In the decades following the launch of NASA’s , astronomers have tallied over 1 trillion galaxies in the universe. But only one galaxy stands out as the most important nearby stellar island to our Milky Way — the Andromeda galaxy. It can be seen with the naked eye on clear autumn nights as a faint oval object roughly the size of the moon.

A century ago, astronomer Edwin Hubble first established that this so-called “spiral nebula” was approximately 2.5 million light years away from our own Milky Way galaxy.

Now, the space telescope named after Hubble has accomplished the most comprehensive survey of this galaxy. The work yields new clues to the evolutionary history of Andromeda — and it looks markedly different from the Milky Way’s history.

91̽�� astronomers presented the findings Jan. 16 in Maryland at a meeting of the , and in an accompanying published the same date in The Astrophysical Journal.

Without Andromeda as an example of a spiral galaxy, astronomers would know much less about the structure and evolution of our own Milky Way. That’s because Earth is embedded inside the Milky Way. This is like trying to understand the layout of New York City by standing in the middle of Central Park.��

“With Hubble we can get into enormous detail about what’s happening on a holistic scale across the entire disk of the galaxy. You can’t do that with any other large galaxy,” said principal investigator , a 91̽��research associate professor of astronomy.����

Hubble’s sharp imaging capabilities can resolve more than 200 million stars in the Andromeda galaxy, detecting only stars brighter than our sun. They look like grains of sand across the beach. But the telescope can’t capture everything. Andromeda’s total population is estimated to be 1 trillion stars, with many less massive stars falling below Hubble’s sensitivity limit.��

Photographing Andromeda was a Herculean task because the galaxy is a much bigger target in the sky than the galaxies Hubble routinely observes, which are often billions of light years away. The full mosaic was carried out under two Hubble programs. In total it required over 1,000 Hubble orbits, spanning more than a decade.��

This panorama started about a decade ago with the . Images were obtained at near-ultraviolet, visible and near-infrared wavelengths using instruments aboard Hubble to photograph the northern half of Andromeda.��

This has now been followed by the newly published . This phase added images of approximately 100 million stars in the southern half of Andromeda. This southern region is structurally unique and more sensitive to the galaxy’s merger history than the northern disk mapped earlier.��

Combined, the two programs collectively cover the entire disk of Andromeda, which is seen almost edge on — tilted by 77 degrees relative to the view we see from Earth. The galaxy is so large that the mosaic is assembled from approximately 600 separate fields of view. The mosaic image is made up of at least 2.5 billion pixels.��

“The asymmetry between the two halves — now visually evident in this image — is incredibly intriguing,” said , a 91̽��postdoctoral researcher in astronomy and lead author of the accompanying . “It’s fascinating to see the detailed structures of an external spiral galaxy mapped over such a large, contiguous area.”��

The complementary Hubble survey programs provide information about the age, heavy-element abundance and stellar masses inside Andromeda. This will allow astronomers to distinguish between competing scenarios where Andromeda merged with one or more galaxies. Hubble’s detailed measurements constrain models of Andromeda’s merger history and disk evolution.��

“This ambitious photography of the Andromeda galaxy sets a new benchmark for precision studies of large spiral galaxies,” Chen said.��

Though the Milky Way and Andromeda galaxies formed presumably around the same time many billions of years ago, observational evidence shows that they have very different evolutionary histories, despite growing up in the same cosmological neighborhood. Andromeda seems to be more highly populated with younger stars and unusual features like coherent streams of stars, researchers say. This implies it has a more active recent star formation and interaction history than the Milky Way.��

“This detailed look at the resolved stars will help us to piece together the galaxy’s past merger and interaction history,” Williams said.��

This research was funded by NASA and the Simons Foundation. A full list of co-authors is listed with the .

For more information, contact Williams at benw1@uw.edu or Chen at zczhuo@uw.edu.��

This article was adapted from a by the Space Telescope Science Institute. See related posts from and the .

The universe is a dynamic place where galaxies are dancing, merging and shifting appearance. Unfortunately, because these changes take millions or billions of years, telescopes can only provide snapshots, squeezed into a human lifetime.

Luckily, galaxies leave behind clues to their histories and origins. ��������’s upcoming will have the capacity to look for these “fossils” of galaxy formation by conducting high-resolution imaging of galaxies in the nearby universe.

Through a grant from NASA, astronomers are designing a set of possible observations called RINGS — the Roman Infrared Nearby Galaxies Survey — that would collect these images, and the team is producing publicly available tools that the astronomy community can use once Roman launches and starts collecting data.

“Roman is the next flagship NASA mission, and it will provide a treasure trove of new data for unraveling the evolutionary histories of galaxies,” said RINGS principal investigator , a 91̽�� research associate professor of astronomy.

Roman is uniquely prepared for RINGS due to its resolution, which is akin to ��������’s Hubble Space Telescope, as well as its wide field of view — 200 times that of Hubble in the infrared — making it a sky survey telescope that complements Hubble’s narrow-field capabilities.

Galaxies leave behind “hints” about how they evolved, embedded in their stellar structures — similar to how living organisms on Earth can leave behind imprints in rock. These “galactic fossils” are groups of ancient stars that hold the history of the galaxy’s formation and evolution, including the complex chemical makeup of the galaxy when those stars formed.

Such cosmic fossils are of particular interest to , the deputy principal investigator of RINGS and an assistant professor at the University of Pennsylvania. She describes the process of analyzing stellar structures in galaxies as “like going through an excavation and trying to sort out bones and put them back together.”

Roman’s high resolution will allow scientists to pick out these galactic fossils, using structures ranging from long tidal tails on a galaxy’s outskirts to stellar streams within it. These large-scale structures, which Roman is uniquely capable of capturing, can give clues to a galaxy’s merger history. The goal is to “reassemble these fossils in order to look back in time and understand how these galaxies came to be,” said Sanderson.

RINGS will also enable further investigations of one of the most mysterious substances in the universe: dark matter, an invisible form of matter that makes up most of a galaxy’s mass. Scientist cannot currently directly detect dark matter and do not know what it consists of. Yet there exists a particularly useful class of objects for testing dark matter theories: ultra-faint dwarf galaxies.

“Ultra faint dwarf galaxies are so dark matter-dominated that they have very little normal matter for star formation,” said , professor at the University of California, Santa Cruz. “With so few stars being created, ultra-faint galaxies can essentially be seen as pure blobs of dark matter to study.”

Roman, thanks to its large field of view and high resolution, will observe these ultra-faint galaxies to help test multiple theories of dark matter. With these new data, the astronomical community will come closer to finding the truth about this unobservable dark matter that vastly outweighs visible matter: Dark matter makes up about 80% of the universe’s matter while normal matter comprises the remaining 20%.

Ultra-faint galaxies are far from the only test of dark matter. Often, just looking in an average-sized galaxy’s backyard is enough. Structures in the halo of stars surrounding a galaxy often give hints to the amount of dark matter present. But, due to the sheer size of galactic halos — they are often 15-20 times as big as the galaxy itself — current telescopes are deeply inefficient at observing them.

At the moment, the only fully resolved galactic halos scientists have to go on are our own Milky Way and Andromeda, our neighbor galaxy.

“We only have reliable measurements of the Milky Way and Andromeda, because those are close enough that we can get measurements of a large number of stars distributed across their stellar halos,” said Williams. “So, with Roman, all of a sudden we’ll have 100 or more of these fully resolved galaxies.”

When Roman launches by May 2027, it is expected to fundamentally alter how scientists understand galaxies. In the process, it will shed some light on our own home galaxy. The Milky Way is easy to study up close, but we do not have a large enough selfie stick to take a photo of our entire galaxy and its surrounding halo. RINGS shows what Roman is capable of should such a survey be approved. By studying the nearby universe, RINGS can examine galaxies similar in size and age to the Milky Way, and shed light on how we came to be here.

For more information, contact Williams at benw1@uw.edu.

Adapted from a by ��������’s Space Telescope Science Institute.

When astronomers peer deep into space, they don’t expect to find something staring back at them.

In this new image, an uncanny pair of glowing eyes glares menacingly in our direction. The piercing “eyes” are the most prominent feature of what resembles the face of an otherworldly creature.

Hubble observed this unique system as part of a “snapshot” program that takes advantage of occasional gaps in the telescope’s observing schedule to squeeze in additional pictures. , a 91̽�� professor of astronomy and chair of the Department of Astronomy, led the team that captured the image, which also included , a 91̽��research associate professor of astronomy, and , a 91̽��doctoral student in astronomy.

The phenomenon that the team captured in this image is no ghostly apparition. Hubble is looking at a titanic head-on collision between two galaxies.

Each “eye” is the bright core of a galaxy, one of which slammed into another. The outline of the face is a ring of young blue stars. Other clumps of new stars form a nose and mouth.

The entire system is catalogued as Arp-Madore 2026-424, or AM 2026-424, from the Arp-Madore “Catalogue of Southern Peculiar Galaxies and Associations.”

Although galaxy collisions are common — especially back in the young universe — most are not head-on smashups, like the collision that likely created this Arp-Madore system. The violent encounter gives the system an arresting “ring” structure for only a short amount of time, about 100 million years. The crash pulled and stretched the galaxies’ disks of gas, dust and stars outward. This action formed the ring of intense star formation that shapes the nose and face.

Ring galaxies are rare; only a few hundred of them reside in our larger cosmic neighborhood. The galaxies have to collide at just the right orientation to create the ring. The galaxies will merge completely in about 1 billion to 2 billion years, hiding their messy past.

The side-by-side juxtaposition of the two central bulges of stars from both galaxies also is unusual. Because the bulges that make the eyes appear to be the same size, it is evidence that two galaxies of nearly equal proportions were involved in the crash, rather than more common collisions where small galaxies are gobbled up by their larger neighbors.

Dalcanton and her collaborators plan to use this innovative Hubble program to take a close look at many other unusual interacting galaxies. The goal is to compile a robust sample of nearby interacting galaxies, which could offer insight into how galaxies grew over time through galactic mergers. By analyzing these detailed Hubble observations, astronomers could then choose which systems are prime targets for follow-up with NASA’s , scheduled to launch in 2021.

Astronomer Halton Arp published his compendium of 338 unusual-looking interacting galaxies in 1966. He later partnered with astronomer Barry Madore to extend the search for unique galactic encounters in the southern sky. Several thousand galaxies are listed in that survey, published in 1987.

The Hubble image of AM 2026-424 was taken June 19, 2019, in visible light by the Advanced Camera for Surveys. The system is 704 million light-years from Earth.

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For more information, contact Dalcanton at jdal@uw.edu.

Adapted from by the Space Telescope Science Institute.

��������’s has produced a stunningly detailed portrait of the Galaxy, displaying a full spiral face aglow with the light of nearly 25 million individually resolved stars. It is the largest high-resolution mosaic image of Triangulum ever assembled, composed of 54 Hubble fields of view spanning an area 19,400 light-years across.

The project behind this mosaic is led by , a 91̽�� professor of astronomy. Other major contributors include 91̽��astronomy graduate student Meredith Durbin and 91̽��astronomy professor .

The images show areas of star birth glow bright blue throughout the galaxy, particularly in nebulas of hot, ionized hydrogen gas.

Triangulum, also known as M33, is a spiral galaxy and one of our neighbors in a collection of dozens of galaxies called the Local Group. Triangulum is oriented with its face toward us, ideal for studying the distribution of stars and gas in its well-defined spiral structure. While astronomers are still delving into the immense trove of data collected by Hubble, a few characteristics stand out immediately, inviting comparisons and contrasts with our own Milky Way galaxy and the third large spiral galaxy in the Local Group, the Andromeda galaxy.

“My first impression on seeing the Hubble images was, wow, that really is a lot of star formation,” said Dalcanton. “The star formation rate intensity is 10 times higher than the area surveyed in 2015.”

Astronomers think that Triangulum has been an introvert, avoiding disruptive interactions with other galaxies, instead spending the eons tending its well-ordered spiral and turning out new generations of stars. Further research may determine if Triangulum is actually a newer member of the Local Group of galaxies, and whether its quiet days will soon be over.

This mosaic was created from images taken by Hubble’s Advanced Camera for Surveys between February 2017 and February 2018. The panoramic image was presented at this week’s of the American Astronomical Society in Seattle.

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For more information, contact Dalcanton at jd@astro.washington.edu��and Williams at ben@astro.washington.edu.

Adapted from press releases by the and the .

, a 91̽�� postdoctoral researcher in the Department of Astronomy and lecturer in the at 91̽��Bothell, spends her days pondering X-rays.

As she and her colleagues report in published Feb. 12 in the Monthly Notices of the Royal Astronomical Society, they recently solved a mystery involving X-rays — a case of X-rays present when they shouldn’t have been. This mystery’s unusual main character — a star that is pretending to be a supernova — illustrates the importance of being in the right place at the right time.

Such was the case in May 2010 when an amateur South African astronomer pointed his telescope toward , a nearby galaxy. He discovered what appeared to be a supernova — a massive star ending its life in a blaze of glory.

“Most supernovae are visible for a short time and then — over a matter of weeks — fade from view,” said Binder.

After a star explodes as a supernova, it usually leaves behind either a black hole or what’s called a neutron star — the collapsed, high-density core of the former star. Neither should be visible to Earth after a few weeks. But this supernova — SN 2010da — still was.

“SN 2010da is what we call a ‘‘ — something initially thought to be a supernova based on a bright emission of light, but as a massive star that for some reason is showing this enormous flare of activity,” said Binder.

Many supernova impostors appear to be massive stars in a binary system — two stars in orbit of one another. Stellar astrophysicists think that the impostor’s occasional flare-ups might be due to perturbations from its neighbor.

For SN 2010da, the story appeared to be over until September 2010 — four months after it was confirmed as an impostor — when Binder pointed NASA’s toward NGC300 and found something unexpected.

“There was just this massive amount of X-rays coming from SN 2010da, which you should not see coming from a supernova impostor,” she said.

Binder considered a variety of explanations. For example, material from the star’s corona could be hitting a nearby dust cloud. But that would not produce the level of X-rays she had observed. Instead, the intensity of the X-rays coming from SN 2010da were consistent with a neutron star — the dense, collapsed core remnant of a supernovae.

“A neutron star at this location would be surprising,” said Binder, “since we already knew that this star was a supernova impostor — not an actual supernova.”

In 2014, Binder and her colleagues looked at this system again with Chandra and, for the first time, the . They found the impostor star and those puzzling X-ray emissions. Based on these new data, they concluded that, like many other supernovae impostors, SN 2010da likely has a companion. But, unlike any other supernovae impostor binary reported to date, SN 2010da is probably paired with a neutron star.

“If this star’s companion truly is a neutron star, that would mean that the neutron star was once a giant, massive star that underwent its own supernova explosion in the past,” said Binder. “The fact that this supernova event didn’t expel the other star, which is 20 to 25 times the mass of our sun, makes this an incredibly rare type of binary system.”

To understand how this unusual binary system could form, Binder and her colleagues considered the age of the stars in this region of space. Looking at stellar size and luminosity, they discovered that most nearby stars were created in two bursts — one 30 million years ago and the other less than 5 million years ago. But neither SN 2010da nor its presumed neutron star companion could’ve been created in the older burst of starbirth.

“Most stars that are as massive as these usually live 10 to 20 million years, not 30 million,” said Binder. “The most massive, hottest stars can form, grow, swell, explode and leave a neutron star emitting X-rays in about 5 million years.”

Surveys of the galaxy as recently as 2007 and 2008 detected no X-ray emissions from the location of SN 2010da. Instead, Binder believes that the X-rays they first found in 2010 represent the neutron star “turning on” for the first time after its formation. The X-rays are likely produced when material from the impostor star is transferred to the neutron star companion.

“That would mean that this is a really rare system at an early stage of formation,” said Binder, “and we could learn a lot about how massive stars form and die by continuing to study this unique pairing.”

One mystery solved, Binder would like to keep looking at SN 2010da, seeing what else she can learn about its formation and evolution. Its home galaxy, which has yielded previously, is sure to keep her busy. She is also planning a follow-up study of other recent supernova impostors with the help of an undergraduate research assistant at 91̽��Bothell.

Co-authors on the paper included 91̽��astronomy professor , at the National Tsing Hua University, and Paul Plucinsky at the Harvard-Smithsonian Center for Astrophysics, at the University of Minnesota and at Raytheon. Their work was funded by NASA.

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For more information, contact Binder at 206-543-9590 or bbinder@uw.edu.

Grant number: G04-15088X.