Andromeda Galaxy Is Slowly Winding Down Its Star Formation, Hubble Study Finds

The Andromeda galaxy is the only nearby major galaxy moving toward the Milky Way instead of receding with the expansion of the universe. Now, a new analysis of data from the Hubble Space Telescope suggests that our galactic neighbor is gradually slowing its production of new stars as it moves toward a possible future collision with the Milky Way.

The study found that Andromeda, also known as M31, has steadily reduced its star formation rate during the past 500 million years. The findings indicate that the giant spiral galaxy may be naturally winding down after a much more active period, giving astronomers new insight into how large galaxies form, evolve and eventually become less active.

Andromeda is a giant spiral galaxy similar to the Milky Way, but it contains roughly twice as much mass. As the closest large spiral galaxy to Earth, Andromeda offers scientists an exceptional opportunity to study galaxy evolution. Understanding where and when stars are born helps researchers explain how galaxies change over billions of years.

Published July 27 in The Astrophysical Journal, the research combines two major Hubble surveys covering approximately two-thirds of Andromeda’s disk and around 200 million individual stars.

Astronomers already knew that Andromeda experienced a powerful burst of star formation roughly 2 billion years ago. That event may have produced about one-fifth of the galaxy’s stars and is thought to have been triggered by a merger with another galaxy, possibly the compact elliptical galaxy M32, which now orbits Andromeda.

The new analysis shows that Andromeda formed stars at a rate of approximately one solar mass per year 500 million years ago. About 40 million years ago, that rate had fallen by half. Today, the galaxy forms only around one-fifth of a solar mass in new stars each year.

Most of Andromeda’s recent star formation has occurred in a ring approximately 32,000 light-years from the galaxy’s center. The decline in activity within this region appears to account for much of the overall slowdown. This pattern suggests that Andromeda is gradually settling down after its earlier starburst rather than suddenly exhausting its supply of star-forming material.

Researchers also examined whether M32 could have influenced Andromeda’s declining star formation. Areas closest to M32 show a more recent decrease in stellar activity, suggesting that the smaller companion galaxy may have played a role. However, the evidence is not yet conclusive.

“We need to measure the individual stars because they are the fossil record of the galaxy’s formation,” study co-author Ben Williams, an astronomer at the University of Washington, said in a NASA statement. “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.”

The research team plans to continue examining Hubble’s extensive archive while also using observations from NASA’s Nancy Grace Roman Space Telescope. Scheduled to launch on a SpaceX Falcon Heavy rocket as soon as Aug. 30, Roman will have a field of view at least 100 times larger than Hubble’s. This will allow scientists to survey the entire Andromeda galaxy and its surrounding halo in unprecedented detail.

The Andromeda galaxy is easiest to observe in November and can be seen with the naked eye from locations with moderate light pollution. Although Andromeda and the Milky Way may collide in approximately 5 billion years, astronomers say the galaxy’s ultimate trajectory remains uncertain.