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Star formation in Andromeda is winding down
The Andromeda Galaxy is our next door neighbor. It’s the closest large spiral galaxy to the Milky Way, 2.5 million light-years away. And on July 27, 2026, NASA said star formation in Andromeda is winding down.
A new study showed that over the past 500 million years, the rate of star formation has been slowing. Plus, there’s been a particularly steep drop over the past 40 million years.
The Andromeda Galaxy and our Milky Way are the two most massive and dominant galaxies within the Local Group of galaxies. Andromeda is about twice as large as the Milky Way, though, and it’s a bit younger than our galaxy. But observing Andromeda provides valuable insight into our Milky Way.
The researchers published their peer-reviewed study in The Astrophysical Journal on July 27, 2026.
Hubble’s view of Andromeda’s stars
Andromeda is absolutely teeming with stars. Estimates are that it contains some 1 trillion stars. The Hubble Space Telescope has mapped much of the galaxy. Two surveys alone mapped 2/3 of Andromeda’s disk. And these data allowed astronomers to take measurements of 200 million individual stars.
Co-author Ben Williams of the University of Washington said:
We need to measure the individual stars because they are the fossil record of the galaxy’s formation. Hubble is the only telescope that can give you high enough spatial resolution over a large enough area to be able to do that in Andromeda.
Using the data from Hubble, the team divided these ultra-sharp images into thousands of squares. The length of each side of the square measured 300 light-years. Then they analyzed each of these squares to determine how much star formation was occurring within it.
Astronomers can tell a star’s general age by assessing its color. Younger and more massive stars are short-lived and bluer. Older and less massive stars that live longer are redder. So, redder squares contain older stars and bluer squares show more star formation.
Star formation in Andromeda
Astronomers have calculated that the last time Andromeda saw a big burst of star formation was about 2 billion years ago. It likely happened after an interaction or merger with another galaxy. The tidal forces push gases together, which can then coalesce and trigger star formation.
So astronomers can calculate the rate of star formation by how much gas and dust can convert into stars. And they found that, 500 million years ago, Andromeda was forming stars at a rate of about one solar mass per year. That might not seem like much. But it fell even lower around 40 million years ago, to about half that rate. And now, scientists said, Andromeda is only cooking up stars at a rate of about 1/5 the mass of our sun per year.
Most of the star-forming activity has been in a ring about 32,000 light-years from the galaxy’s center. It’s that activity that is now slowing over time.
Watch how star formation slows over time in the Andromeda Galaxy. Brighter colors indicate where star formation was highest during a given time. Video via Tobin Wainer (UWashington); Image Processing: Joseph DePasquale (STScI); Video: NASA, ESA, STScI, Gregory Bacon (STScI).
Why is star formation dying down?
Astronomers were curious to know if one of its satellite galaxies, M32, might be playing a role in the slowing of its star formation. M32 is a dwarf satellite galaxy that appears as a fuzzy oval next to Andromeda in images. Astronomers are unsure how much these two galaxies – separated by 16,000 light-years – have interacted in the past.
Co-author Zhuo Chen of the University of Washington said:
One of the major motivations for this program was to probe potential interactions between M32 and Andromeda’s disk.
What they found was the region of Andromeda closest to M32 did show signs of decreased star formation compared with other regions. And the timing of this decrease extends to about 60 million years ago. Wainer said:
We can’t explicitly say that we are seeing a decrease in star formation because of M32. But it’s right there, and it’s definitely the most likely suspect.
What’s next?
Next, the team plans to combine ground-based observations with the Hubble data. They’re hoping to learn even more about our next door neighbor. Co-author Raja GuhaThakurta of the University of California Santa Cruz said:
There’s a strong scientific value to this archival data. Andromeda is important because it’s a neighbor that is close enough that we can see it in great detail while also getting a global perspective.
With the upcoming Nancy Grace Roman Space Telescope, astronomers hope to get an even better look at the giant spiral next door. Roman’s field of view is around 100 times as large as Hubble’s with a single observation. One of Roman’s observing programs will be to image Andromeda’s entire disk and parts of its halo. This will give astronomers hundreds of millions of more stars to analyze.
Bottom line: Star formation in Andromeda – the giant spiral next door to us – is winding down, astronomers said. And it could be related to M32, the dwarf satellite galaxy that lies nearby.
Source: The Panchromatic Hubble Andromeda Southern Treasury (PHAST). II. The Spatially Resolved Recent Star Formation History in M31
Via NASA