Astounding stream of stars caught escaping from nearby galaxy

Back on June 23, 2025, the “first look observations” from the Vera C. Rubin Observatory were released, highlighting the power of the United States’s and the National Science Foundation’s newest telescope. Designed to survey a large portion of the entire sky over and over, more deeply and in a speedier fashion than ever before, its science goals are stupendous. Armed with capabilities that no other observatory can match, it hopes to:

• discover enormous numbers of new objects within our Solar System,
• look for transient events, or changes in distant stars, galaxies, and nebulae, to greater precision than ever before,
• to find new novae, supernovae, tidal disruption events, plus flares and eruptions,
• and to measure variable objects in distant galaxies, helping to resolve the Hubble tension,

along with many other endeavors.

However, the greatest thing it can bring to us — like any new observatory with unprecedented capabilities — is something known as discovery potential: the ability to discover what’s out there because you dared to look at the Universe in a novel, unprecedented fashion. The Vera Rubin Observatory is the first flagship-class telescope (8.4 meters in diameter) that leverages the most complex and sophisticated telescope mount and the largest, most sensitive, highest-resolution camera (3200 megapixels) of all-time to survey the whole sky in rapid fashion.

Although it’s only November of its debut year, 2025, those first-look observations, even though they weren’t designed to optimize the science that could be gleaned from them, are already paying off with unexpected scientific dividends. Here’s the first big major discovery.

This close-up of spiral galaxy Messier 61 isn’t the first time this galaxy has been imaged with a flagship-class telescope, as both the ground-based Very Large Telescope and the space-based Hubble have seen it before. However, the large stellar stream emanating from it, visible faintly at the top-center of the image, was a new discovery with the advent of Rubin data.

What you see, above, is an example of the power of the Vera C. Rubin observatory. Sure, you might look at this and think there’s little that’s remarkable about this image: it’s just a face-on spiral galaxy, fairly typical of the galaxies we see and know very well in the Universe today. In fact, it’s part of the original legendary catalogue of bright nebulae that could have been confused with a comet back when we were observing the skies with 18th Century technology: the Messier catalogue. This galaxy, the 61st object in the catalogue (Messier 61), was actually discovered in 1779 by Barnaba Oriani, who beat Messier to it by 6 days (and, unlike Messier, did not initially mis-identify it as a comet!)

This galaxy is about 50 million light-years away, is slightly smaller than the Milky Way in extent (at about 90,000 light-years across), and is surrounded by many other galaxies in its field, as well as countless more in the distant background. It’s easy to overlook it, but if you look directly above the galaxy in the image above, you’ll see what appears to be a faint “line” of stars emanating from it; it’s so faint you could easily miss it. In fact, up until 2025, everyone had missed it, as telescopes were insensitive to such a low-surface-brightness feature. Instead, Messier 61 was instead notable, up until today, for being such a prolific hotbed for supernovae, with 8 supernovae, all but one of which were of the core-collapse variety, observed within it over the past 100 years.

This 2020 image of galaxy Messier 61 and its surroundings was acquired by an amateur astronomer with an 11″ diameter telescope. In addition to the “normal” features typically found in Messier 61, an extra point of light, identified as the 8th supernova to be spotted within Messier 61, can easily be seen.

Why are some galaxies relatively quiet when it comes to supernova, displaying just one-per-century or fewer, while other galaxies have them going off so frequently? The answer seems to be closely tied to recent or ongoing episodes of star-formation within those galaxies. In order to form new stars, galaxies require reservoirs of cold molecular gas: large clouds or clumps of gas that can collapse, under their own gravity, to trigger the formation of new stars. This typically occurs in galaxies that have massive, dusty disks, are irregularly shaped, or that either are now interacting with or have recently gravitationally interacted with a massive neighbor.

Messier 61 appears to have a massive, dusty disk, but doesn’t appear — at least at first glance — to be exhibiting an ongoing or a recent gravitational interaction. It looks more like a quiet spiral galaxy: something similar to our own Milky Way, which falls into the one-supernova-per-century class of spiral galaxies. The recent PHANGS survey looked at the galaxy, and discovered evidence for a starburst (rapid star-forming episode) that happened in the galactic nucleus about 10 million years ago.

But has our own galaxy been the victim of a recent interaction? If you were looking at our own galaxy in extreme detail, however, you might think to search for evidence of faint stellar streams around it. This is very difficult to do for a distant galaxy. But for our own galaxy, if you look directly above or below the galactic plane, a recent interaction should leave a telltale sign behind: a stream of stars appearing to flow away from the galaxy itself.

This image shows a map of stars in the outer regions of the Milky Way, from the northern celestial hemisphere, with several galactic streams visible. The color-coding indicates the distance to the stars, and the brightness indicates the density of stars in that patch of sky. In the white circles are faint companions of the Milky Way discovered by the SDSS: only two are globular clusters, the rest are all dwarf galaxies.

Indeed, we find several streams and even a few rings extending away from our own Milky Way. What these typically represent is smaller galaxies than our own — dwarf galaxies, satellite galaxies, potentially even large globular clusters — that have passed not only close to the Milky Way, but around or actually through the galactic plane itself on one or more occasions. In fact, as of 2025, there are officially at least 25 known streams in the the Milky Way alone, and they fall into two categories.

• There are stellar streams, like the Magellanic Stream and the Sagittarius Steam, that have an identifiable origin with a still-existing galaxy or globular cluster, with masses that range from 100 solar masses or so up to hundreds of millions of solar masses.
• And there are orphaned stellar streams, like the Lamost stream, the Helmi stream, or the Arcturus stream, that have either a no-longer-extant galaxy or cluster of origin, or whose origin is known but the object that gave rise to the stream has now been either disrupted or is completely defunct.

These streams range from around 1000 light-years in extent to over a million light-years long, with the longest and largest ones making several loops or arcs around the entire Milky Way. The hope of many researchers, when it came to the Vera C. Rubin observatory, was that it would allow many such streams to be observed for more distant galaxies: the ones found well outside of the Local Group.

This section of space, imaged in exquisite detail by the Vera C. Rubin Observatory as part of their “First Look” observations, is known as the Cosmic Treasure Chest. Inside, large galaxies, evidence of extended structure and low-surface brightness features, numerous galaxy groups and galaxy clusters, and tens of thousands of distant background galaxies all appear together.

What you see, above, is an example of a portion of the Vera C. Rubin Observatory’s “first look” observations. Specifically, it’s of a dense region of space known as the Cosmic Treasure Chest: named so because it ought to be a place where the first “riches” of the Vera C. Rubin Observatory reveal themselves. Earlier, we mentioned looking for galaxies that could be hotbeds for supernovae, which typically arise in the immediate and near aftermath of a gravitational interaction, merger, or collision occurring. And as we stated earlier, one hallmark of a signature you could look for that signifies a recent merger or interaction is an extended feature of a stellar stream or trail prominently connected to one of the galaxies.

In an image as rich as this, there re bound to be several. You might look just slightly above the center of the image and spot a giant elliptical galaxy that clearly has a trail of stars coming off of its right side; that’s clearly a candidate. On the far right side, a giant elliptical galaxy appears to have a stream that connects it to a group of galaxies that exhibit spiral and disk-like features, with stellar streams abounding in that galaxy group or cluster. And at the lower-right of the image, barely noticeable at this scale, is the galaxy Messier 61: a face-on spiral galaxy that appears to have a faint “line” emanating from it in the direction of a glowing red foreground star.

Are these really stellar streams? There’s a remarkable technique we can leverage to help us find out.

Shown here are two views of the nearby, Milky Way-like galaxy Messier 63: the Sunflower galaxy. While the stellar extent of the galaxy is normally only shown to overlap with the bright dusty disk, superior techniques can reveal the low surface-brightness halo and streams around it. Those extended features show the true stellar extent of the galaxy, and are required to make an accurate determination of its total stellar mass.

The key, as you can see above, is to devise a method that can optimally image the low-surface-brightness Universe, even in the presence of very bright objects and sources. If you want to see a stream of stars that’s much, much fainter than the galaxy that it’s connected to, one remarkable technique involves the following steps.

• You create a new “absolute sky” flat field, where you use the data to define what “no brightness” actually is.
• You develop an algorithm for perfoming dedicated sky background subtraction, which allows you to identify what the “true background” levels are in each frame, before doing any co-adding together.
• You identify and remove sources of bias that affect the overall signal-to-noise ratios of galaxies you obtain by co-adding the relevant frames.
• And then, below a certain brightness, you perform a color inversion, where “bright white” indicates zero brightness and then gets fainter, almost black-looking, above a certain threshold.

This allows faint features (like a stellar stream) that appear alongside bright features (like a face-on galaxy) to be easily identified; a technique that’s proven to be remarkably successful here in the 21st century under a variety of conditions.

So, you might wonder, what would we find if we used this technique to focus on the galaxy Messier 61 in the Vera Rubin Observatory data?

Applying several astronomical imaging techniques to the raw Vera C. Rubin data that was released as their First Look observations enables the construction of brightness-sensitive maps: where even faint, low-surface brightness features like stellar streams can be visualized alongside much brighter features, like galaxies and dense collections of stars. Several features that are not readily visible in a black-background image easily appear in this view.

That’s exactly what a team led by Aaron Romanowsky accomplished when they applied this technique with the First Look imaging provided by the Vera C. Rubin Observatory. Above, you see that technique applied to this galaxy, as well as to the candidate stellar stream that we could visually identify — well, kind of — emanating from it. When you examine the region surrounding this galaxy, you can see how the transition works from the high-brightness disk-like region, where the background is dark and the luminous sources are shown illuminated, to the low-brightness region farther away, where the background is white and the luminous sources appear darkened.

And then, looking above the galaxy, you can see a large stellar stream, clearly tracing out a “dark” stream-like patch in the low-brightness areas, emanating upwards in this image: away from the galaxy. All told, this identified stream, which appears to come to a diffuse, plume-like end just below and to the right of the “a” in the upper right-hand corner, is estimated to be about 160,000 light-years long, or nearly twice the diameter of its parent galaxy: Messier 61. Using the color information obtained from the First Look observations, they estimate the stellar mass of the stream to be 200 million solar masses, or about half-a-percent of the full stellar mass of Messier 61 itself.

This brightness-stretched image shows the blocked out disk of the main galaxy of Messier 61, as imaged with the Vera C. Rubin Observatory in their “First Look” release, with regions of higher stellar density shown in black, then blues, then greens, then yellows, and then whites. There is some evidence, as pointed to by the yellow arrows, that there are plumes in the opposite direction of the stellar stream, which could be further evidence for a galactic smash-up.

The study authors then conducted an image-stretch, shown above, in order to highlight the lower surface brightness features found on the outskirts of the galaxy. If you take a look at where the yellow arrows are (added by the authors), you’ll see locations where there are candidate plumes of extended star formation coming off of the opposite side of the galaxy from where the stream is emanating. These candidate plumes would extend for anywhere from 20,000 to 50,000 light-years beyond the extent of the main disk of the galaxy, but will require the full Rubin dataset, not just the incomplete “First Look” data that is accessible so far, to confirm their existence.

But such a set of features, if confirmed, would be vitally important. When a smaller galaxy punches through a larger one, it doesn’t just leave a trail of stars behind the smaller galaxy, and it doesn’t just trigger an episode of rapid new star-formation in the main galaxy’s nucleus. Instead, it also creates a “splash” phenomenon where large quantities of gaseous normal matter, including matter that can form new stars, is ejected in the reverse direction of the impact. If there was a galaxy that smashed into and passed through the large, main galaxy of Messier 61, then there should indeed be “plumes” of new stars that appear on the opposite side of the main galaxy.

This exquisitely imaged portion of space contains the terminus of the plume spotted by the Vera C. Rubin Observatory that emanates from Messier 61. This location in space is 160,000 light-years away from the galaxy, and may be coincident with an older population of dwarf galaxy stars: potentially the stream’s progenitor.

And then, when you look at the terminal end of the plume that arrives where the large stellar stream ceases, you should see:

• a bevy of new stars,
• a continuous decline in surface brightness from the plume’s beginning to its end,
• a collection of older stars that represents what was inside the smaller galaxy prior to the collision,
• a diffuse end to the stellar distribution from the stream, perhaps ending in windings or knots,
• and a lack of the stellar stream continuing past that terminus.

This image, as shown above, likely displays all of these features. There is a large collection of bright stars that appears in a cloud-like distribution, over and above the background brightness of other galaxies. The brightness of the stream declines by 1.4 astronomical magnitudes from the edge of the disk to the end of the plume, and the colors of the plume behave like a quenched dwarf galaxy, with a similar luminosity to the Sagittarius stream found surrounding the Milky Way.

The stream, however, appears to be very narrow: much narrower than comparably massive streams around the Milky Way, like Sagittarius. The authors highlight how remarkable it is that here, in Messier 61, we have a galaxy that’s been:

• known for nearly 250 years,
• observed at very high resolution for decades, including with Hubble and 8+ meter ground-based observatories,
• pinned down to be just over 50 million light-years away,

and yet has a stellar stream that’s been unnoticed for all this time. What’s potentially even more remarkable is likely to come from the full suite of Vera C. Rubin Observatory data, when it’s available: the possibility of not only discovering more streams around this and other previously well-studied galaxies and more information about this stream, but also of uncovering whether there is a connection between this stream and a large collection of potentially related stars located just a little bit farther away in space.

The stellar stream that’s emanating from the galaxy Messier 61, at lower right, appears to terminate in the green boxed region (center) that’s blown up and highlighted in the inset panel. However, just slightly beyond that identified end, there appears to be a large source of stellar light, despite being outshone by a foreground star. The full suite of Rubin data will be needed to progress further.

Just based on the early data we have from the Vera C. Rubin Observatory’s “First Look” observations, we’re already now beginning to ask questions that we didn’t know we needed to ask earlier in 2025. Some of these include:

• How common is the presence of stellar streams around Milky Way-like galaxies?
• How many of the nearby, Milky Way-like spirals actually display one or more of these streams?
• How massive are these streams, and what is the prime cause of them?
• What does the stellar population within these streams tell us about their nature?
• Do all of the galaxies with such streams display enhanced supernova rates, recent starbursts, and other properties we typically associate with the creation of such streams?
• Do the galaxies or globular clusters that lead to these streams remain intact? And if so, for how long and under what circumstances?

This latest result highlights some wonderful aspects of how science gets done. We have our theories of how the Universe works, and that sets expectations for what we suspet we’re going to find out there. But when we go and look in a novel way — with new tools, new technology, new equipment, new techniques, or with improvements on any pre-existing methods or instrumentation — the Universe has the potential to surprise us. It’s by being open to those surprises, and in letting what we discover guide our way, that we not only learn new cosmic lessons, but learn what the right “next step” questions are to be asking. The Vera C. Rubin Observatory may just be getting started, but the science lessons it’s teaching us now will provide the foundation for so much future science still to come!