Space & Physics

NASA’s Hubble Shows Star Formation in Andromeda Galaxy Winding Down

NASA’s Hubble finds that star formation in the Andromeda galaxy has undergone a 500-million-year decline.

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Jul 27, 2026 - 22:00

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NASA’s Hubble Shows Star Formation in Andromeda Galaxy Winding Down

The left half of the disk of the Andromeda galaxy, with a jagged outer edge at the top and bottom on a black background. At right, the galaxy’s center is a horizontal yellow oval extending across about a third of the image. Two crescent shaped clumpy, dark dust lanes with abundant stars throughout extend out from the center. The galaxy becomes bluer as you move outward. At top middle, the satellite galaxy M32 appears as a small, fuzzy yellow oval. Within the disk of Andromeda, two regions at upper right and lower middle are outlined in small white boxes labeled 1 and 2, respectively. At the left side of the image, those two regions are enlarged in pullouts. Both contain a dense, grainy-appearing field of thousands of stars. Region 1 is noticeably bluer than region 2 and contains blue star-forming clumps.

NASA Hubble’s view of part of the Andromeda galaxy.

Credits:
Image: NASA, ESA, Benjamin Williams (UWashington), Zhuo Chen (UWashington), L. Clifton Johnson (Northwestern); Image Processing: Joseph DePasquale (STScI)

A new study using data from NASA’s Hubble Space Telescope finds that star formation in the nearby Andromeda galaxy has undergone a 500-million-year decline, with an even steeper drop in the last 40 million years. Andromeda, a spiral galaxy comparable in size to our Milky Way, is close enough to be seen with the unaided eye from areas with dark skies. Located about 2.5 million light-years from Earth, practically our cosmic backyard, Andromeda offers an opportunity for astronomers to examine its stellar populations in detail, leading to better understanding of the past of galaxies like our own.

The results published Monday in The Astrophysical Journal.

To reach this conclusion, the researchers combined data from two Hubble surveys: the Panchromatic Hubble Andromeda Treasury and the Panchromatic Hubble Andromeda Southern Treasury. Together, these two surveys mapped two-thirds of the disk of Andromeda in ultra-sharp detail. In total, the team measured about 200 million individual stars across the galaxy, giving them a detailed picture of Andromeda’s past activity.

“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,” said Ben Williams, astronomer at the University of Washington and a co-author on the study.

Massive stars are bluer and short-lived, while less massive stars are redder and longer-lived. As a result, areas that have experienced recent star formation tend to have a larger fraction of blue stars, while areas with less recent star formation typically have a redder population. The team divided the Andromeda images into thousands of squares, spanning 300 light-years on each side of the square, and determined the history of star formation within each parcel to gain a comprehensive view of the galaxy’s past.

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NASA’s Hubble Space Telescope has provided a detailed view of millions of stars in the Andromeda galaxy. Regions that have experienced recent star formation (1) appear significantly bluer than regions with less recent star formation (2).

NASA, ESA, Benjamin Williams (UWashington), Zhuo Chen (UWashington), L. Clifton Johnson (Northwestern); Image Processing: Joseph DePasquale (STScI)

Steady decline

Previous research showed the Andromeda galaxy experienced a dramatic burst of star formation about 2 billion years ago, likely due to a past interaction or merger with another galaxy. Since that time, star formation has been steadily declining.

Astronomers measure the rate of star formation in terms of the mass, or amount of gas and dust, converted into stars per year. The researchers calculated that, 500 million years ago, Andromeda formed stars at a rate of about one solar mass per year. However, the formation rate dropped to about half that by 40 million years ago. The current rate has plummeted even farther, to about one-fifth the mass of our Sun per year.

The team also examined whether that decline was consistent across the galaxy or concentrated in certain areas. They found that much of the recent star formation has occurred in a star-forming ring located about 32,000 light-years from the galaxy’s center. As a result, much of the decline they measure is driven by decreasing activity within that ring.

Rather than being the result of reduced material from which new stars can form, the decline is more likely to be a natural winding down from its previous, more active state.

“It’s just like after running a marathon, sometimes you’ve got to take a bit of a breather,” said Tobin Wainer, lead author, University of Washington.

Likely suspect

The team also investigated whether there was any connection between the decrease of activity in Andromeda and its proximity to satellite galaxy M32 (Messier 32). The M32 galaxy is separated from Andromeda by about 16,000 light-years in the plane of the sky; however, its 3D location in space is uncertain. As a result, astronomers are unsure if or when it might have interacted with Andromeda in the past.

“One of the major motivations for this program was to probe potential interactions between M32 and Andromeda’s disk,” said Zhuo Chen, co-author, University of Washington.

Survey data from the Panchromatic Hubble Andromeda Southern Treasury allowed the team to study the history of star formation in Andromeda near M32. They found that this area showed signs of decreased star formation compared with other regions. The timing of this decrease, which this study finds began roughly 60 million years ago, could help constrain when the M32 galaxy interacted with Andromeda’s disk.

“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,” said Wainer.

This movie, created from an analysis of data from NASA’s Hubble Space Telescope, demonstrates how the rate of star formation in the Andromeda galaxy has declined over the past 500 million years. Brighter colors indicate where star formation was highest during a given time.

Visualization: Tobin Wainer (UWashington); Image Processing: Joseph DePasquale (STScI); Video: NASA, ESA, STScI, Gregory Bacon (STScI)

The team plans to continue analyzing the Hubble data and combine it with data from ground-based observatories to gain additional insights into the history of Andromeda.

“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, co-author, University of California Santa Cruz.

An even greater global perspective is likely to come from NASA’s Nancy Grace Roman Space Telescope after it launches as early as Sunday, Aug. 30. Roman’s gigantic field of view can cover at least 100 times as much area as Hubble at near-infrared wavelengths in a single observation. A newly approved Roman observing program will image the entirety of Andromeda’s disk and areas of its surrounding halo, allowing astronomers to measure hundreds of millions of stars and enabling groundbreaking new science.

The Hubble Space Telescope has been operating for more than three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.

Related Images & Videos

Andromeda Galaxy

NASA’s Hubble Space Telescope has provided a detailed view of millions of stars in the Andromeda galaxy.

On a black background, a pixelated, partial ring of bright pink and purple extends from 8 ou2019clock clockwise to 3 ou2019clock. There is a black gap at the bottom of the ring as well as in the middle. Below the ring, a timeline labeled millions of years ago lists the numbers 500, 200, 100, 50, 25, 0. The numbers 500 and 200 are yellow with a yellow line between them, indicating that the image shows the rate of star formation that was occurring in that timeframe.

Star Formation Rate in Andromeda Over Time

This movie, created from an analysis of data from NASA’s Hubble Space Telescope, demonstrates how the rate of star formation in the Andromeda galaxy has declined over the past 500 million years.