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Hubble reveals a 500-million-year slowdown in the galaxy next door.
Something has been squeezing the life out of Andromeda’s stellar nurseries.
A new study using NASA’s Hubble Space Telescope finds that star formation in Andromeda, the great spiral galaxy nearest to the Milky Way, has declined for the past 500 million years and dropped sharply over the last 40 million.
It’s a rare, detailed look at how a galaxy similar to our own changes over time — and hints that a small neighboring galaxy, M32, may have bullied Andromeda into a slower era.
Andromeda sits about 2.5 million light-years from Earth. On a dark night, it can appear as a faint smudge to the unaided eye. Through Hubble, it becomes a vast body of stars close enough for astronomers to study one star at a time.
That makes Andromeda a stand-in for the Milky Way. We live inside our own galaxy, which makes its full shape and history hard to see. NASA has compared the problem to trying to understand New York City while standing in Central Park.
Andromeda gives astronomers the outside view.
Across two major Hubble surveys—the Panchromatic Hubble Andromeda Treasury (PHAT) and the Panchromatic Hubble Andromeda Southern Treasury (PHAST)—researchers measured about 200 million individual stars across much of Andromeda’s disk. The galaxy likely contains about 1 trillion stars, but even this partial census offered an extensive record of its recent past.
“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,” Ben Williams, an astronomer at the University of Washington and a co-author of the study, said in NASA’s press release.
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The researchers divided Hubble’s images into a grid made up of thousands of squares, each about 300 light-years wide. Then they studied the colors of the stars in each square.
Blue stars tend to be young, massive and short-lived. Redder stars tend to be older and longer-lived. By mapping those colors, the team reconstructed where Andromeda had been forming stars—and when.
The picture that emerged was one of decline.
About two billion years ago, Andromeda experienced a major burst of star formation, likely after an encounter or merger with another galaxy. Such collisions can squeeze clouds of gas and dust, creating dense pockets where stars ignite.
But that burst did not last.
The new study found that 500 million years ago, Andromeda formed stars at a rate of about one solar mass per year—roughly the mass of our sun turned into new stars annually. By 40 million years ago, that rate had fallen to about half a solar mass per year. Today, it has dropped to about one-fifth of the sun’s mass per year.
Much of the recent star-making activity has taken place in a ring about 32,000 light-years from Andromeda’s center. That ring, once one of the galaxy’s active stellar nurseries, has also slowed.
The researchers do not think Andromeda simply lost all the raw material for new stars. Instead, the galaxy may be settling after a more active period.
“It’s just like after running a marathon, sometimes you’ve got to take a bit of a breather,” said Tobin Wainer, the study’s lead author and a University of Washington graduate student.
A pattern stood out. The slowdown was not evenly spread across Andromeda. It appeared especially strong on the side closest to Messier 32 (M32), a compact satellite galaxy about 16,000 light-years away in the plane of the sky.
M32 looks like the stripped core of a former spiral galaxy—as if a larger neighbor tore away its outer parts long ago. Astronomers have suspected for years that M32 and Andromeda may have interacted. The new timeline adds another reason to look closely.
“One of the major motivations for this program was to probe potential interactions between M32 and Andromeda’s disk,” Zhuo Chen, a co-author at the University of Washington, said in NASA’s release.
The researchers found that the area of Andromeda near M32 showed reduced star formation compared with other regions. The drop there appears to have begun about 60 million years ago.
That does not prove M32 caused the slowdown. Astronomers still do not know M32’s exact three-dimensional position relative to Andromeda, or the precise path it may have taken in the past.
“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,” Wainer added.
The study also shows the lasting power of Hubble’s archive. The Andromeda surveys took more than a decade and more than 1,000 Hubble orbits, producing a giant mosaic assembled from hundreds of overlapping fields of view.
“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,” said Raja GuhaThakurta, a co-author at the University of California, Santa Cruz.
The next leap may come from NASA’s Nancy Grace Roman Space Telescope, scheduled to launch as early as Aug. 30, 2026. Roman will see a much wider patch of sky than Hubble in a single observation—at least 100 times more area at near-infrared wavelengths, according to NASA.
A newly approved Roman program will image Andromeda’s entire disk and parts of its halo. That survey could add hundreds of millions of more stars to the record.
For now, Hubble has shown that the galaxy next door is changing fast by cosmic standards. Andromeda still shines with a trillion stars. But its stellar nurseries are dimming, and astronomers may have caught the moment when a giant spiral began to slow down.
The study was published in The Astrophysical Journal.

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© 2007-2025 ZME Science – Not exactly rocket science. All Rights Reserved.