Andromeda galaxy: Large spiral formation of stars, gas and dust in space, angled from upper left to lower right. Many other stars are in the background.
View at EarthSky Community Photos. | Steven Bellavia in Mattituck, New York, created this impressive composite of the Andromeda galaxy on January 27, 2024. Thank you, Steven! You can also see 2 of the brighter dwarf satellite galaxies, along the top edge and just below Andromeda.
Andromeda is a huge spiral galaxy near our own Milky Way galaxy. It’s about twice as massive as the Milky Way and has 36 known dwarf satellite galaxies.
NASA’s Hubble Space Telescope has obtained a new bird’s-eye view and 3D map of this Andromeda ecosystem.
Hubble can trace the motions and evolution of Andromeda and its satellites going back nearly 14 billion years.
Andromeda galaxy and its satellites are a whole ecosystem
The Andromeda galaxy is our Milky Way’s nearest galactic neighbor (other than our own small satellite galaxies). It’s close enough that you can see it, albeit faintly, with the unaided eye. But like the Milky Way, Andromeda isn’t alone. In fact, it has three dozen dwarf satellite galaxies that orbit it, making up a galactic ecosystem. On February 27, 2025, NASA said the Hubble Space Telescope is building a new 3D map of the satellite galaxies and studying how they have formed stars over the nearly 14-billion-year lifetime of the universe. And researchers have found the process of star formation has continued much longer than first thought.
The Hubble researchers published their peer-reviewed results in The Astrophysical Journal on January 28, 2025.
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Bird’s-eye view of the Andromeda galaxy ecosystem
When Hubble looks at Andromeda, 2.5 million light-years away, it can see the entire galaxy and its surroundings. A sort of bird’s-eye view. We can’t do that with the Milky Way, since we are embedded inside it. This also makes it more difficult to see all of the Milky Way’s satellite galaxies. But we do know that Andromeda has many more satellites than our galaxy does. Why?
The new observations provide clues as to how both galaxies evolved over time. For example, it seems that Andromeda’s evolution has been more dynamic than that of the Milky Way. Andromeda looks quite similar to the Milky Way, although it is up to twice as massive. So that may be one reason. But Hubble’s newest observations provide more clues. As lead author Alessandro Savino at the University of California at Berkeley said:
We see that the duration for which the satellites can continue forming new stars really depends on how massive they are and on how close they are to the Andromeda galaxy. It is a clear indication of how small-galaxy growth is disturbed by the influence of a massive galaxy like Andromeda.
View of thousands of stars with a small oblong smudge in the center. Numerous tiny rings are also in various places. Four oval and circular patches of light are shown along the right side of the image.
View larger. | Wide-angle view from Hubble of the Andromeda galaxy ecosystem. Andromeda has 36 known dwarf satellites that orbit it. Image via NASA/ ESA/ Alessandro Savino (UC Berkeley)/ Joseph DePasquale (STScI)/ Akira Fujii DSS2.
Andromeda system asymmetric and perturbed
Andromeda is also more asymmetric than the Milky Way. And the whole system appears to have been disrupted somehow. What caused that? Astronomers think Andromeda merged with another galaxy billions of years ago. That could also help explain why the galaxy is so massive and has so many satellite galaxies. Hubble principal investigator Daniel Weisz at the University of California at Berkeley explained:
Everything scattered in the Andromeda system is very asymmetric and perturbed. It does appear that something significant happened not too long ago. There’s always a tendency to use what we understand in our own galaxy to extrapolate more generally to the other galaxies in the universe. There’s always been concerns about whether what we are learning in the Milky Way applies more broadly to other galaxies. Or is there more diversity among external galaxies? Do they have similar properties? Our work has shown that low-mass galaxies in other ecosystems have followed different evolutionary paths than what we know from the Milky Way satellite galaxies.
Not only are there more satellite galaxies around Andromeda, they also orbit differently too. About half of them are all on the same plane. That’s unusual, as Weisz noted:
That’s weird. It was actually a total surprise to find the satellites in that configuration and we still don’t fully understand why they appear that way.
Animation of the neighboring Andromeda galaxy, zooming through foreground stars and the blackness of intergalactic space. It crosses 2.5 million light-years, with 36 dwarf satellite galaxies orbiting the massive Andromeda galaxy. Video via NASA/ ESA/ Christian Nieves (STScI)/ Alessandro Savino (UC Berkeley)/ Joseph DePasquale (STScI)/ Frank Summers (STScI)/ Robert Gendler.
Star formation lasted a long time
One of the biggest puzzles is that the formation of new stars appears to have lasted much longer in Andromeda’s satellite galaxies than astronomers assumed. As with other galaxies, they formed new stars early on. But, surprisingly, star formation continued for almost the entire age of the universe. Astronomers don’t know why. Savino said:
Star formation really continued to much later times, which is not at all what you would expect for these dwarf galaxies. This doesn’t appear in computer simulations. No one knows what to make of that so far.
Weisz added:
We do find that there is a lot of diversity that needs to be explained in the Andromeda satellite system. The way things come together matters a lot in understanding this galaxy’s history.
Messier 32 (M32), for example, is the brightest of the Andromeda satellite galaxies. Like other galaxies, it does contain older stars. But it also has many younger ones as well. Somehow, star formation continued in M32, with a big burst of activity a few billion years ago. Apart from such bursts, star formation continued, although at a slower rate. But the fact that it continued for so long at all is what astronomers are trying to figure out.
In addition, M32 might be the remaining core of the larger galaxy that merged with Andromeda.
More observations of Andromeda in 5 years
These are just the first new observations of the Andromeda ecosystem. Astronomers are planning to do the second observations in about five years, with either Hubble or the Webb space telescope. This will help astronomers reconstruct the motions of all 36 dwarf satellite galaxies since they formed billions of years ago.
Earlier this year, Hubble scientists also assembled the largest photomosaic so far of Andromeda. It consists of 600 images and shows 200 million stars. Check it out!
Bottom line: The Hubble Space Telescope has captured a new bird’s-eye view and 3D map of the Andromeda galaxy and its ecosystem of 36 dwarf satellite galaxies.
Source: The Hubble Space Telescope Survey of M31 Satellite Galaxies. IV. Survey Overview and Lifetime Star Formation Histories
Via NASA
Read more: Andromeda galaxy: Hubble’s largest photomosaic yet
Read more: Are the Andromeda and Milky Way galaxies already merging?
Paul Scott Anderson
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About the Author:
Paul Scott Anderson has had a passion for space exploration that began when he was a child when he watched Carl Sagan’s Cosmos. He studied English, writing, art and computer/publication design in high school and college. He later started his blog The Meridiani Journal in 2005, which was later renamed Planetaria. He also later started the blog Fermi Paradoxica, about the search for life elsewhere in the universe. While interested in all aspects of space exploration, his primary passion is planetary science and SETI. In 2011, he started writing about space on a freelance basis with Universe Today. He has also written for SpaceFlight Insider and AmericaSpace and has also been published in The Mars Quarterly. He also did some supplementary writing for the iOS app Exoplanet. He has been writing for EarthSky since 2018, and also assists with proofing and social media.