark matter tells astronomers something about the invisible substance. "In our 2018 paper, we suggested that if you have a galaxy without dark matter, and other similar galaxies seem to have it, that means that dark matter is actually real and it exists," van Dokkum said. "It's not a mirage."

- The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

• June 16, 2021: NASA is working to resolve an issue with the payload computer on the Hubble Space Telescope. The computer halted on Sunday, June 13, shortly after 4 p.m. EDT. After analyzing the data, the Hubble operations team is investigating whether a degrading memory module led to the computer halt. The team is preparing to switch to one of several backup modules on Wednesday, June 16. The computer will then be allowed to run for approximately one day to verify that the problem has been solved. The team would then restart all science instruments and return the telescope to normal science operations. 43)

- The purpose of the payload computer is to control and coordinate the science instruments onboard the spacecraft. After the halt occurred on Sunday, the main computer stopped receiving a "keep-alive" signal, which is a standard handshake between the payload and main spacecraft computers to indicate all is well. The main computer then automatically placed all science instruments in a safe mode configuration. Control center personnel at NASA's Goddard Space Flight Center in Greenbelt, Maryland restarted the payload computer on Monday, June 14, but it soon experienced the same problem.

- The payload computer is a NASA Standard Spacecraft Computer-1 (NSSC-1) system built in the 1980s. It is part of the Science Instrument Command and Data Handling module, which was replaced during the last astronaut servicing mission in 2009. The module has various levels of redundancy which can be switched on to serve as the primary system when necessary.

• June 11, 2021: The image of Figure 39, taken with Hubble's Wide Field Camera 3 (WFC3), features the spiral galaxy NGC 4680. At 2 o'clock and 7 o'clock two other galaxies can be seen flanking NGC 4680. NGC 4680 enjoyed a wave of attention in 1997, as it played host to a supernova explosion known as SN 1997bp. Amazingly, the supernova was identified by an Australian amateur astronomer named Robert Evans, who has identified an extraordinary 42 supernova explosions. 44)


Figure 39: NGC 4680 is actually a rather tricky galaxy to classify. It is sometimes referred to as a spiral galaxy, but it is also sometimes classified as a lenticular galaxy. Lenticular galaxies fall somewhere in between spiral galaxies and elliptical galaxies. Whilst NGC 4680 does have distinguishable spiral arms, they are not clearly defined, and the tip of one arm appears very diffuse. Galaxies are not static, and their morphologies (and therefore their classifications) vary throughout their lifetimes. Spiral galaxies are thought to evolve into elliptical galaxies, most likely by merging with one another, causing them to lose their distinctive spiral structures (image credit: ESA/Hubble & NASA, A. Riess et al.; CC BY 4.0)

Minimize Hubble continued

• June 9, 2021: Brown dwarfs are the cosmic equivalent of tweeners. They're too massive to be planets and too small to sustain nuclear fusion in their cores, which powers stars. Many brown dwarfs are nomadic. They do not orbit stars but drift among them as loners. 45)

- Astronomers would like to know how these wayward objects are put together. Do they share any kind of kinship with bloated gas-giant planets like Jupiter? Studying brown dwarfs is much more difficult than studying nearby Jupiter for making comparisons. We can send spacecraft to Jupiter. But astronomers need to look across many light-years to peer down into a brown dwarf's atmosphere.

- Researchers used the giant W. M. Keck Observatory in Hawaii to observe a nearby brown dwarf in infrared light. Unlike Jupiter, the young brown dwarf is still so hot it glows from the inside out, and looks like a carved Halloween pumpkin. Because the brown dwarf has scattered clouds, light shining up from deep down in the dwarf's atmosphere fluctuates, which the researchers measured. They found that the dwarf's atmosphere has a layer-cake structure with clouds having different composition at different altitudes.

- Jupiter may be the bully planet of our solar system because it's the most massive planet. But it's actually a runt compared to many of the giant planets found around other stars.

- These alien worlds, called super-Jupiters, weigh up to 13 times Jupiter's mass. Astronomers have analyzed the composition of some of these monsters. But it has been difficult to study their atmospheres in detail because these gas giants get lost in the glare of their parent stars.

- Researchers, however, have a substitute: the atmospheres of brown dwarfs, so-called failed stars that are up to 80 times Jupiter's mass. These hefty objects form out of a collapsing cloud of gas, as stars do, but lack the mass to become hot enough to sustain nuclear fusion in their cores, which powers stars.

- Instead, brown dwarfs share a kinship with super-Jupiters. Both types of objects have similar temperatures and are extremely massive. They also have complex, varied atmospheres. The only difference, astronomers think, is their pedigree. Super-Jupiters form around stars; brown dwarfs often form in isolation.

- A team of astronomers, led by Elena Manjavacas of the Space Telescope Science Institute in Baltimore, Maryland, has tested a new way to peer through the cloud layers of these nomadic objects. The researchers used an instrument at the W. M. Keck Observatory in Hawaii to study in near-infrared light the colors and brightness variations of the layer-cake cloud structure in the nearby, free-floating brown dwarf known as 2MASS J22081363+2921215.

- The Keck Observatory instrument, called the Multi-Object Spectrograph for Infrared Exploration (MOSFIRE), also analyzed the spectral fingerprints of various chemical elements contained in the clouds and how they change with time. This is the first time astronomers have used the MOSFIRE instrument in this type of study.

- These measurements offered Manjavacas a holistic view of the brown dwarf's atmospheric clouds, providing more detail than previous observations of this object. Pioneered by Hubble observations, this technique is difficult for ground-based telescopes to do because of contamination from Earth's atmosphere, which absorbs certain infrared wavelengths. This absorption rate changes due to the weather.


Figure 40: Observations of a nearby brown dwarf suggest that it has a mottled atmosphere with scattered clouds and mysterious dark spots reminiscent of Jupiter's Great Red Spot, as shown in this artist's concept. The nomadic object, called 2MASS J22081363+2921215, resembles a carved Halloween pumpkin, with light escaping from its hot interior. Brown dwarfs are more massive than planets but too small to sustain nuclear fusion, which powers stars. - Though only roughly 115 light-years away, the brown dwarf is too distant for any features to be photographed. Instead, researchers used the Multi-Object Spectrograph for Infrared Exploration (MOSFIRE) at the W. M. Keck Observatory in Hawaii to study the colors and brightness variations of the brown dwarf's layer-cake cloud structure, as seen in near-infrared light. MOSFIRE also collected the spectral fingerprints of various chemical elements contained in the clouds and how they change with time [image credit: Artwork: NASA, ESA, STScI, Leah Hustak (STScI)]


Figure 41: This graphic shows successive layers of clouds in the atmosphere of a nearby, free-floating brown dwarf. Breaks in the upper cloud layers allowed astronomers to probe deeper into the atmosphere of the brown dwarf called 2MASS J22081363+2921215. Brown dwarfs are more massive than planets but too small to sustain nuclear fusion, which powers stars. - This illustration is based on infrared observations of the clouds' colors and brightness variations, as well as the spectral fingerprints of various chemical elements contained in the clouds and atmospheric modeling [Illustration: NASA, ESA, STScI, Andi James (STScI)]

- "The only way to do this from the ground is using the high-resolution MOSFIRE instrument because it allows us to observe multiple stars simultaneously with our brown dwarf," Manjavacas explained. "This allows us to correct for the contamination introduced by the Earth's atmosphere and measure the true signal from the brown dwarf with good precision. So, these observations are a proof-of-concept that MOSFIRE can do these types of studies of brown-dwarf atmospheres."

- Manjavacas will present her results June 9 in a press conference at the virtual meeting of the American Astronomical Society.

- The researcher decided to study this particular brown dwarf because it is very young and therefore extremely bright and has not cooled off yet. Its mass and temperature are similar to those of the nearby giant exoplanet Beta Pictoris b, discovered in 2008 near-infrared images taken by the European Southern Observatory's VLT (Very Large Telescope) in northern Chile.

- "We don't have the ability yet with current technology to analyze in detail the atmosphere of Beta Pictoris b," Manjavacas said. "So, we're using our study of this brown dwarf's atmosphere as a proxy to get an idea of what the exoplanet's clouds might look like at different heights of its atmosphere."

- Both the brown dwarf and Beta Pictoris b are young, so they radiate heat strongly in the near-infrared. They are both members of a flock of stars and sub-stellar objects called the Beta Pictoris moving group, which shares the same origin and a common motion through space. The group, which is about 33 million years old, is the closest grouping of young stars to Earth. It is located roughly 115 light-years away.

- While they're cooler than bona fide stars, brown dwarfs are still extremely hot. The brown dwarf in Manjavacas' study is a sizzling 2,780º Fahrenheit (1,527º Celsius).

- The giant object is about 12 times heavier than Jupiter. As a young body, it is spinning incredibly fast, completing a rotation every 3.5 hours, compared to Jupiter's 10-hour rotation period. So, clouds are whipping it, creating a dynamic, turbulent atmosphere.

- Keck Observatory's MOSFIRE instrument stared at the brown dwarf for 2.5 hours, watching how the light filtering up through the atmosphere from the dwarf's hot interior brightens and dims over time. Bright spots that appear on the rotating object indicate regions where researchers can see deeper into the atmosphere, where it is hotter. Infrared wavelengths allow astronomers to peer deeper into the atmosphere. The observations suggest the brown dwarf has a mottled atmosphere with scattered clouds. If viewed close-up, it might resemble a carved Halloween pumpkin, with light escaping from its hot interior.

- Its spectrum reveals clouds of hot sand grains and other exotic elements. Potassium iodide traces the object's upper atmosphere, which also includes magnesium silicate clouds. Moving down in the atmosphere is a layer of sodium iodide and magnesium silicate clouds. The final layer consists of aluminum oxide clouds. The atmosphere's total depth is 446 miles (718 km). The elements detected represent a typical part of the composition of brown dwarf atmospheres, Manjavacas said.

- The researcher and her team used computer models of brown dwarf atmospheres to determine the location of the chemical compounds in each cloud layer.

- Manjavacas' plan is to use Keck Observatory's MOSFIRE to study other atmospheres of brown dwarfs and compare them to those of gas giants. Future telescopes such as NASA's James Webb Space Telescope, an infrared observatory scheduled to launch later this year, will provide even more information about a brown dwarf's atmosphere. "JWST will give us the structure of the entire atmosphere, providing more coverage than any other telescope," Manjavacas said.

- The researcher hopes that MOSFIRE can be used in tandem with JWST to sample a wide range of brown dwarfs. The goal is a better understanding of brown dwarfs and giant planets.

• June 4, 2021: Objects such as NGC 691 are observed by Hubble using a range of filters. Each filter only allows certain wavelengths of light to reach Hubble's WFC3. The images collected using different filters are then colored by specialized visual artists who can make informed choices about which color best corresponds to which filter. By combining the colored images from individual filters, a full-color image of the astronomical object can be recreated. In this way, we can get remarkably good insight into the nature and appearance of these objects. 46)


Figure 42: This image features the spiral galaxy NGC 691, imaged in fantastic detail by Hubble's Wide Field Camera 3 (WFC3). This galaxy is the eponymous member of the NGC 691 galaxy group, a group of gravitationally bound galaxies that lie about 120 million light-years from Earth (image credit: ESA/Hubble & NASA, A. Riess; CC BY 4.0 Acknowledgement: M. Zamani)

• May 28, 2021: WFC3 (Wide Field Camera 3) is a very versatile camera, as it can collect ultraviolet, visible, and infrared light, thereby providing a wealth of information about the objects it observes. WFC3 was installed on Hubble by astronauts in 2009, during Servicing Mission 4 (SM4). SM4 was Hubble's final Space Shuttle servicing mission, expected to prolong Hubble's life for at least another five years. Twelve years later, both Hubble and WFC3 remain very active and scientifically productive. 47) 48)


Figure 43: This image shows the spiral galaxy NGC 5037, in the constellation of Virgo. First documented by William Herschel in 1785, the galaxy lies about 150 million light-years away from Earth. Despite this distance, we can see the delicate structures of gas and dust within the galaxy in extraordinary detail. This detail is possible using Hubble's WFC3, whose combined exposures created this image (image credit: ESA/Hubble & NASA, D. Rosario; Acknowledgment: L. Shatz)

• May 27, 2021: The myriad spiral galaxies in our universe almost all look like fried eggs. A central bulge of aging stars is like the egg yolk, surrounded by a disk of stars that are the egg white. The galaxy in this Hubble photo looks like it is sliding off the frying pan. The central bulge is off in one corner relative to the surrounding disk of bright young blue stars. In reality, the stars on the right side of the galaxy are being pulled like taffy by the gravitational tug of a neighboring galaxy, not seen in this close-up view. Galaxies are not solid objects but tenuous agglomerations of tens of billions of stars. When two galaxies come close to each other they feel each other's gravity and are distorted, like pulling on cotton candy. It's the universe's equivalent of the 19th century children's poem about two stuffed animals – the gingham dog and calico cat — who got into a spat and ate each other. It's not so dramatic in this case. The galaxies are only getting a little chewed up because of their close proximity. 49) 50)

What's going on?

- In reality, a neighboring galaxy to the right of NGC 2276 (NGC 2300, not seen here) is gravitationally tugging on its disk of blue stars, pulling the stars on one side of the galaxy outward to distort the galaxy's normal fried-egg appearance.

- This sort of "tug of war" between galaxies that pass close enough to feel each other's gravitational pull is not uncommon in the universe. But, like snowflakes, no two close encounters look exactly alike.

- In addition, newborn and short-lived massive stars form a bright, blue arm along the upper left edge of NGC 2276. They trace out a lane of intense star formation. This may have been triggered by a prior collision with a dwarf galaxy. It could also be due to NGC 2276 plowing into the superheated gas that lies among galaxies in galaxy clusters. This would compress the gas to precipitate into stars, and trigger a firestorm of starbirth.

- The spiral galaxy lies 120 million light-years away, in the northern constellation Cepheus.


Figure 44: The magnificent spiral galaxy NGC 2276 looks a bit lopsided in this Hubble Space Telescope snapshot. A bright hub of older yellowish stars normally lies directly in the center of most spiral galaxies. But the bulge in NGC 2276 looks offset to the upper left. This image was taken as part of the Hubble observation program #15615 (PI: P. Sell), a collaboration between the University of Florida (USA), the University of Crete/FORTH (Greece), INAF-Brera (Italy), and the Center for Astrophysics | Harvard & Smithsonian (USA) [image credit: NASA, ESA, STScI, Paul Sell (University of Florida), acknowledgement: Leo Shatz]

• May 21, 2021: Hubble image of the week. 51)

- The galaxy cluster dominates the center of this image, both visually and physically. The cluster's huge mass has gravitationally lensed the light from background galaxies, distorting and smearing their shapes. In addition to providing astronomers with a natural magnifying glass with which to study distant galaxies, gravitational lensing has subtly framed the center of this image, producing a visually striking scene.


Figure 45: This packed ESA/Hubble Picture of the Week showcases the galaxy cluster ACO S 295, as well as a jostling crowd of background galaxies and foreground stars. Galaxies of all shapes and sizes populate this image, ranging from stately spirals to fuzzy ellipticals. As well as a range of sizes, this galactic menagerie boasts a range of orientations, with spiral galaxies such as the one at the centre of this image appearing almost face on, and some edge-on spiral galaxies visible only as thin slivers of light (image credit: ESA/Hubble & NASA, F. Pacaud, D. Coe; CC BY 4.0)

• May 20, 2021: Astronomers using NASA's Hubble Space Telescope have traced the locations of five brief, powerful radio blasts to the spiral arms of five distant galaxies. 52)

- Called fast radio bursts (FRBs), these extraordinary events generate as much energy in a thousandth of a second as the Sun does in a year. Because these transient radio pulses disappear in much less than the blink of an eye, researchers have had a hard time tracking down where they come from, much less determining what kind of object or objects is causing them. Therefore, most of the time, astronomers don't know exactly where to look.

- Locating where these blasts are coming from, and in particular, what galaxies they originate from, is important in determining what kinds of astronomical events trigger such intense flashes of energy. The new Hubble survey of eight FRBs helps researchers narrow the list of possible FRB sources.

They come from anywhere in the sky: mysterious flashes of radio energy that disappear in the blink of an eye. They're called fast radio bursts (FRBs), and astronomers have spotted roughly 1,000 of them over the past 20 years. But they come and go so quickly that researchers have only been able to trace about 15 of them to their home galaxies, all are massive and far from Earth. After that, their trail runs cold. Astronomers haven't been able to track the bursts to the neighborhoods where the radio waves were beamed. Their location could offer clues to the cause of one of the most enigmatic events in modern astronomy.

Astronomers are now using the Hubble Space Telescope as an intergalactic sleuth on the trail of this cosmic mystery. With Hubble they have tracked five FRBs to the spiral arms of five distant galaxies. But surprisingly, these powerful events don't come from the brightest regions, which blaze with the light from hefty stars. These clues help researchers rule out several possible explanations for the brilliant flares, such as the explosive deaths of the youngest, most massive stars. The researchers' results favor an increasingly popular theory, that the bursts come from magnetars, intensely magnetic remnants of collapsed dead stars.

Table 1: Summary: These brilliant flares originate from young, massive galaxies

Flash in the Night

- The first FRB was discovered in archived data recorded by the Parkes radio observatory (New South Wales, Australia) on July 24, 2001. Since then astronomers have uncovered up to 1,000 FRBs, but they have only been able to associate roughly 15 of them to particular galaxies.

- "Our results are new and exciting. This is the first high-resolution view of a population of FRBs, and Hubble reveals that five of them are localized near or on a galaxy's spiral arms," said Alexandra Mannings of the University of California, Santa Cruz, the study's lead author. "Most of the galaxies are massive, relatively young, and still forming stars. The imaging allows us to get a better idea of the overall host-galaxy properties, such as its mass and star-formation rate, as well as probe what's happening right at the FRB position because Hubble has such great resolution."


Figure 46: Astronomers using the Hubble Space Telescope have tracked down two brief, powerful radio bursts to the spiral arms of the two galaxies shown at top and bottom of this image. The catalogue names of the bursts are FRB 190714, top row, and FRB 180924, bottom row. The galaxies are far from Earth, appearing as they looked billions of years ago. The dotted oval lines in each of the four images mark the location of the brilliant radio flares. The two images at left show the full Hubble snapshots of each galaxy. - To study each galaxy's spiral structure in more detail, the researchers overlaid a computer model of the galaxies' starlight onto the images at left. They then subtracted the smoother, more diffuse starlight from each of those images. The resulting two images at right reveal each galaxy's spiral arms more clearly, which were harder to see in the original images. Because these radio pulses disappear in much less than the blink of an eye, researchers have had a hard time tracking down where they come from. These galaxies are part of a survey to determine the origin of fast radio bursts, which can release as much energy in a thousandth of a second as the Sun does in a year. - Identifying the radio bursts' location helped researchers narrow the list of possible FRB sources that can generate such prodigious tsunamis of energy. One of the leading possible explanations is a torrential blast from a young magnetar. Magnetars are a type of neutron star with extraordinarily powerful magnetic fields. The observations were made in ultraviolet and near-infrared light with Hubble's Wide Field Camera 3. The images were taken between November 2019 and April 2020 [Credits: Science: NASA, ESA, Alexandra Mannings (UC Santa Cruz), Wen-fai Fong (Northwestern); Image processing: Alyssa Pagan (STScI)]

- In the Hubble study, astronomers not only pinned all of them to host galaxies, but they also identified the kinds of locations they originated from. Hubble observed one of the FRB locations in 2017 and the other seven in 2019 and 2020.

- "We don't know what causes FRBs, so it's really important to use context when we have it," said team member Wen-fai Fong of Northwestern University in Evanston, Illinois. "This technique has worked very well for identifying the progenitors of other types of transients, such as supernovae and gamma-ray bursts. Hubble played a big role in those studies, too."

- The galaxies in the Hubble study existed billions of years ago. Astronomers, therefore, are seeing the galaxies as they appeared when the universe was about half its current age.

- Many of them are as massive as our Milky Way. The observations were made in ultraviolet and near-infrared light with Hubble's Wide Field Camera 3.

- Ultraviolet light traces the glow of young stars strung along a spiral galaxy's winding arms. The researchers used the near-infrared images to calculate the galaxies' mass and find where older populations of stars reside.

Location, Location, Location

- The images display a diversity of spiral-arm structure, from tightly wound to more diffuse, revealing how the stars are distributed along these prominent features. A galaxy's spiral arms trace the distribution of young, massive stars. However, the Hubble images reveal that the FRBs found near the spiral arms do not come from the very brightest regions, which blaze with the light from hefty stars. The images help support a picture that the FRBs likely do not originate from the youngest, most massive stars.

- These clues helped the researchers rule out some of the possible triggers of types of these brilliant flares, including the explosive deaths of the youngest, most massive stars, which generate gamma-ray bursts and some types of supernovae. Another unlikely source is the merger of neutron stars, the crushed cores of stars that end their lives in supernova explosions. These mergers take billions of years to occur and are usually found far from the spiral arms of older galaxies that are no longer forming stars.

Magnetic Monsters

- The team's Hubble results, however, are consistent with the leading model that FRBs originate from young magnetar outbursts. Magnetars are a type of neutron star with powerful magnetic fields. They're called the strongest magnets in the universe, possessing a magnetic field that is 10 trillion times more powerful than a refrigerator door magnet. Astronomers last year linked observations of an FRB spotted in our Milky Way galaxy with a region where a known magnetar resides.

- "Owing to their strong magnetic fields, magnetars are quite unpredictable," Fong explained. "In this case, the FRBs are thought to come from flares from a young magnetar. Massive stars go through stellar evolution and becomes neutron stars, some of which can be strongly magnetized, leading to flares and magnetic processes on their surfaces, which can emit radio light. Our study fits in with that picture and rules out either very young or very old progenitors for FRBs."

- The observations also helped the researchers strengthen the association of FRBs with massive, star-forming galaxies. Previous ground-based observations of some possible FRB host galaxies did not as clearly detect underlying structure, such as spiral arms, in many of them. Astronomers, therefore, could not rule out the possibility that FRBs originate from a dwarf galaxy hiding underneath a massive one. In the new Hubble study, careful image processing and analysis of the images allowed researchers to rule out underlying dwarf galaxies, according to co-author Sunil Simha of the University of California, Santa Cruz.

- Although the Hubble results are exciting, the researchers say they need more observations to develop a more definitive picture of these enigmatic flashes and better pinpoint their source. "This is such a new and exciting field," Fong said. "Finding these localized events is a major piece to the puzzle, and a very unique puzzle piece compared to what's been done before. This is a unique contribution of Hubble."

- The team's results will appear in an upcoming issue of The Astrophysical Journal.

• May 14, 2021: The language that astronomers use changes as we become better acquainted with the Universe, and astronomical history is littered with now-obsolete phrases to describe objects in the night sky, such as "spiral nebulae" for spiral galaxies or "inferior planets" for Mercury and Venus. 53)

- While modern astronomical terminology has become steadily more precise, the nature of objects in astronomical exposures can still occasionally puzzle astronomers. For example, if you look very closely, you can see a faint bluish streak across the center of this image to the bottom right of the blue region. This could be an asteroid, but seems to be travelling far too quickly for such an object — making this one of the remaining mysteries of the night sky.


Figure 47: This Picture of the Week showcases the emission nebula NGC 2313. The bright star V565 — surrounded by four prominent diffraction spikes — illuminates a silvery, fan-shaped veil of gas and dust, while the right half of this image is obscured by a dense cloud of dust. Nebulae with similar shapes — a star accompanied by a bright fan of gas — were once referred to as cometary nebulae, though the name is no longer used (image credit: ESA/Hubble, R. Sahai; CC BY 4.0)

• May 7, 2021: Looking at this cluster of hundreds of galaxies, it is amazing to recall that until less than 100 years ago, many astronomers believed that the Milky Way was the only galaxy in the Universe. The possibility of other galaxies had been debated previously, but the matter was not truly settled until Edwin Hubble confirmed that the Great Andromeda Nebula was in fact far too distant to be part of the Milky Way. The Great Andromeda Nebula became the Andromeda Galaxy, and astronomers recognized that our Universe was much, much bigger than humanity had imagined. We can only imagine how Edwin Hubble — after whom the Hubble Space Telescope was named — would have felt if he'd seen this spectacular image of Abell 3827. 54)


Figure 48: This detailed image features Abell 3827, a galaxy cluster that offers a wealth of exciting possibilities for study. It was observed by Hubble in order to study dark matter, which is one of the greatest puzzles cosmologists face today. The science team used Hubble's Advanced Camera for Surveys (ACS) and Wide Field Camera 3 (WFC3) to complete their observations. The two cameras have different specifications and can observe different parts of the electromagnetic spectrum, so using them both allowed the astronomers to collect more complete information. Abell 3827 has also been observed previously by Hubble, because of the interesting gravitational lens at its core (image credit: ESA/Hubble & NASA, R. Massey, CC BY 4.0)

• April 30, 2021: The Necklace Nebula — which also goes by the less glamorous name of PN G054.2-03.4 — was produced by a pair of tightly orbiting Sun-like stars. Roughly 10,000 years ago, one of the aging stars expanded and engulfed its smaller companion, creating something astronomers call a "common envelope". The smaller star continued to orbit inside its larger companion, increasing the bloated giant's rotation rate until large parts of it spun outwards into space. This escaping ring of debris formed the Necklace Nebula, with particularly dense clumps of gas forming the bright "diamonds" around the ring. 55)

- The pair of stars which created the Necklace Nebula remain so close together — separated by only a few million kilometers — that they appear as a single bright dot in the centre of this image. Despite their close encounter the stars are still furiously whirling around each other, completing an orbit in just over a day.

- The Necklace Nebula was featured in a previously released Hubble image, but now this new image has been created by applying advanced processing techniques, making for a new and improved view of this intriguing object. The composite image includes several exposures from Hubble's Wide Field Camera 3.


Figure 49: The interaction of two doomed stars has created this spectacular ring adorned with bright clumps of gas — a diamond necklace of cosmic proportions. Fittingly known as the Necklace Nebula, this planetary nebula is located 15,000 light-years away from Earth in the small, dim constellation of Sagitta (The Arrow), image credit: ESA/Hubble & NASA, K. Noll; CC BY 4.0)

• April 29, 2021: NASA's Hubble Space Telescope is giving astronomers a rare look at a Jupiter-sized, still-forming planet that is feeding off material surrounding a young star. 56)

- "We just don't know very much about how giant planets grow," said Brendan Bowler of the University of Texas at Austin. "This planetary system gives us the first opportunity to witness material falling onto a planet. Our results open up a new area for this research."

- Though over 4,000 exoplanets have been cataloged so far, only about 15 have been directly imaged to date by telescopes. And the planets are so far away and small, they are simply dots in the best photos. The team's fresh technique for using Hubble to directly image this planet paves a new route for further exoplanet research, especially during a planet's formative years.

- This huge exoplanet, designated PDS 70b, orbits the orange dwarf star PDS 70, which is already known to have two actively forming planets inside a huge disk of dust and gas encircling the star. The system is located 370 light-years from Earth in the constellation Centaurus.

- "This system is so exciting because we can witness the formation of a planet," said Yifan Zhou, also of the University of Texas at Austin. "This is the youngest bona fide planet Hubble has ever directly imaged." At a youthful five million years, the planet is still gathering material and building up mass.

- Hubble's ultraviolet light (UV) sensitivity offers a unique look at radiation from extremely hot gas falling onto the planet. "Hubble's observations allowed us to estimate how fast the planet is gaining mass," added Zhou.


Figure 50: This illustration of the newly forming exoplanet PDS 70b shows how material may be falling onto the giant world as it builds up mass. By employing Hubble's ultraviolet light (UV) sensitivity, researchers got a unique look at radiation from extremely hot gas falling onto the planet, allowing them to directly measure the planet's mass growth rate for the first time. The planet PDS 70b is encircled by its own gas-and-dust disk that's siphoning material from the vastly larger circumstellar disk in this solar system. The researchers hypothesize that magnetic field lines extend from its circumplanetary disk down to the exoplanet's atmosphere and are funneling material onto the planet's surface. The illustration shows one possible magnetospheric accretion configuration, but the magnetic field's detailed geometry requires future work to probe. The remote world has already bulked up to five times the mass of Jupiter over a period of about five million years, but is anticipated to be in the tail end of its formation process. PDS 70b orbits the orange dwarf star PDS 70 approximately 370 light-years from Earth in the constellation Centaurus [image credits: NASA, ESA, STScI, Joseph Olmsted (STScI)]

- The UV observations, which add to the body of research about this planet, allowed the team to directly measure the planet's mass growth rate for the first time. The remote world has already bulked up to five times the mass of Jupiter over a period of about five million years. The present measured accretion rate has dwindled to the point where, if the rate remained steady for another million years, the planet would only increase by approximately an additional 1/100th of a Jupiter-mass.

- Zhou and Bowler emphasize that these observations are a single snapshot in time – more data are required to determine if the rate at which the planet is adding mass is increasing or decreasing. "Our measurements suggest that the planet is in the tail end of its formation process."

- The youthful PDS 70 system is filled with a primordial gas-and-dust disk that provides fuel to feed the growth of planets throughout the entire system. The planet PDS 70b is encircled by its own gas-and-dust disk that's siphoning material from the vastly larger circumstellar disk. The researchers hypothesize that magnetic field lines extend from its circumplanetary disk down to the exoplanet's atmosphere and are funneling material onto the planet's surface.


Figure 51: ESO's (European Southern Observatory's) VLT (Very Large Telescope) caught the first clear image of a forming planet, PDS 70b, around a dwarf star in 2018. The planet stands out as a bright point to the right of the center of the image, which is blacked out by the coronagraph mask used to block the light of the central star [image credits: ESO, VLT, André B. Müller (ESO)]

- "If this material follows columns from the disk onto the planet, it would cause local hot spots," Zhou explained. "These hot spots could be at least 10 times hotter than the temperature of the planet." These hot patches were found to glow fiercely in UV light.


Figure 52: Hubble observations pinpoint planet PDS 70b. A coronagraph on Hubble's camera blocks out the glare of the central star for the planet to be directly observed. Though over 4,000 exoplanets have been cataloged so far, only about 15 have been directly imaged to date by telescopes. The team's fresh technique for using Hubble to directly image this planet paves a new route for further exoplanet research, especially during a planet's formative years ([image credits: Joseph DePasquale (STScI)]

- These observations offer insights into how gas giant planets formed around our Sun 4.6 billion years ago. Jupiter may have bulked up on a surrounding disk of infalling material. Its major moons would have also formed from leftovers in that disk.

- A challenge to the team was overcoming the glare of the parent star. PDS 70b orbits at approximately the same distance as Uranus does from the Sun, but its star is more than 3,000 times brighter than the planet at UV wavelengths. As Zhou processed the images, he very carefully removed the star's glare to leave behind only light emitted by the planet. In doing so, he improved the limit of how close a planet can be to its star in Hubble observations by a factor of five.

- "Thirty-one years after launch, we're still finding new ways to use Hubble," Bowler added. "Yifan's observing strategy and post-processing technique will open new windows into studying similar systems, or even the same system, repeatedly with Hubble. With future observations, we could potentially discover when the majority of the gas and dust falls onto their planets and if it does so at a constant rate."

- The researchers' results were published in April 2021 in The Astronomical Journal. 57)

• April 23, 2021: The giant star featured in this latest Hubble Space Telescope anniversary image is waging a tug-of-war between gravity and radiation to avoid self-destruction. The star, called AG Carinae, is surrounded by an expanding shell of gas and dust — a nebula — that is shaped by the powerful winds of the star. The nebula is about five light-years wide, which equals the distance from here to our nearest star, Alpha Centauri. 58)

- The huge structure was created from one or more giant eruptions several thousand years ago. The star's outer layers were blown into space, the expelled material amounting to roughly 10 times the mass of our Sun. These outbursts are typical in the life of a rare breed of star called a Luminous Blue Variable (LBV), a brief unstable phase in the short life of an ultra-bright, glamorous star that lives fast and dies young. These stars are among the most massive and brightest stars known. They live for only a few million years, compared to the roughly 10-billion-year lifetime of our own Sun. AG Carinae is a few million years old and resides 20,000 light-years away inside our Milky Way galaxy. The star's expected lifetime is between 5 million and 6 million years.

- LBVs have a dual personality. They appear to spend years in semi-quiescent bliss and then they erupt in a petulant outburst, during which their luminosity increases — sometimes by several orders of magnitude. These behemoths are stars in the extreme, far different from normal stars like our Sun. In fact AG Carinae is estimated to be up to 70 times more massive than our Sun and shines with the blinding brilliance of 1 million suns.

- Major outbursts such as the one that produced the nebula featured in this image occur a few times during a LBV's lifetime. A LBV star only casts off material when it is in danger of self-destruction. Because of their massive forms and super-hot temperatures, luminous blue variable stars like AG Carinae are in a constant battle to maintain stability. It's an arm-wrestling contest between radiation pressure from within the star pushing outward and gravity pressing inward. This arm-wrestling match results in the star's expanding and contracting. The outward pressure occasionally wins the battle, and the star expands to such an immense size that it blows off its outer layers, like a volcano erupting. But this outburst only happens when the star is on the verge of coming apart. After the star ejects the material, it contracts to its normal (large) size, settles back down, and becomes stable again.

- LBV stars are rare: fewer than 50 are known among the galaxies in our local group of neighboring galaxies. These stars spend tens of thousands of years in this phase, a blink of an eye in cosmic time. Some are expected to end their lives in titanic supernova blasts, which enrich the Universe with the heavier elements beyond iron.

- Like many other LBVs, AG Carinae remains unstable. It has experienced lesser outbursts that have not been as powerful as the one that created the present nebula. Although AG Carinae is semi-quiescent now, its searing radiation and powerful stellar wind (streams of charged particles) have been shaping the ancient nebula, sculpting intricate structures as outflowing gas slams into the slower-moving outer nebula. The wind is travelling at up to 1 million kilometers per hour, about 10 times faster than the expanding nebula. Over time, the hot wind catches up with the cooler expelled material, ploughs into it, and pushes it farther away from the star. This "snowplough" effect has cleared a cavity around the star.

- The red material is glowing hydrogen gas laced with nitrogen gas. The diffuse red material at upper left pinpoints where the wind has broken through a tenuous region of material and swept it into space. The most prominent features, highlighted in blue, are filamentary structures shaped like tadpoles and lopsided bubbles. These structures are dust clumps illuminated by the star's light. The tadpole-shaped features, most prominent at left and bottom, are denser dust clumps that have been sculpted by the stellar wind. Hubble's sharp vision reveals these delicate-looking structures in great detail.


Figure 53: In celebration of the 31st anniversary of the launch of the NASA/ESA Hubble Space Telescope, astronomers aimed the celebrated observatory at one of the brightest stars seen in our galaxy to capture its beauty. The image was taken in visible and ultraviolet light. Hubble is ideally suited for observations in ultraviolet light because this wavelength range can only be viewed from space (image credit: NASA, ESA and STScI)

• April 23, 2021: This image shows a close-up portrait of the magnificent spiral galaxy NGC 4603, which lies over 100 million light-years away in the constellation of Centaurus (The Centaur). Bright bands of blue young stars make up the arms of this galaxy, which wind lazily outwards from the luminous core. The intricate red-brown filaments threading through the spiral arms are known as dust lanes, and consist of dense clouds of dust which obscure the diffuse starlight from the galaxy. 59)


Figure 54: This galaxy is a familiar subject for Hubble. In the last years of the twentieth century, NGC 4063 was keenly and closely watched for signs of a peculiar class of stars known as Cepheid variables. These stars have a luminosity closely tied to the period with which they darken and brighten, allowing astronomers to accurately measure how far they are from Earth. Distance measurements from Cepheid variables are key to measuring the furthest distances in the Universe, and were one of the factors used by Georges Lemaître and Edwin Hubble to show that the Universe is expanding (image credit: ESA/Hubble & NASA, J. Maund; CC BY 4.0)

• April 16, 2021: This extraordinary image from the NASA/ESA Hubble Space Telescope of the galaxy cluster Abell 2813 (also known as ACO 2813) has an almost delicate beauty, which also illustrates the remarkable physics at work within it. The image spectacularly demonstrates the concept of gravitational lensing. 60)

- In amongst the tiny dots, spirals and ovals that are the galaxies that belong to the cluster, there are several distinct crescent shapes. These curved arcs of light are strong examples of a phenomenon known as gravitational lensing. The image was compiled using observations taken with the Hubble Space Telescope's Advanced Camera for Surveys (ACS) and Wide Field Camera 3 (WFC3).

- This very visual evidence that mass causes light to bend has been famously used as a proof of one of the most famous scientific theories: Einstein's theory of general relativity.


Figure 55: Gravitational lensing occurs when an object's mass causes light to bend. The curved crescents and s-shapes of light in this image are not curved galaxies, but are light from galaxies that actually lie beyond Abell 2813. The galaxy cluster has so much mass that it acts as a gravitational lens, causing light from more distant galaxies to bend around it. These distortions can appear as many different shapes, such as long lines or arcs (image credit: ESA/Hubble & NASA, D. Coe; CC BY 4.0)

• April 9, 2021: M61 appears almost face-on, making it a popular subject for astronomical images, even though the galaxy lies more than 52 million light-years from Earth. This particular astronomical image incorporates data from not only Hubble, but also the FORS camera at the European Southern Observatory's VLT (Very Large Telescope), together revealing M61 in unprecedented detail. This striking image is one of many examples of telescope teamwork — astronomers frequently combine data from ground-based and space-based telescopes to learn more about the Universe. 61)

- Though the gleaming spiral of this galaxy makes for a spectacular sight, one of the most interesting features of M61 lurks unseen at the center of this image. As well as widespread pockets of star formation, M61 hosts a supermassive black hole more than 5 million times as massive as the Sun.


Figure 56: The luminous heart of the galaxy M61 dominates this image, framed by its winding spiral arms threaded with dark tendrils of dust. As well as the usual bright bands of stars, the spiral arms of M61 are studded with ruby-red patches of light. Tell-tale signs of recent star formation, these glowing regions lead to M61's classification as a starburst galaxy (image credit: ESA/Hubble & NASA, ESO, J. Lee and the PHANGS-HST Team; CC BY 4.0)

April 8, 2021: Forty years ago in 1981, the first space shuttle launched, the Voyager 2 space probe encountered Saturn, and in Baltimore, Maryland, the Space Telescope Science Institute (STScI) was founded. In that year, NASA selected a proposal by the Association of Universities for Research in Astronomy to establish STScI on the Johns Hopkins University Homewood campus. 62)

- STScI would serve as the science operations center for NASA's Hubble Space Telescope (then known as the Large Space Telescope). Nine years later in 1990 Hubble launched, and for the past 31 years STScI has provided Hubble data to the astronomical community, and publicized Hubble's revolutionary discoveries and inspirational images to the world. As it celebrates its 40th anniversary STScI is looking forward to the future, including the October 2021 launch of NASA's next flagship mission, the James Webb Space Telescope, as well as other endeavors.


Figure 57: The STScI (Space Telescope Science Institute) is headquartered in the Muller building on the Johns Hopkins University's Homewood campus. This photo shows the original building shortly after its completion in 1983 (image credit: STScI)

- "For the past 40 years, STScI has partnered with NASA and the astronomical community to advance scientific discovery," said STScI director Kenneth Sembach. "Much has changed in the field of astronomy over that time as Hubble has revolutionized our understanding of astrophysical phenomena. We've grown and changed as well to meet the needs of the astronomical community, create new avenues for exploration, and engage the public in the wonders of the universe. I can hardly wait to see what the future holds as we look ahead to many more years of Hubble operations, the launch of Webb, and the upcoming Nancy Grace Roman Space Telescope."

- STScI is best known for its role in the Hubble mission. As science operations center, the institute enables scientists around the world to make maximum use of Hubble's unique capabilities to conduct cutting-edge science. STScI personnel strive not only to maintain but also continuously improve Hubble operations, ensuring that the telescope will provide quality data for years into the future. Members of the institute's scientific staff also conduct their own research, producing hundreds of peer-reviewed articles each year, and help lead initiatives guiding the future of astrophysics research.

- "In 1976, a committee of the National Academy of Sciences proposed a radical idea that STScI should run Hubble. Working in partnership with the scientific community and NASA, the new organization's sole job was to advocate for the science," commented Matt Mountain, President of the Association of Universities for Research in Astronomy (AURA), which runs STScI. "Today, no one doubts the value of that prescient decision by NASA to create STScI to run the science program for the Hubble Space Telescope. Driven by the science of the astronomical community, Hubble has become the scientific ‘gold standard' and a global brand, precisely because STScI has retained the scientific independence and integrity entrusted to AURA and its partner Johns Hopkins University."

- In planning for Hubble's launch and science operations, STScI was instrumental in making a transformative change to how astronomy is conducted. Unlike previous space missions, Hubble was opened to observers around the world. Accordingly, the institute fostered the growth of astronomer teams, which provided opportunities to more researchers. Under the guidance of its first director, Riccardo Giacconi, STScI made a pioneering effort by taking a novel approach to opening up astronomy to general users. The world's first digitized sky catalog was created for aiming the telescope, and complex automation was developed for planning, scheduling, and archiving observations. This became the guide for future NASA space astrophysics missions.

- In 2001 STScI was selected to oversee the science and mission operations of NASA's James Webb Space Telescope, planned to launch later this year. Webb will be the largest, most powerful and complex space telescope ever built and launched into space. It will complement and extend the discoveries of Hubble, with infrared detectors that will allow it to observe the first galaxies, as well as look inside dust clouds where stars and planetary systems are forming today.

- STScI will also play a key role in the science operations for NASA's Nancy Grace Roman Space Telescope, which is planned to launch in the mid-2020s. The Roman Space Telescope will provide a panoramic field of view that is 100 times greater than Hubble's, leading to the first wide-field maps of the universe at space-based resolution.

- A key element of the institute's work is the Barbara A. Mikulski Archive for Space Telescopes (MAST). Established at the outset of the Hubble mission in 1990, it was expanded in 1997 to include data from other ultraviolet and optical space astronomy missions. Today, MAST provides astronomers access to data from more than 20 space missions and ground-based observatories.

- STScI also plays a vital role in the development of technologies for future observatories. The institute's Russell B. Makidon Optics Laboratory, created in 2013, conducts research focused on enabling direct images of exoplanets using large segmented telescopes in space, including high-contrast coronagraphy, optical mirror alignments, applications of deformable mirrors for wavefront sensing and control, and digital micromirror devices for multi-object spectroscopy.

- In addition to its scientific leadership, STScI strives to be a leader in diversity, equity and inclusion (DE&I). The Institute pioneered the use of a dual-anonymous review process, in which scientists reviewing requests for Hubble observing time do not know the names or locations of the proposers. The process proved so successful in achieving gender parity that NASA is mandating it for all its astrophysics missions in the future. The review process is only one element of a broad commitment to DE&I as STScI strives to model the workplace of the future, while also broadening participation in the exploration of the universe.

- STScI is a leader in the field of astronomy communications and outreach. The institute's public outreach team leverages unique access to scientific discoveries, data, and mission experts to produce a broad variety of materials ranging from awe-inspiring images and press releases to videos and in-depth articles. Additional products and learning experiences, grounded in evidence-based learning strategies and externally evaluated, are used by museums, libraries, and other organizations nationwide, as well as the general public. STScI's outreach team maintains a web presence for core science missions, provides support for the Space Astronomy Summer Program (SASP) for undergraduates, and leverages new technologies to create virtual reality (VR) and other interactive experiences. STScI also leads the multi-institutional NASA's Universe of Learning project.

• April 6, 2021: NASA's Hubble Space Telescope is "seeing double." Peering back 10 billion years into the universe's past, Hubble astronomers found a pair of quasars that are so close to each other they look like a single object in ground-based telescopic photos, but not in Hubble's crisp view. 63)

- The researchers believe the quasars are very close to each other because they reside in the cores of two merging galaxies. The team went on to win the "daily double" by finding yet another quasar pair in another colliding galaxy duo.

- A quasar is a brilliant beacon of intense light from the center of a distant galaxy that can outshine the entire galaxy. It is powered by a supermassive black hole voraciously feeding on inflating matter, unleashing a torrent of radiation.

- "We estimate that in the distant universe, for every 1,000 quasars, there is one double quasar. So finding these double quasars is like finding a needle in a haystack," said lead researcher Yue Shen of the University of Illinois at Urbana-Champaign.

- The discovery of these four quasars offers a new way to probe collisions among galaxies and the merging of supermassive black holes in the early universe, researchers say.

- Quasars are scattered all across the sky and were most abundant 10 billion years ago. There were a lot of galaxy mergers back then feeding the black holes. Therefore, astronomers theorize there should have been many dual quasars during that time.

- "This truly is the first sample of dual quasars at the peak epoch of galaxy formation with which we can use to probe ideas about how supermassive black holes come together to eventually form a binary," said research team member Nadia Zakamska of Johns Hopkins University in Baltimore, Maryland.

- The team's results appeared in the April 1 online issue of the journal Nature Astronomy. 64)

- Shen and Zakamska are members of a team that is using Hubble, the European Space Agency's Gaia space observatory, and the Sloan Digital Sky Survey, as well as several ground-based telescopes, to compile a robust census of quasar pairs in the early universe.

- The observations are important because a quasar's role in galactic encounters plays a critical part in galaxy formation, the researchers say. As two close galaxies begin to distort each other gravitationally, their interaction funnels material into their respective black holes, igniting their quasars.

- Over time, radiation from these high-intensity "light bulbs" launch powerful galactic winds, which sweep out most of the gas from the merging galaxies. Deprived of gas, star formation ceases, and the galaxies evolve into elliptical galaxies.

- "Quasars make a profound impact on galaxy formation in the universe," Zakamska said. "Finding dual quasars at this early epoch is important because we can now test our long-standing ideas of how black holes and their host galaxies evolve together."

- Astronomers have discovered more than 100 double quasars in merging galaxies so far. However, none of them is as old as the two double quasars in this study.


Figure 58: This artist's conception shows the brilliant light of two quasars residing in the cores of two galaxies that are in the chaotic process of merging. The gravitational tug-of-war between the two galaxies stretches them, forming long tidal tails and igniting a firestorm of starbirth. - Quasars are brilliant beacons of intense light from the centers of distant galaxies. They are powered by supermassive black holes voraciously feeding on infalling matter. This feeding frenzy unleashes a torrent of radiation that can outshine the collective light of billions of stars in the host galaxy. - In a few tens of millions of years, the black holes and their galaxies will merge, and so will the quasar pair, forming an even more massive black hole. A similar sequence of events will happen a few billion years from now when our Milky Way galaxy merges with the neighboring Andromeda galaxy [image credit: NASA/ESA and J. Olmsted (STScI)]

- The Hubble images show that quasars within each pair are only about 10,000 light-years apart. By comparison, our Sun is 26,000 light-years from the supermassive black hole in the center of our galaxy.

- The pairs of host galaxies will eventually merge, and then the quasars also will coalesce, resulting in an even more massive, single solitary black hole.

- Finding them wasn't easy. Hubble is the only telescope with vision sharp enough to peer back to the early universe and distinguish two close quasars that are so far away from Earth. However, Hubble's sharp resolution alone isn't good enough to find these dual light beacons.

- Astronomers first needed to figure out where to point Hubble to study them. The challenge is that the sky is blanketed with a tapestry of ancient quasars that flared to life 10 billion years ago, only a tiny fraction of which are dual. It took an imaginative and innovative technique that required the help of the European Space Agency's Gaia satellite and the ground-based Sloan Digital Sky Survey to compile a group of potential candidates for Hubble to observe.


Figure 59: Hubble Resolves Two Pairs of Quasars. These two Hubble Space Telescope images reveal two pairs of quasars that existed 10 billion years ago and reside at the hearts of merging galaxies. Each of the four quasars resides in a host galaxy. These galaxies, however, cannot be seen because they are too faint, even for Hubble. The quasars within each pair are only about 10,000 light-years apart—the closest ever seen at this cosmic epoch. -Quasars are brilliant beacons of intense light from the centers of distant galaxies that can outshine their entire galaxies. They are powered by supermassive black holes voraciously feeding on infalling matter, unleashing a torrent of radiation. - The quasar pair in the left-hand image is catalogued as J0749+2255; the pair on the right, as J0841+4825. The two pairs of host galaxies inhabited by each double quasar will eventually merge. The quasars will then tightly orbit each other until they eventually spiral together and coalesce, resulting in an even more massive, but solitary black hole. - The image for J0749+2255 was taken Jan. 5, 2020. The J0841+4825 snapshot was taken Nov. 30, 2019. Both images were taken in visible light with Wide Field Camera 3 [NASA/ESA, H. Hwang and N. Zakamska (Johns Hopkins University), and Y. Shen (University of Illinois, Urbana-Champaign)]

- Located at Apache Point Observatory in New Mexico, the Sloan telescope produces three-dimensional maps of objects throughout the sky. The team poured through the Sloan survey to identify the quasars to study more closely.

- The researchers then enlisted the Gaia observatory to help pinpoint potential double-quasar candidates. Gaia measures the positions, distances, and motions of nearby celestial objects very precisely. But the team devised a new, innovative application for Gaia that could be used for exploring the distant universe. They used the observatory's database to search for quasars that mimic the apparent motion of nearby stars. The quasars appear as single objects in the Gaia data. However, Gaia can pick up a subtle, unexpected "jiggle" in the apparent position of some of the quasars it observes.

- The quasars aren't moving through space in any measurable way, but instead their jiggle could be evidence of random fluctuations of light as each member of the quasar pair varies in brightness. Quasars flicker in brightness on timescales of days to months, depending on their black hole's feeding schedule.

- This alternating brightness between the quasar pair is similar to seeing a railroad crossing signal from a distance. As the lights on both sides of the stationary signal alternately flash, the sign gives the illusion of "jiggling."

- When the first four targets were observed with Hubble, its crisp vision revealed that two of the targets are two close pairs of quasars. The researchers said it was a "light bulb moment" that verified their plan of using Sloan, Gaia, and Hubble to hunt for the ancient, elusive double powerhouses.

- Team member Xin Liu of the University of Illinois at Urbana-Champaign called the Hubble confirmation a "happy surprise." She has long hunted for double quasars closer to Earth using different techniques with ground-based telescopes. "The new technique can not only discover dual quasars much further away, but it is much more efficient than the methods we've used before," she said.

- Their Nature Astronomy article is a "proof of concept that really demonstrates that our targeted search for dual quasars is very efficient," said team member Hsiang-Chih Hwang, a graduate student at Johns Hopkins University and the principal investigator of the Hubble program. "It opens a new direction where we can accumulate a lot more interesting systems to follow up, which astronomers weren't able to do with previous techniques or datasets."

- The team also obtained follow-up observations with the National Science Foundation NOIRLab's Gemini telescopes. "Gemini's spatially-resolved spectroscopy can unambiguously reject interlopers due to chance superpositions from unassociated star-quasar systems, where the foreground star is coincidentally aligned with the background quasar," said team member Yu-Ching Chen, a graduate student at the University of Illinois at Urbana-Champaign.

- Although the team is convinced of their result, they say there is a slight chance that the Hubble snapshots captured double images of the same quasar, an illusion caused by gravitational lensing. This phenomenon occurs when the gravity of a massive foreground galaxy splits and amplifies the light from the background quasar into two mirror images. However, the researchers think this scenario is highly unlikely because Hubble did not detect any foreground galaxies near the two quasar pairs.

- Galactic mergers were more plentiful billions of years ago, but a few are still happening today. One example is NGC 6240, a nearby system of merging galaxies that has two and possibly even three supermassive black holes. An even closer galactic merger will occur in a few billion years when our Milky Way galaxy collides with neighboring Andromeda galaxy. The galactic tussle would likely feed the supermassive black holes in the core of each galaxy, igniting them as quasars.

- Future telescopes may offer more insight into these merging systems. NASA's James Webb Space Telescope, an infrared observatory scheduled to launch later this year, will probe the quasars' host galaxies. Webb will show the signatures of galactic mergers, such as the distribution of starlight and the long streamers of gas pulled from the interacting galaxies.

- The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

• April 2, 2021: The Veil Nebula lies around 2,100 light-years from Earth in the constellation of Cygnus (the Swan), making it a relatively close neighbor in astronomical terms. Only a small portion of the nebula was captured in this image. 65)

- The Veil Nebula is the visible portion of the nearby Cygnus Loop, a supernova remnant formed roughly 10,000 years ago by the death of a massive star. That star – which was 20 times the mass of the Sun – lived fast and died young, ending its life in a cataclysmic release of energy. Despite this stellar violence, the shockwaves and debris from the supernova sculpted the Veil Nebula's delicate tracery of ionized gas – creating a scene of surprising astronomical beauty.

- To create this colorful image, observations were taken by Hubble's Wide Field Camera 3 instrument using five different filters. The new post-processing methods have further enhanced details of emissions from doubly ionized oxygen (seen here in blues), ionized hydrogen, and ionized nitrogen (seen here in reds).


Figure 60: This image taken by the NASA/ESA Hubble Space Telescope revisits the Veil Nebula, which was featured in a previous Hubble image release. In this image, new processing techniques have been applied, bringing out fine details of the nebula's delicate threads and filaments of ionized gas (image credit: ESA/Hubble & NASA, Z. Levay)

• March 31, 2021: The NHFP (NASA Hubble Fellowship Program) is one of the highlights of NASA's pursuit of excellence in astrophysics. The program enables outstanding postdoctoral scientists to pursue independent research in any area of NASA Astrophysics, using theory, observation, experimentation, or instrument development. Over 400 applicants vied for the fellowships. Each fellowship provides the awardee up to three years of support. 66)

- NASA has selected 24 new Fellows for its prestigious NASA Hubble Fellowship Program (NHFP). The NHFP is one of the highlights of NASA's pursuit of excellence in astrophysics. The program enables outstanding postdoctoral scientists to pursue independent research in any area of NASA Astrophysics, using theory, observation, experimentation, or instrument development. Over 400 applicants vied for the fellowships. Each fellowship provides the awardee up to three years of support.

- Once selected, Fellows are named to one of three sub-categories corresponding to three broad scientific questions NASA seeks to answer about the universe:

a) How does the universe work? – Einstein Fellows

b) How did we get here? – Hubble Fellows

c) Are we alone? – Sagan Fellows

- "The annual selection of NASA Hubble Fellows always fills me with hope," said Paul Hertz, Director of Astrophysics at NASA Headquarters in Washington. "These outstanding young scientists are the future of astrophysics, and their impact on our understanding of the cosmos will be felt for decades to come."

- The newly selected NHFP Fellows will begin their programs in the fall of 2021 at a university or research center of their choosing in the United States. The list below provides the names of the 2021 awardees, their fellowship host institutions, and their proposed research topics.

An important part of the NHFP are the Symposia, which allow Fellows the opportunity to present results of their research, and to meet each other and the scientific and administrative staff who manage the program. A lively and very successful virtual symposium was held in the fall of 2020, and organizers are waiting to make a decision on whether the 2021 symposium will be virtual or in-person.

The Space Telescope Science Institute administers the NHFP on behalf of NASA, in collaboration with the Chandra X-ray Center at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, and the NASA Exoplanet Science Institute at Caltech/IPAC in Pasadena, California.

Kishalay De, Massachusetts Institute of Technology, Unveiling the Local Stellar Graveyard

Sara Issaoun, Smithsonian Astrophysical Observatory, Connecting Black Hole Shadows to Multi-Wavelength Accretion and Outflow Physics

Mikhail Ivanov, Institute for Advanced Study, Fundamental Cosmology from Galaxy Clustering

Lea Marcotulli, Yale University, Unveiling the Hidden Connection Between Supermassive Black Holes and Jet Triggering Mechanisms

Ariadna Murguia-Berthier, Northwestern University, The Study of Hyper-Accreting Black Holes and the Origin of Gold in the Universe

Antonella Palmese, University of California, Berkeley, Gravitational Wave Cosmology Through the Eyes of Dark Energy Experiments

Anowar Shajib, University of Chicago, Internal Structure of Massive Elliptical Galaxies: The Missing Piece at the Intersection of Astrophysics and Cosmology

Alexandra Tetarenko, Texas Tech University, Unraveling the Complex Nature of Black Holes and How They Power Explosive Outflows with Time-Domain Observations

Table 2: How does the universe work? – Einstein Fellows

Kirk Barrow, Harvard University, High-Cadence Radiative Transfer Modeling on Galactic Scales

Pradip Gatkine, California Institute of Technology, Probing the CGM-Galaxy Connection Using Multi-Object Spectroscopy and Astrophotonics

Michael Grudic, Carnegie Observatories, Star Formation Physics from Cosmos to Cores

Thales Gutcke, Princeton University, Linking ISM Physics and Galaxy Formation

Erika Holmbeck, Carnegie Observatories, The R-Process Refinery: Distilling Stellar Signatures to Characterize the Astrophysical Production Site of the Heavy Elements

Sinclaire Manning, University of Massachusetts, Amherst, Revealing Optically Invisible Dusty Star-Forming Galaxies in the Early Universe

Viraj Pandya, Columbia University, Towards a Fully Predictive Standard Model of Galaxy Formation

Melinda Soares, University of Wisconsin, Madison, Devoured Worlds—The Signatures of Substellar Ingestion

Catherine Zucker, Space Telescope Science Institute, Knitting Together the Milky Way: Tracing Gas Flows in the Era of Big Data

Table 3: How did we get here? – Hubble Fellows

Marta Bryan, University of California, Berkeley, The Celestial Movers and Shakers: Gas Giant Planets Provide Key Insights into the Formation Histories of Exoplanet Systems

Megan Mansfield, University of Arizona, Steward Observatory, Atmospheres as Windows to the Diversity of Extrasolar Planets

Diana Powell, Smithsonian Astrophysical Observatory, Origins: Relating Protoplanetary Disks to Planetary Atmospheres

Raluca Rufu, Southwest Research Institute, A New Paradigm for Compact Exoplanetary System Origin

Jake Turner, Cornell University, Studying Exoplanetary Magnetic Fields Using Radio and High-Resolution Spectropolarimetry Observations

Luis Welbanks, Arizona State University, High-Definition Exo-Atmospheric Characterization with Transit Spectroscopy

Jon Zink, California Institute of Technology, Expanding Exoplanet Demographics to Identify Key Components of Planet Formation

Table 4: Are we alone? – Sagan Fellows

• March 26, 2021: This week's Hubble/ESA Picture of the Week features NGC 7678 — a galaxy located approximately 164 million light-years away in the constellation of Pegasus (The Winged Horse). With a diameter of around 115,000 light-years, this bright spiral galaxy is a similar size to our own galaxy (the Milky Way), and was discovered in 1784 by the German-British astronomer William Herschel. 67)


Figure 61: The Atlas of Peculiar Galaxies is a catalogue which was produced in 1966 by the American astronomer Halton Arp. NGC 7678 is among the 338 galaxies presented in this catalogue, which organizes peculiar galaxies according to their unusual features. Catalogued here as Arp 28, this galaxy is listed together with six others in the group "spiral galaxies with one heavy arm" (image credit: ESA/Hubble & NASA, A. Riess et al.; CC BY 4.0)

This week's Hubble/ESA Picture of the Week features NGC 7678 — a galaxy located approximately 164 million light-years away in the constellation of Pegasus (The Winged Horse). With a diameter of around 115 000 light-years, this bright spiral galaxy is a similar size to our own galaxy (the Milky Way), and was discovered in 1784 by the German-British astronomer William Herschel

• March 19, 2021: Located around 5000 light-years away in the constellation of Cygnus (The Swan), Abell 78 is an unusual type of planetary nebula. 68)

- After exhausting the nuclear fuel in their cores, stars with a mass of around 0.8 to 8 times the mass of our Sun collapse to form dense and hot white dwarf stars. As this process occurs, the dying star will throw off its outer layers of material, forming an elaborate cloud of gas and dust known as a planetary nebula. This phenomenon is not uncommon, and planetary nebulae are a popular focus for astrophotographers because of their often beautiful and complex shapes. However, a few like Abell 78 are the result of a so-called "born again" star.

- Although the core of the star has stopped burning hydrogen and helium, a thermonuclear runaway at its surface ejects material at high speeds. This ejecta shocks and sweeps up the material of the old nebula, producing the filaments and irregular shell around the central star seen in this Picture of the Week, which features data from Hubble's Wide Field Camera 3 and Pan-STARSS.


Figure 62: Planetary nebula Abell 78 captured by the Hubble Space Telescopes's Wide Field Camera 3 and PANSTARSS (image credit: ESA/Hubble & NASA, M. Guerrero, CC BY 40, Acknowledgement: Judy Schmidt)

• March 18, 2021: NASA's Hubble Space Telescope is giving astronomers a view of changes in Saturn's vast and turbulent atmosphere as the planet's northern hemisphere summer transitions to fall as shown in this series of images taken in 2018, 2019, and 2020 (left to right). 69)

- "These small year-to-year changes in Saturn's color bands are fascinating," said Amy Simon, planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "As Saturn moves towards fall in its northern hemisphere, we see the polar and equatorial regions changing, but we are also seeing that the atmosphere varies on much shorter timescales." Simon is lead author of a paper on these observations published March 11 in Planetary Science Journal.


Figure 63: "What we found was a slight change from year-to-year in color, possibly cloud height, and winds - not surprising that the changes aren't huge, as we're only looking at a small fraction of a Saturn year," added Simon. "We expect big changes on a seasonal timescale, so this is showing the progression towards the next season." (image credit: NASA/ESA/STScI, text credit: Bill Steigerwald)

Figure 64: Hubble Space Telescope images of Saturn taken in 2018, 2019, and 2020 as the planet's northern hemisphere summer transitions to fall (image credits: NASA/ESA/STScI, A. Simon, R. Roth)

- The Hubble data show that from 2018 to 2020 the equator got 5 to 10 percent brighter, and the winds changed slightly. In 2018, winds measured near the equator were about 1,000 miles per hour (roughly 1,600 kilometers per hour), higher than those measured by NASA's Cassini spacecraft during 2004-2009, when they were about 800 miles per hour (roughly 1,300 kilometers per hour). In 2019 and 2020 they decreased back to the Cassini speeds. Saturn's winds also vary with altitude, so the change in measured speeds could possibly mean the clouds in 2018 were around 37 miles (~ 60 km) deeper than those measured during the Cassini mission. Further observations are needed to tell which is happening.

- Saturn is the sixth planet from our Sun and orbits at a distance of about 886 million miles (1.4 billion kilometers) from the Sun. It takes around 29 Earth years to orbit the Sun, making each season on Saturn more than seven Earth years long. Earth is tilted with respect to the Sun, which alters the amount of sunlight each hemisphere receives as our planet moves in its orbit. This variation in solar energy is what drives our seasonal changes. Saturn is tilted also, so as the seasons change on that distant world, the change in sunlight could be causing some of the observed atmospheric changes.

- Like Jupiter, the solar system's largest planet, Saturn is a "gas giant" made mostly of hydrogen and helium, although there may be a rocky core deep inside. Enormous storms, some almost as large as Earth, occasionally erupt from deep within the atmosphere. Since many of the planets discovered around other stars are gas giants as well, astronomers are eager to learn more about how gas giant atmospheres work.

- Saturn is the second largest planet in the solar system, over 9 times wider than Earth, with more than 50 moons and a spectacular system of rings made primarily of water ice. Two of these moons, Titan and Enceladus, appear to have oceans beneath their icy crusts that might support life. Titan, Saturn's largest moon, is the only moon in our solar system with a thick atmosphere, including clouds that rain liquid methane and other hydrocarbons on to the surface, forming rivers, lakes, and seas. This mix of chemicals is thought to be similar to that on Earth billions of years ago when life first emerged. NASA's Dragonfly mission will fly over the surface of Titan, touching down in various locations to search for the primal building blocks of life.

- The Saturn observations are part of Hubble's Outer Planets Atmospheres Legacy (OPAL) program. "The OPAL program allows us to observe each of the outer planets with Hubble every year, enabling new discoveries and watching how each planet is changing over time," said Simon, principal investigator for OPAL.

- The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

• March 18, 2021: Though our galaxy is an immense city of at least 200 billion stars, the details of how they formed remain largely cloaked in mystery. 70)

- Scientists know that stars form from the collapse of huge hydrogen clouds that are squeezed under gravity to the point where nuclear fusion ignites. But only about 30 percent of the cloud's initial mass winds up as a newborn star. Where does the rest of the hydrogen go during such a terribly inefficient process?


Figure 65: These four images taken by NASA's Hubble Space Telescope reveal the chaotic birth of stars in the Orion complex, the nearest major star-forming region to Earth. The snapshots show fledgling stars buried in dusty gaseous cocoons announcing their births by unleashing powerful winds and pairs of spinning, lawn-sprinkler-style jets shooting off in opposite directions. Near-infrared light pierces the dusty region to unveil details of the birthing process. The stellar outflows are carving out cavities within the hydrogen gas cloud. This relatively brief birthing stage lasts about 500,000 years. Although the stars themselves are shrouded in dust, they emit powerful radiation, which strikes the cavity walls and scatters off dust grains, illuminating in infrared light the gaps in the gaseous envelopes. Astronomers found that the cavities in the surrounding gas cloud sculpted by a forming star's outflow did not grow regularly as they matured, as theories propose. The protostars were photographed in near-infrared light by Hubble's Wide Field Camera 3. The images were taken Nov. 14, 2009, and Jan. 25, Feb. 11, and Aug. 11, 2010 [image credits: NASA, ESA, STScI, N. Habel and S. T. Megeath (University of Toledo)]

- It has been assumed that a newly forming star blows off a lot of hot gas through lightsaber-shaped outflowing jets and hurricane-like winds launched from the encircling disk by powerful magnetic fields. These fireworks should squelch further growth of the central star. But a new, comprehensive Hubble survey shows that this most common explanation doesn't seem to work, leaving astronomers puzzled.

- Researchers used data previously collected from NASA's Hubble and Spitzer space telescopes and the European Space Agency's Herschel Space Telescope to analyze 304 developing stars, called protostars, in the Orion Complex, the nearest major star-forming region to Earth. (Spitzer and Herschel are no longer operational).

- In this largest-ever survey of nascent stars to date, researchers are finding that gas-clearing by a star's outflow may not be as important in determining its final mass as conventional theories suggest. The researchers' goal was to determine whether stellar outflows halt the infall of gas onto a star and stop it from growing.

- Instead, they found that the cavities in the surrounding gas cloud sculpted by a forming star's outflow did not grow regularly as they matured, as theories propose.

- "In one stellar formation model, if you start out with a small cavity, as the protostar rapidly becomes more evolved, its outflow creates an ever-larger cavity until the surrounding gas is eventually blown away, leaving an isolated star," explained lead researcher Nolan Habel of the University of Toledo in Ohio, USA.

- "Our observations indicate there is no progressive growth that we can find, so the cavities are not growing until they push out all of the mass in the cloud. So, there must be some other process going on that gets rid of the gas that doesn't end up in the star."

- The team's results will appear in an upcoming issue of The Astrophysical Journal. 71)


Figure 66: This ground-based image offers a wide view of the entire Orion cloud complex, the closest major star-forming region to Earth. The red material is hydrogen gas ionized and heated by ultraviolet radiation from massive stars in Orion. The stars are forming in clouds of cold hydrogen gas that are either invisible or appear as dark regions in this image. The crescent shape is called Barnard's Loop and partly wraps around the winter constellation figure of Orion the Hunter. The hunter's belt is the diagonal chain of three stars at image center. His feet are the bright stars Saiph (bottom left) and Rigel (bottom right). This landscape encompasses tens of thousands of newly forming stars bursting to life. Many are still encased in their natal cocoons of gas and dust and only seen in infrared light. The undulating line of yellow dots, beginning at lower left, is a superimposed image of 304 nascent stars taken by NASA's Hubble Space Telescope. This landscape encompasses tens of thousands of newly forming stars bursting to life. Many are still encased in their natal cocoons of gas and dust and only seen in infrared light. Researchers used NASA's Hubble and Spitzer space telescopes and the European Space Agency's Herschel Space Telescope to analyze how young stars' powerful outflows carve out cavities in the vast gas clouds. The study is the largest-ever survey of developing stars [image credits: Image courtesy of R. B. Andreo,; Data Overlay: NASA, ESA, STScI, N. Habel and S. T. Megeath (University of Toledo)]

A Star is Born

- During a star's relatively brief birthing stage, lasting only about 500,000 years, the star quickly bulks up on mass. What gets messy is that, as the star grows, it launches a wind, as well as a pair of spinning, lawn-sprinkler-style jets shooting off in opposite directions. These outflows begin to eat away at the surrounding cloud, creating cavities in the gas.

- Popular theories predict that as the young star evolves and the outflows continue, the cavities grow wider until the entire gas cloud around the star is completely pushed away. With its gas tank empty, the star stops accreting mass – in other words, it stops growing.

- To look for cavity growth, the researchers first sorted the protostars by age by analyzing Herschel and Spitzer data of each star's light output. The protostars in the Hubble observations were also observed as part of the Herschel telescope's Herschel Orion Protostar Survey.

- Then the astronomers observed the cavities in near-infrared light with Hubble's Near-infrared Camera and Multi-object Spectrometer and Wide Field Camera 3. The observations were taken between 2008 and 2017. Although the stars themselves are shrouded in dust, they emit powerful radiation which strikes the cavity walls and scatters off dust grains, illuminating the gaps in the gaseous envelopes in infrared light.

- The Hubble images reveal the details of the cavities produced by protostars at various stages of evolution. Habel's team used the images to measure the structures' shapes and estimate the volumes of gas cleared out to form the cavities. From this analysis, they could estimate the amount of mass that had been cleared out by the stars' outbursts.

Figure 67: Though our galaxy is an immense city of at least 200 billion stars, the details of how they formed remain largely cloaked in mystery. Scientists know that stars form from the collapse of huge hydrogen clouds that are squeezed under gravity to the point where nuclear fusion ignites. But only about 30 percent of the cloud's initial mass winds up as a newborn star. Where does the rest of the hydrogen go during such a terribly inefficient process? (video credit: NASA Goddard Space Flight Center)

- "We find that at the end of the protostellar phase, where most of the gas has fallen from the surrounding cloud onto the star, a number of young stars still have fairly narrow cavities," said team member Tom Megeath of the University of Toledo. "So, this picture that is still commonly held of what determines the mass of a star and what halts the infall of gas is that this growing outflow cavity scoops up all of the gas. This has been pretty fundamental to our idea of how star formation proceeds, but it just doesn't seem to fit the data here."

- Future telescopes such as NASA's upcoming James Webb Space Telescope will probe deeper into a protostar's formation process. Webb spectroscopic observations will observe the inner regions of disks surrounding protostars in infrared light, looking for jets in the youngest sources. Webb also will help astronomers measure the accretion rate of material from the disk onto the star, and study how the inner disk is interacting with the outflow.

• March 12, 2021: NASA's Hubble Space Telescope resumed observations March 11 after a software error placed it in a protective safe mode several days earlier, but the incident is a reminder of the telescope's mortality. 72)

- NASA said Hubble resumed observations at 8 p.m. Eastern March 11, more than four and a half days after a software error caused the spacecraft to go into a safe mode, suspending normal operations of the nearly 31-year-old space telescope. 73)

- The software error was traced to what an agency statement called an "enhancement" recently uploaded to the spacecraft. That enhancement was intended to compensate for fluctuations from one of the telescope's gyroscopes, but a glitch in the software caused a broader problem with Hubble's main computer, triggering the safe mode early March 7.

- Controllers resolved the problem for now by disabling that software enhancement, and plan to correct the flaw and test the new software further before uploading it again.

- That safe mode, though, caused two other problems with Hubble. The telescope's aperture door, a cover on top of the telescope, is designed to automatically close when the spacecraft enters safe mode to prevent stray sunlight from entering, which could damage instruments and optics. During this safe mode, though, the door did not swing shut, a problem never before seen with Hubble.

- Engineers troubleshooting the problem found that the door did close once they switched to a backup motor. They have now set that motor as the primary one as they continue to study the problem with the other motor.

- One of Hubble's instruments, the Wide Field Camera 3, "experienced an unexpected error" during the recovery from safe mode. NASA did not elaborate on the error but said that observations using that instrument will remain on hold as engineers study the problem. The spacecraft's other instruments, including a camera and two spectrographs, are functioning.

- The safe mode, and related issues, is a reminder of Hubble's age. The spacecraft was launched in April 1990 and serviced by the space shuttle five times, most recently in May 2009. With the shuttle long since retired, astronomers know that, at some point, Hubble will suffer an unrecoverable failure that will end its historic mission.

- "Right now we're in the middle of what I think is a very good news story about Hubble," Jennifer Wiseman, an astronomer at NASA's Goddard Space Flight Center, said at a conference last year about the space telescope. She and others believe that the telescope can remain operational through much of this decade, based on trends in the performance of key components, such as its gyroscopes and batteries.

- Some have argued for a new servicing mission to Hubble using either a robotic or crewed spacecraft. John Grunsfeld, a former NASA astronaut who flew on three Hubble servicing missions and later served as the agency's associate administrator for science, presented a concept last year for a crewed servicing mission using an Orion spacecraft and a module equipped with a robotic arm and airlock. That spacecraft would dock with Hubble, with astronauts then performing repairs much as they did on past servicing missions.

- "We have the technology to go back to Hubble," he said in a presentation last June to the Space Transportation Association, noting that a commercial crew vehicle, like Crew Dragon, could be used in place of Orion. "We could keep Hubble going for another few decades."

- NASA, though, has shown no public interest in such a servicing mission, whose expense would run in the hundreds of millions to billions of dollars. "It's not currently on the books. Nobody is really talking about it a lot, at least publicly," Grunsfeld acknowledged.

- Without a servicing mission, Hubble could last for many years, or fail tomorrow, astronomers like Wiseman acknowledge. "We don't know how long Hubble's going to last," she said.

• March 11, 2021: For the first time, scientists using the NASA/ESA Hubble Space Telescope have found evidence of volcanic activity reforming the atmosphere on a rocky planet around a distant star. The planet, GJ 1132 b, has a similar density, size, and age to those of Earth. 74) 75)

- The planet GJ 1132 b appears to have begun life as a gaseous world with a thick blanket of atmosphere. Starting out at several times the radius of Earth, this so-called "sub-Neptune" quickly lost its primordial hydrogen and helium atmosphere, which was stripped away by the intense radiation from its hot, young star. In a short period of time, it was reduced to a bare core about the size of Earth.

- To the surprise of astronomers, new observations from Hubble have uncovered a secondary atmosphere that has replaced the planet's first atmosphere. It is rich in hydrogen, hydrogen cyanide, methane and ammonia, and also has a hydrocarbon haze. Astronomers theorize that hydrogen from the original atmosphere was absorbed into the planet's molten magma mantle and is now being slowly released by volcanism to form a new atmosphere. This second atmosphere, which continues to leak away into space, is continually being replenished from the reservoir of hydrogen in the mantle's magma.

- "This second atmosphere comes from the surface and interior of the planet, and so it is a window onto the geology of another world," explained team member Paul Rimmer of the University of Cambridge, UK. "A lot more work needs to be done to properly look through it, but the discovery of this window is of great importance."

- "We first thought that these highly radiated planets would be pretty boring because we believed that they lost their atmospheres," said team member Raissa Estrela of the Jet Propulsion Laboratory at the California Institute of Technology in Pasadena, California, USA. But we looked at existing observations of this planet with Hubble and realized that there is an atmosphere there."

- "How many terrestrial planets don't begin as terrestrials? Some may start as sub-Neptunes, and they become terrestrials through a mechanism whereby light evaporates the primordial atmosphere. This process works early in a planet's life, when the star is hotter," said team leader Mark Swain of the Jet Propulsion Laboratory. "Then the star cools down and the planet's just sitting there. So you've got this mechanism that can cook off the atmosphere in the first 100 million years, and then things settle down. And if you can regenerate the atmosphere, maybe you can keep it."

- In some ways, GJ 1132 b has various parallels to Earth, but in some ways it is also very different. Both have similar densities, similar sizes, and similar ages, being about 4.5 billion years old. Both started with a hydrogen-dominated atmosphere, and both were hot before they cooled down. The team's work even suggests that GJ 1132 b and Earth have similar atmospheric pressure at the surface.

- However, the planets' formation histories are profoundly different. Earth is not believed to be the surviving core of a sub-Neptune. And Earth orbits at a comfortable distance from our yellow dwarf Sun. GJ 1132 b is so close to its host red dwarf star that it completes an orbit the star once every day and a half. This extremely close proximity keeps GJ 1132 b tidally locked, showing the same face to its star at all times — just as our moon keeps one hemisphere permanently facing Earth.

- "The question is, what is keeping the mantle hot enough to remain liquid and power volcanism?" asked Swain. "This system is special because it has the opportunity for quite a lot of tidal heating."


Figure 68: This image is an artist's impression of the exoplanet GJ 1132 b. To the surprise of astronomers, new observations from Hubble have uncovered a second atmosphere that has replaced the planet's first atmosphere. It is rich in hydrogen, hydrogen cyanide, methane and ammonia, and also has a hydrocarbon haze. Astronomers theorize that hydrogen from the original atmosphere was absorbed into the planet's molten magma mantle and is now being slowly released by volcanism to form a new atmosphere. This second atmosphere, which continues to leak away into space, is continually being replenished from the reservoir of hydrogen in the mantle's magma [image credit: NASA, ESA, and R. Hurt (IPAC/Caltech)]

- The phenomenon of tidal heating occurs through friction, when energy from a planet's orbit and rotation is dispersed as heat inside the planet. GJ 1132 b is in an elliptical orbit, and the tidal forces acting on it are strongest when it is closest to or farthest from its host star. At least one other planet in the host star's system also exerts a gravitational pull on the planet. The consequences are that the planet is squeezed or stretched by this gravitational "pumping." That tidal heating keeps the mantle liquid for a long time. A nearby example in our own Solar System is the Jovian moon, Io, which has continuous volcanism as a result of a tidal tug-of-war between Jupiter and the neighboring Jovian moons.

- The team believes the crust of GJ 1132 b is extremely thin, perhaps only hundreds of feet thick. That's much too feeble to support anything resembling volcanic mountains. Its flat terrain may also be cracked like an eggshell by tidal flexing. Hydrogen and other gases could be released through such cracks.

- "This atmosphere, if it's thin — meaning if it has a surface pressure similar to Earth — probably means you can see right down to the ground at infrared wavelengths. That means that if astronomers use the James Webb Space Telescope to observe this planet, there's a possibility that they will see not the spectrum of the atmosphere, but rather the spectrum of the surface," explained Swain. "And if there are magma pools or volcanism going on, those areas will be hotter. That will generate more emission, and so they'll potentially be looking at the actual geological activity — which is exciting!"


Figure 69: This plot shows the spectrum of the atmosphere of an Earth sized rocky exoplanet, GJ 1132 b, which is overlaid on an artist's impression of the planet. The orange line represents the model spectrum. In comparison, the observed spectrum is shown as blue dots representing averaged data points, along with their error bars. - This analysis is consistent with GJ 1132 b being predominantly a hydrogen atmosphere with a mix of methane and hydrogen cyanide. The planet also has aerosols which cause scattering of light. - This is the first time a so-called "secondary atmosphere," which was replenished after the planet lost its primordial atmosphere, has been detected on a world outside of our solar system [image credit: NASA, ESA, and P. Jeffries (STScI)]

• March 5, 2021: Hubble Space Telescope image of the week of the NGC2336 galaxy. Its spiral arms are glittered with young stars, visible in their bright blue light. In contrast, the redder central part of the galaxy is dominated by older stars. 76) 77)


Figure 70: NGC 2336 is the quintessential galaxy — big, beautiful and blue — and it is captured here by the NASA/ESA Hubble Space Telescope. The barred spiral galaxy stretches an immense 200,000 light-years across and is located approximately 100 million light years away in the northern constellation of Camelopardalis (The Giraffe), image credit: ESA/Hubble & NASA, V. Antoniou; CC BY 4.0 – Acknowledgement: Judy Schmidt

- NGC 2336 was discovered in 1876 by German astronomer Wilhelm Tempel, using a 28-centimeter telescope. This Hubble image is so much better than the view Tempel would have had — Hubble's main mirror is 2.4 meters across, nearly ten times the size of the telescope Tempel used. In 1987, NGC 2336 experienced a Type-Ia supernova, the only observed supernova in the galaxy since its discovery 111 years earlier.

• March 4, 2021: The red hypergiant VY Canis Majoris is enshrouded in huge clouds of dust. Stars come in an extraordinary range of sizes. One of the most colossal is VY Canis Majoris. If placed in the middle of our solar system it would engulf all the planets out to Saturn's orbit. This monster, appropriately called a red hypergiant, is as bright as 300,000 Suns. Yet it is so far away that, 200 years ago, it could be seen only as a faint star in the winter constellation of the Great Dog. Since then, it has faded and is no longer visible to the naked eye. Astronomers used Hubble to get a close-up look at the star and discovered the reason for the dimming. The star is expelling huge clouds of dust in the final stages of its life. Eventually, the bloated star may explode as a supernova, or may simply collapse and form a black hole. 78)


Figure 71: This zoom into VY Canis Majoris is a combination of Hubble imaging and an artist's impression. The left panel is a multicolor Hubble image of the huge nebula of material cast off by the hypergiant star. This nebula is approximately a trillion miles across. The middle panel is a close-up Hubble view of the region around the star. This image reveals close-in knots, arcs, and filaments of material ejected from the star as it goes through its violent process of casting off material into space. VY Canis Majoris is not seen in this view, but the tiny red square marks the location of the hypergiant, and represents the diameter of the solar system out to the orbit of Neptune, which is 5.5 billion miles across. The final panel is an artist's impression of the hypergiant star with vast convection cells and undergoing violent ejections. VY Canis Majoris is so large that if it replaced the Sun, the star would extend for hundreds of millions of miles, to between the orbits of Jupiter and Saturn [image credit: NASA, ESA, and R. Humphreys (University of Minnesota), and J. Olmsted (STScI)]

- Last year, astronomers were puzzled when Betelgeuse, the bright red supergiant star in the constellation Orion, dramatically faded, but then recovered. The dimming lasted for weeks. Now, astronomers have turned their sights toward a monster star in the adjoining constellation Canis Major, the Great Dog.

- The red hypergiant VY Canis Majoris—which is far larger, more massive, and more violent than Betelgeuse—experiences much longer, dimmer periods that last for years. New findings from NASA's Hubble Space Telescope suggest the same processes that occurred on Betelgeuse are happening in this hypergiant, but on a much grander scale.

- "VY Canis Majoris is behaving a lot like Betelgeuse on steroids," explained the study's leader, astrophysicist Roberta Humphreys of the University of Minnesota, Minneapolis.

- As with Betelgeuse, Hubble data suggest the answer for why this bigger star is dimming. For Betelgeuse, the dimming corresponded to a gaseous outflow that may have formed dust, which briefly obstructed some of Betelgeuse's light from our view, creating the dimming effect.

- "In VY Canis Majoris we see something similar, but on a much larger scale. Massive ejections of material which correspond to its very deep fading, which is probably due to dust that temporarily blocks light from the star," said Humphreys.

- The enormous red hypergiant is 300,000 times brighter than our Sun. If it replaced the Sun in our own solar system, the bloated monster would extend out for hundreds of millions of miles, between the orbits of Jupiter and Saturn.

- "This star is absolutely amazing. It's one of the largest stars that we know of—a very evolved, red supergiant. It has had multiple, giant eruptions," explained Humphreys.

- Giant arcs of plasma surround the star at distances from it that are thousands of times farther away than the Earth is from the Sun. These arcs look like the solar prominences from our own Sun, only on a much grander scale. Also, they're not physically connected to the star, but rather, appear to have been thrown out and are moving away. Some of the other structures close to the star are still relatively compact, looking like little knots and nebulous features.

- In previous Hubble work, Humphreys and her team were able to determine when these large structures were ejected from the star. They found dates ranging over the past several hundred years, some as recently as the past 100 to 200 years.

- Now, in new work with Hubble, researchers resolved features much closer to the star that may be less than a century old. By using Hubble to determine the velocities and motions of the close-in knots of hot gas and other features, Humphreys and her team were able to date these eruptions more accurately. What they found was remarkable: many of these knots link to multiple episodes in the 19th and 20th centuries when VY Canis Majoris faded to one-sixth its usual brightness.

- Unlike Betelgeuse, VY Canis Majoris is now too faint to be seen by the naked eye. The star was once visible but has dimmed so much that it can now only be seen with telescopes.

- The hypergiant sheds 100 times as much mass as Betelgeuse. The mass in some of the knots is more than twice the mass of Jupiter. "It's amazing the star can do it," Humphreys said. "The origin of these high mass-loss episodes in both VY Canis Majoris and Betelgeuse is probably caused by large-scale surface activity, large convective cells like on the Sun. But on VY Canis Majoris, the cells may be as large as the whole Sun or larger."

- "This is probably more common in red supergiants than scientists thought and VY Canis Majoris is an extreme example," Humphreys continued. "It may even be the main mechanism that's driving the mass loss, which has always been a bit of a mystery for red supergiants."

- Though other red supergiants are comparably bright and eject a lot of dust, none of them is as complex as VY Canis Majoris. "So what's special about it? VY Canis Majoris may be in a unique evolutionary state that separates it from the other stars. It's probably this active over a very short period, maybe only a few thousand years. We're not going to see many of those around," said Humphreys.

- The star began life as a super-hot, brilliant, blue supergiant star perhaps as much as 35 to 40 times our Sun's mass. After a few million years, as the hydrogen fusion burning rate in its core changed, the star swelled up to a red supergiant. Humphreys suspects that the star may have briefly returned to a hotter state and then swelled back up to a red supergiant stage.

- "Maybe what makes VY Canis Majoris so special, so extreme, with this very complex ejecta, might be that it's a second-stage red supergiant," explained Humphreys. VY Canis Majoris may have already shed half of its mass. Rather than exploding as a supernova, it might simply collapse directly to a black hole.

- The team's findings appear in the February 4, 2021 edition of The Astronomical Journal. 79)

- The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.

• February 26, 2021: This NASA/ESA Hubble Space Telescope Picture of the Week features NGC4826 — a spiral galaxy located 17 million light-years away in the constellation of Coma Berenices (Berenice's Hair). This galaxy is often referred to as the "Black Eye", or "Evil Eye", galaxy because of the dark band of dust that sweeps across one side of its bright nucleus. 80) 81)

- NGC4826 is known by astronomers for its strange internal motion. The gas in the outer regions of this galaxy and the gas in its inner regions are rotating in opposite directions, which might be related to a recent merger. New stars are forming in the region where the counter rotating gases collide.

- This galaxy was first discovered in 1779 by the English astronomer Edward Pigott.


Figure 72: NGC4826 — a spiral galaxy located 17 million light-years away in the constellation of Coma Berenices (Berenice's Hair) — captured by the Hubble Space Telescope (image credit: ESA/Hubble & NASA, J. Lee and the PHANGS-HST Team, Acknowledgement: Judy Schmidt)

• February 25, 2021: For the first time, a wayward comet-like object has been spotted near the family of ancient asteroids. 82)

- After traveling several billion miles toward the Sun, a wayward young comet-like object orbiting among the giant planets has found a temporary parking place along the way. The object has settled near a family of captured ancient asteroids, called Trojans, that are orbiting the Sun alongside Jupiter. This is the first time a comet-like object has been spotted near the Trojan population.

- The unexpected visitor belongs to a class of icy bodies found in space between Jupiter and Neptune. Called Centaurs, they become active for the first time when heated as they approach the Sun, and dynamically transition into becoming more comet-like.

- Visible-light snapshots by NASA's Hubble Space Telescope reveal that the vagabond object shows signs of comet activity, such as a tail, outgassing in the form of jets, and an enshrouding coma of dust and gas. Earlier observations by NASA's Spitzer Space Telescope gave clues to the composition of the comet-like object and the gasses driving its activity.

- "Only Hubble could detect active comet-like features this far away at such high detail, and the images clearly show these features, such as a roughly 400,000-mile-long broad tail and high-resolution features near the nucleus due to a coma and jets," said lead Hubble researcher Bryce Bolin of Caltech in Pasadena, California.

- Describing the Centaur's capture as a rare event, Bolin added, "The visitor had to have come into the orbit of Jupiter at just the right trajectory to have this kind of configuration that gives it the appearance of sharing its orbit with the planet. We're investigating how it was captured by Jupiter and landed among the Trojans. But we think it could be related to the fact that it had a somewhat close encounter with Jupiter."

- The team's paper appears in Feb. 11 issue of The Astronomical Journal. 83)

- The research team's computer simulations show that the icy object, called P/2019 LD2 (LD2), probably swung close to Jupiter about two years ago. The planet then gravitationally punted the wayward visitor to the Trojan asteroid group's co-orbital location, leading Jupiter by about 437 million miles.


Figure 73: Hubble Trojan comet. NASA's Hubble Space Telescope snapped this image of the young comet P/2019 LD2 as it orbits near Jupiter's captured ancient asteroids, which are called Trojans. The Hubble view reveals a 400,000-mile-long tail of dust and gas flowing from the wayward comet's bright solid nucleus. This Hubble visible-light image is a combination of exposures taken April 1 and May 8, 2020, with the Wide Field Camera 3 (image credit: NASA/ESA/J. Olmsted/STScI)

Bucket Brigade

- The nomadic object was discovered in early June 2019 by the University of Hawaii's Asteroid Terrestrial-impact Last Alert System (ATLAS) telescopes located on the extinct volcanoes, one on Mauna Kea and one on Haleakala. Japanese amateur astronomer Seiichi Yoshida tipped off the Hubble team to possible comet activity. The astronomers then scanned archival data from the Zwicky Transient Facility, a wide-field survey conducted at Palomar Observatory in California, and realized that the object was clearly active in images from April 2019.


Figure 74: The main asteroid belt lies between Mars and Jupiter, whereas Trojan asteroids both lead and follow Jupiter. Scientists now know that asteroids in the early solar system (4.6 billion years ago) adhered together and eventually formed the inner planets, including Earth (image credit: NASA/ESA/J. Olmsted/STScI)

- They followed up with observations from the Apache Point Observatory in New Mexico, which also hinted at the activity. The team observed the comet using Spitzer just days before the observatory's retirement in January 2020, and identified gas and dust around the comet nucleus. These observations convinced the team to use Hubble to take a closer look. Aided by Hubble's sharp vision, the researchers identified the tail, coma structure, the size of the dust particles, and their ejection velocity. These images helped them confirm that the features are due to relatively new comet-like activity.

- Although LD2's location is surprising, Bolin wonders whether this pit stop could be a common pull-off for some sunward-bound comets. "This could be part of the pathway from our solar system through the Jupiter Trojans to the inner solar system," he said.

- The unexpected guest probably will not stay among the asteroids for very long. Computer simulations show that it will have another close encounter with Jupiter in about another two years. The hefty planet will boot the comet from the system, and it will continue its journey to the inner solar system.

- "The cool thing is that you're actually catching Jupiter flinging this object around and changing its orbital behavior and bringing it into the inner system," said team member Carey Lisse of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland. "Jupiter controls what's going on with comets once they get into the inner system by altering their orbits."

- The icy interloper is most likely one of the latest members of the so-called "bucket brigade" of comets to get kicked out of its frigid home in the Kuiper Belt and into the giant planet region through interactions with another Kuiper Belt object. Located beyond Neptune's orbit, the Kuiper Belt is a haven of icy, leftover debris from our planets' construction 4.6 billion years ago, containing millions of objects, and occasionally these objects have near misses or collisions that drastically alter their orbits from the Kuiper Belt inward into the giant planet region.

- The bucket brigade of icy relics endure a bumpy ride during their journey sunward. They bounce gravitationally from one outer planet to the next in a game of celestial pinball before reaching the inner solar system, warming up as they come closer to the Sun. The researchers say the objects spend as much or even more time around the giant planets, which are gravitationally pulling on them – about 5 million years – than they do crossing into the inner system where we live.

- "Inner-system, ‘short-period' comets break up about once a century," Lisse explained. "So, in order to maintain the number of local comets we see today, we think the bucket brigade has to deliver a new short-period comet about once every 100 years."

An Early Bloomer

- Seeing outgassing activity on a comet 465 million miles away from the Sun (where the intensity of sunlight is 1/25th as strong as on Earth) surprised the researchers. "We were intrigued to see that the comet had just started to become active for the first time so far away from the Sun at distances where water ice is barely starting to sublimate," said Bolin.

- Water remains frozen on a comet until it reaches about 200 million miles from the Sun, where heat from sunlight converts water ice to gas that escapes from the nucleus in the form of jets. So the activity signals that the tail might not be made of water. In fact, observations by Spitzer indicated the presence of carbon monoxide and carbon dioxide gas, which could be driving the creation of the tail and jets seen on the Jupiter-orbiting comet. These volatiles do not need much sunlight to heat their frozen form and convert them to gas.

- Once the comet gets kicked out of Jupiter's orbit and continues its journey, it may meet up with the giant planet again. "Short-period comets like LD2 meet their fate by being thrown into the Sun and totally disintegrating, hitting a planet, or venturing too close to Jupiter once again and getting thrown out of the solar system, which is the usual fate," Lisse said. "Simulations show that in about 500,000 years, there's a 90% probability that this object will be ejected from the solar system and become an interstellar comet."

- The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C. NASA's Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, managed the Spitzer mission for NASA's Science Mission Directorate in Washington, D.C. Science operations were conducted at the Spitzer Science Center at IPAC at Caltech. Spitzer's entire science catalogue is available via the Spitzer data archive, housed at the Infrared Science Archive at IPAC. Spacecraft operations were based at Lockheed Martin Space in Littleton, Colorado.

• February 19, 2021: Tantrums of a baby star - a Milky War star with circumstellar material outflow at a distance of 1400 light years.. Week in images. 84)


Figure 75: Herbig-Haro objects are some of the rarer sights in the night sky, taking the form of thin spindly jets of matter floating amongst the surrounding gas and stars. The two Herbig-Haro objects catalogued as HH46 and HH47, seen in this image taken with the NASA/ESA Hubble Space Telescope, were spotted in the constellation of Vela (The Sails), at a distance of over 1400 light-years from Earth. Prior to its discovery in 1977 by the American astronomer R. D. Schwartz, the exact mechanism by which these multi-colored objects formed was unknown (image credit: ESA/Hubble & NASA, B. Nisini)

- Before 1997 it was theorized by Schwartz and others that the objects could be a type of reflection nebula, or a type of shock wave formed from the gas emitted from a star interacting with the surrounding matter. The mystery was finally solved when a protostar, unseen in this image, was discovered at the centre of the long jets of matter. The outflows of matter, some 10 light-years across, were ejected from the newly born star and violently propelled outwards at speeds of over 150 kilometers per second. Upon reaching the surrounding gas, the collision created the bright shock waves seen here.

• February 12, 2021: This week's NASA/ESA Hubble Space Telescope Picture of the Week features an impressive portrait of M1-63, a beautifully captured example of a bipolar planetary nebula located in the constellation of Scutum (the Shield). A nebula like this one is formed when the star at its center sheds huge quantities of material from its outer layers, leaving behind a spectacular cloud of gas and dust. 85)

- It is believed that a binary system of stars at the center of the bipolar nebula is capable of creating hourglass or butterfly-like shapes like the one in this image. This is because the material from the shedding star is funneled towards its poles, with the help of the companion, creating the distinctive double-lobed structure seen in nebulae such as M1-63.


Figure 76: Portrait of M1-63, a beautiful example of a bipolar planetary nebula located in the constellation of Scutum, captured by the Hubble Space Telescope (image credit: ESA/Hubble & NASA, L. Stanghellini)

• February 11, 2021: Scientists were expecting to find an intermediate-mass black hole at the heart of the globular cluster NGC 6397, but instead they found evidence of a concentration of smaller black holes lurking there. New data from the NASA/ESA Hubble Space Telescope have led to the first measurement of the extent of a collection of black holes in a core-collapsed globular cluster. 86) 87)

- Globular clusters are extremely dense stellar systems, in which stars are packed closely together. They are also typically very old — the globular cluster that is the focus of this study, NGC 6397, is almost as old as the Universe itself. It resides 7800 light-years away, making it one of the closest globular clusters to Earth. Because of its very dense nucleus, it is known as a core-collapsed cluster.

- When Eduardo Vitral and Gary A. Mamon of the Institut d'Astrophysique de Paris set out to study the core of NGC 6397, they expected to find evidence for an "intermediate-mass" black hole (IMBH). These are smaller than the supermassive black holes that lie at the cores of large galaxies, but larger than stellar-mass black holes formed by the collapse of massive stars. IMBH are the long-sought "missing link" in black hole evolution and their mere existence is hotly debated, although a few candidates have been found (see Notes [1]).

- To look for the IMBH, Vitral and Mamon analyzed the positions and velocities of the cluster's stars. They did this using previous estimates of the stars' proper motions [2] from Hubble images of the cluster spanning several years [3], in addition to proper motions provided by ESA's Gaia space observatory, which precisely measures the positions, distances and motions of stars. Knowing the distance to the cluster allowed the astronomers to translate the proper motions of these stars into velocities.

- "Our analysis indicated that the orbits of the stars are close to random throughout the globular cluster, rather than systematically circular or very elongated," explained Mamon.

- "We found very strong evidence for invisible mass in the dense central regions of the cluster, but we were surprised to find that this extra mass is not point-like but extended to a few percent of the size of the cluster," added Vitral.

- This invisible component could only be made up of the remnants (white dwarfs, neutron stars, and black holes) of massive stars whose inner regions collapsed under their own gravity once their nuclear fuel was exhausted. The stars progressively sank to the cluster's centre after gravitational interactions with nearby less massive stars, leading to the small extent of the invisible mass concentration. Using the theory of stellar evolution, the scientists concluded that the bulk of the unseen concentration is made of stellar-mass black holes, rather than white dwarfs or neutron stars that are too faint to observe.

- Two recent studies had also proposed that stellar remnants and in particular, stellar-mass black holes, could populate the inner regions of globular clusters.

- "Our study is the first finding to provide both the mass and the extent of what appears to be a collection of mostly black holes in a core-collapsed globular cluster," said Vitral.

- "Our analysis would not have been possible without having both the Hubble data to constrain the inner regions of the cluster and the Gaia data to constrain the orbital shapes of the outer stars, which in turn indirectly constrain the velocities of foreground and background stars in the inner regions," added Mamon, attesting to an exemplary international collaboration.

- The astronomers also note that this discovery raises the question of whether mergers of these tightly packed black holes in core-collapsed globular clusters may be an important source of gravitational waves recently detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) experiment.


[1] In 2020, new data from the NASA/ESA Hubble Space Telescope provided the strongest evidence to date for a mid-sized black hole. Read the full press release on this result here.

[2] Proper motion describes how fast objects move in the sky.

[3] The Hubble data for this study were provided by A. Bellini, who measured the proper motions for over 1.3 million stars in 22 globular clusters, including NGC 6397.


Figure 77: This ancient stellar jewelry box, a globular cluster called NGC 6397, glitters with the light from hundreds of thousands of stars. Astronomers used the NASA/ESA Hubble Space Telescope to gauge the cluster's distance at 7800 light-years away. NGC 6397 is one of the closest globular clusters to Earth. The cluster's blue stars are near the end of their lives. These stars have used up their hydrogen fuel that makes them shine. Now they are converting helium to energy in their cores, which fuses at a higher temperature and appears blue. - The reddish glow is from red giant stars that have consumed their hydrogen fuel and have expanded in size. The myriad small white objects include stars like our Sun. This image is composed of a series of observations taken from July 2004 to June 2005 with Hubble's Advanced Camera for Surveys. The research team used Hubble's Wide Field Camera 3 to measure the distance to the cluster [image credit: NASA, ESA, and T. Brown and S. Casertano (STScI), Acknowledgement: NASA, ESA, and J. Anderson (STScI)]

• February 5, 2021: Picture of the Week: The Modest Galaxy (Astronomy). 88)

- Many young blue stars are sprinkled throughout the circular patterns of UGC 3885's arms, contrasted and complemented by dark lanes of dust also following the spiral structure. A glancing look at UGC 3885 (Uppsala General Catalogue) of Galaxies may only leave you with an impression of the galaxy, but spare a moment longer and the intricacies of the galaxy begin to emerge. Located in the constellation of Lynx, a spiral galaxy some 180 million light-years away. UGC 3885 is a cosmic beauty to behold.


Figure 78: A bright foreground star isn't enough to distract from the grandeur of the galaxy UGC 3885, captured here by the NASA/ESA Hubble Space Telescope. While this foreground star is incredibly bright to Hubble's eye, it does not outshine the details of the background galaxy (image credit: ESA/Hubble & NASA, J. Walsh; CC BY 4.0)

• January 29, 2021: The lives of planetary nebulae are often chaotic, from the death of their parent star to the scattering of its contents far out into space. Captured here by the NASA/ESA Hubble Space Telescope, ESO 455-10 is one such planetary nebula, located in the constellation of Scorpius (The Scorpion). 89)

- The oblate shells of ESO 455-10, previously held tightly together as layers of its central star, not only give this planetary nebula its unique appearance, but also offer information about the nebula. Seen in a field of stars, the distinct asymmetrical arc of material over the north side of the nebula is a clear sign of interactions between ESO 455-10 and the interstellar medium.


Figure 79: Image of the week. The interstellar medium is the material — consisting of matter and radiation — between star systems and galaxies. The star at the centre of ESO 455-10 allows Hubble to see the interaction with the gas and dust of the nebula, the surrounding interstellar medium, and the light from the star itself. Planetary nebulae are thought to be crucial in galactic enrichment as they distribute their elements, particularly the heavier metal elements produced inside a star, into the interstellar medium which will in time form the next generation of stars (image credit: ESA/Hubble & NASA, L. Stanghellini; CC BY 4.0)

• January 22, 2021: Located in the constellation of Virgo (The Virgin), around 50 million light-years from Earth, NGC 4535 is truly a stunning sight to behold. Despite the incredible quality of this image, taken from the NASA/ESA Hubble Space Telescope, NGC 4535 has a hazy, somewhat ghostly, appearance when viewed from a smaller telescope. This led amateur astronomer Leland S. Copeland to nickname NGC 4535 the "Lost Galaxy" in the 1950s. 90) 91)

- This galaxy was studied as part of the PHANGS (Physics at High Angular resolution in Nearby GalaxieS) survey, which aims to clarify many of the links between cold gas clouds, star formation, and the overall shape and other properties of galaxies. On 11 January 2021 the first release of the PHANGS-HST Collection was made publicly available. 92)


Figure 80: The bright colors in this image aren't just beautiful to look at, as they actually tell us about the population of stars within this barred spiral galaxy. The bright blue-ish colors, seen nestled amongst NGC 4535's long, spiral arms, indicate the presence of a greater number of younger and hotter stars. In contrast, the yellower tones of this galaxy's bulge suggest that this central area is home to stars which are older and cooler (image credit: ESA/Hubble & NASA, J. Lee and the PHANGS-HST Team; CC BY 4.0)

• January 15, 2021: Images of two iconic planetary nebulae taken by the Hubble Space Telescope are revealing new information about how they develop their dramatic features. Researchers from Rochester Institute of Technology (RIT) and Green Bank Observatory presented new findings about the Butterfly Nebula (NGC 6302) and the Jewel Bug Nebula (NGC 7027) at the 237th meeting of the American Astronomical Society on Friday, Jan. 15. 93)


Figure 81: On the left is an image of the Jewel Bug Nebula (NGC 7027) captured by the Hubble Space Telescope in 2019 and released in 2020. Further analysis by researchers produced the RGB image on the right, which shows extinction due to dust, as inferred from the relative strength of two hydrogen emission lines, as red; emission from sulfur, relative to hydrogen, as green; and emission from iron as blue (image credit: STScI, Alyssa Pagan; P. Moraga (RIT) et al.)

- Hubble's Wide Field Camera 3 observed the nebulae in 2019 and early 2020 using its full, panchromatic capabilities, and the astronomers involved in the project have been using emission line images from near-ultraviolet to near-infrared light to learn more about their properties. The studies were first-of-their-kind panchromatic imaging surveys designed to understand the formation process and test models of binary-star-driven planetary nebula shaping.

- "We're dissecting them," said Joel Kastner, a professor in RIT's Chester F. Carlson Center for Imaging Science and School of Physics and Astronomy. "We're able to see the effect of the dying central star in how it's shedding and shredding its ejected material. We're now seeing where material that the central star has tossed away is being dominated by ionized gas, where it's dominated by cooler dust, and even how the hot gas is being ionized, whether by the star's UV or by collisions caused by its present, fast winds."

- Kastner said analysis of the new HST images of the Butterfly Nebula is confirming that the nebula was ejected only about 2,000 years ago—an eyeblink by the standards of astronomy – and established that the S-shaped iron emission that helps give it the "wings" of gas is even younger. Surprisingly, they found that while astronomers previously believed they had located the nebula's central star, that previously-identified star is actually not associated with the nebula and is instead much closer to Earth than the Butterfly Nebula. Kastner said he hopes that future studies with the James Webb Space Telescope could help locate the real dying star at the heart of the nebula.


Figure 82: On top is an image of the Butterfly Nebula (NGC 6302) captured by the Hubble Space Telescope in 2019 and released in 2020. Further analysis by researchers produced the RGB image on the bottom, which shows extinction due to dust, as inferred from the relative strength of two hydrogen emission lines, as red; emission from nitrogen, relative to hydrogen, as green; and emission from iron as blue (image credit: STScI, APOD/J. Schmidt; J. Kastner (RIT) et al.)

• January 15, 2021: First discovered in 1798 by German-English astronomer William Hershel, NGC 613 is a galaxy which lies in the southern constellation of Sculptor 67 million light-years away. 94)

- Recent studies have shown that bars are more common in galaxies now than they were in the past, which gives us important clues about galaxy formation and evolution.


Figure 83: Featured here in a new image from the NASA/ESA Hubble Space Telescope, NGC 613 is a lovely example of a barred spiral galaxy. It is easily distinguishable as such because of its well defined central bar and long arms, which spiral loosely around its nucleus. As revealed by surveys, about two thirds of spiral galaxies, including our own Milky Way galaxy, contain a bar (image credit: ESA/Hubble & NASA, G. Folatelli; CC BY 4.0)

• January 14, 2021: Astronomers are winding back the clock on the expanding remains of a nearby, exploded star. By using NASA's Hubble Space Telescope, they retraced the speedy shrapnel from the blast to calculate a more accurate estimate of the location and time of the stellar detonation. 95) 96)

- The victim is a star that exploded long ago in the Small Magellanic Cloud, a satellite galaxy to our Milky Way. The doomed star left behind an expanding, gaseous corpse, a supernova remnant named 1E 0102.2-7219, which NASA's Einstein Observatory first discovered in X-rays. Like detectives, researchers sifted through archival images taken by Hubble, analyzing visible-light observations made 10 years apart.

- The research team, led by John Banovetz and Danny Milisavljevic of Purdue University in West Lafayette, Indiana, measured the velocities of 45 tadpole-shaped, oxygen-rich clumps of ejecta flung by the supernova blast. Ionized oxygen is an excellent tracer because it glows brightest in visible light.

- To calculate an accurate explosion age, the astronomers picked the 22 fastest moving ejecta clumps, or knots. The researchers determined that these targets were the least likely to have been slowed down by passage through interstellar material. They then traced the knots' motion backward until the ejecta coalesced at one point, identifying the explosion site. Once that was known, they could calculate how long it took the speedy knots to travel from the explosion center to their current location.

- According to their estimate, light from the blast arrived at Earth 1,700 years ago, during the decline of the Roman Empire. However, the supernova would only have been visible to inhabitants of Earth's southern hemisphere. Unfortunately, there are no known records of this titanic event.

- The researchers' results differ from previous observations of the supernova's blast site and age. Earlier studies, for example, arrived at explosion ages of 2,000 and 1,000 years ago. However, Banovetz and Milisavljevic say their analysis is more robust.

- "A prior study compared images taken years apart with two different cameras on Hubble, the Wide Field Planetary Camera 2 and the Advanced Camera for Surveys (ACS)," Milisavljevic said. "But our study compares data taken with the same camera, the ACS, making the comparison much more robust; the knots were much easier to track using the same instrument. It's a testament to the longevity of Hubble that we could do such a clean comparison of images taken 10 years apart."

- The astronomers also took advantage of the sharp ACS images in selecting which ejecta clumps to analyze. In prior studies, researchers averaged the speed of all of the gaseous debris to calculate an explosion age. However, the ACS data revealed regions where the ejecta slowed down because it was slamming into denser material shed by the star before it exploded as a supernova. Researchers didn't include those knots in the sample. They needed the ejecta that best reflected their original velocities from the explosion, using them to determine an accurate age estimate of the supernova blast.

- Hubble also clocked the speed of a suspected neutron star—the crushed core of the doomed star—that was ejected from the blast. Based on their estimates, the neutron star must be moving at more than 2 million miles per hour from the center of the explosion to have arrived at its current position. The suspected neutron star was identified in observations with the European Southern Observatory's VLT (Very Large Telescope) in Chile, in combination with data from NASA's Chandra X-ray Observatory.


Figure 84: Hubble Captures the Supernova Remnant 1E 0102.2-7219. This Hubble Space Telescope portrait reveals the gaseous remains of an exploded massive star that erupted approximately 1,700 years ago. The stellar corpse, a supernova remnant named 1E 0102.2-7219, met its demise in the Small Magellanic Cloud, a satellite galaxy of our Milky Way [image credits: NASA, ESA, and J. Banovetz and D. Milisavljevic (Purdue University)]

- "That is pretty fast and at the extreme end of how fast we think a neutron star can be moving, even if it got a kick from the supernova explosion," Banovetz said. "More recent investigations call into question whether the object is actually the surviving neutron star of the supernova explosion. It is potentially just a compact clump of supernova ejecta that has been lit up, and our results generally support this conclusion."

- So the hunt may still be on for the neutron star. "Our study doesn't solve the mystery, but it gives an estimate of the velocity for the candidate neutron star," Banovetz said.

• January 8, 2021: The galaxy NGC 6946 is nothing short of spectacular. In the last century alone, NGC 6946 has experienced 10 observed supernovae, earning its nickname as the Fireworks Galaxy. In comparison, our Milky Way averages just one to two supernova events per century. This NASA/ESA Hubble Space Telescope image shows the stars, spiral arms, and various stellar environments of NGC 6946 in phenomenal detail. 97)

- We are able to marvel at NGC 6946 as it is a face-on galaxy, which means that we see the galaxy "facing" us, rather than seeing it from the side (known as edge-on). The Fireworks Galaxy is further classified as an intermediate spiral galaxy and as a starburst galaxy. The former means the structure of NGC 6946 sits between a full spiral and a barred spiral galaxy, with only a slight bar in its center, and the latter means it has an exceptionally high rate of star formation.


Figure 85: The NASA/ESA Hubble Space Telescope image of galaxy NGC 6946. The "Fireworks Galaxy" resides 25.2 million light-years away, along the border of the northern constellations of Cepheus and Cygnus (The Swan), image credit: ESA/Hubble & NASA, A. Leroy, K. S. Long

• January 7, 2021: It is during rare merging events that galaxies undergo dramatic changes in their appearance and in their stellar content. These systems are excellent laboratories to trace the formation of star clusters under extreme physical conditions. 98) 99)


Figure 86: To celebrate a new year, the NASA/ESA Hubble Space Telescope has published a montage of six beautiful galaxy mergers. Each of these merging systems was studied as part of the recent HiPEEC survey to investigate the rate of new star formation within such systems. These interactions are a key aspect of galaxy evolution and are among the most spectacular events in the lifetime of a galaxy (image credits: ESA/Hubble & NASA, A. Adamo, et al.)

- The Milky Way typically forms star clusters with masses that are 10 thousand times the mass of our Sun. This doesn't compare to the masses of the star clusters forming in colliding galaxies, which can reach millions of times the mass of our Sun.

- These dense stellar systems are also very luminous. Even after the collision, when the resulting galactic system begins to fade into a more quiescent phase, these very massive star clusters will shine throughout their host galaxy, as long-lasting witnesses of past merging events.

- By studying the six galaxy mergers shown here, the Hubble imaging Probe of Extreme Environments and Clusters (HiPEEC) survey has investigated how star clusters are affected during collisions by the rapid changes that drastically increase the rate at which new stars are formed in these galaxies. Hubble's capabilities have made it possible to resolve large star-forming "knots" into numerous compact young star clusters. Hubble's ultraviolet and near-infrared observations of these systems have been used to derive star cluster ages, masses, and extinctions and to analyze the star formation rate within these six merging galaxies. The HiPEEC study reveals that the star cluster populations undergo large and rapid variations in their properties, with the most massive clusters formed towards the end of the merger phase.

- Each of the merging systems shown here has been previously published by Hubble, as early as 2008 and as recently as October 2020. To celebrate it's 18th anniversary in 2008, the Hubble Space Telescope released a collection of 59 images of merging galaxies, which can be explored here.

Top left: NGC 3256: This image of NGC 3256 was taken with the Wide Field Camera 3 (WFC3) and the Advanced Camera for Surveys (ACS), both installed on the NASA/ESA Hubble Space Telescope. The galaxy is about 100 million light-years from Earth and provides an ideal target in which to investigate starbursts that have been triggered by galaxy mergers (image credit: ESA/Hubble, NASA)

Top Middle: NGC 1614: The galaxy system NGC 1614 has a bright optical centre and two clear inner spiral arms that are fairly symmetrical. It also has a spectacular outer structure that consists principally of a large one-sided curved extension of one of these arms to the lower right, and a long, almost straight tail that emerges from the nucleus and crosses the extended arm to the upper right. [image credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)]

Top Right: NGC 4194: NGC 4194 is also known as the Medusa merger. An early galaxy consumed a smaller gas-rich system, throwing out streams of stars and dust out into space. These streams, seen rising from the top of the merger galaxy, resemble the writhing snakes that Medusa, a monster in ancient Greek mythology, famously had on her head in place of hair, lending the object its intriguing name. The Medusa merger is located about 130 million light-years away in the constellation of Ursa Major (The Great Bear). [image credit: ESA/Hubble & NASA, A. Adamo]

Bottom Left: NGC 3690: This system consists of a pair of galaxies, dubbed IC 694 and NGC 3690, which made a close pass some 700 million years ago. As a result of this interaction, the system underwent a fierce burst of star formation. In the last fifteen years or so six supernovae have popped off in the outer reaches of the galaxy, making this system a distinguished supernova factory. [image credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)]

Bottom Middle: NGC 6052: Located in the constellation of Hercules, about 230 million light-years away, NGC 6052 is a pair of colliding galaxies. They were first discovered in 1784 by William Herschel and were originally classified as a single irregular galaxy because of their odd shape. However, we now know that NGC 6052 actually consists of two galaxies that are in the process of colliding. This particular image of NGC 6052 was taken using Hubble's Wide Field Camera 3 (image credit: ESA/Hubble & NASA, A. Adamo, et al.)

Bottom Right: NGC 34: Lying in the constellation Cetus (The Sea Monster), NGC 34's outer region appears almost translucent, pin pricked with stars and strange wispy tendrils. This image shows the galaxy's bright centre, a result of this merging event that has created a burst of new star formation and lit up the surrounding gas. As the galaxies continue to intertwine and become one, NGC 34's shape will become more like that of a peculiar galaxy, devoid of any distinct shape (image credit: ESA/Hubble & NASA, A. Adamo, et al.)

Table 5: Hubble Showcases 6 Galaxy Mergers in Figure 86


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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (

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