A Hubble analysis of about 200 million stars shows Andromeda has sharply reduced its star-making rate over the past 500 million years, while NASA's Roman telescope prepares to map the galaxy more completely
Andromeda, or M31, is moving toward the Milky Way while most other major galaxies recede with cosmic expansion. A new analysis of Hubble Space Telescope observations now indicates that this nearby spiral galaxy is also entering a quieter phase: its production of new stars has been declining for hundreds of millions of years.
A fading stellar nursery
The result does not show that Andromeda has suddenly run out of star-forming material. Instead, the evidence points to a gradual reduction in activity after a much more productive period. That makes Andromeda a useful case study for how massive spiral galaxies change after major episodes of growth.
Andromeda is the nearest large spiral galaxy to Earth, at roughly 2.5 million light-years away, and has about twice the mass of the Milky Way. Its relative proximity allows Hubble to distinguish individual stars across a substantial portion of the galaxy's disk, something that is not possible for most large galaxies beyond the Local Group.
What Hubble measured
The study, published July 27, 2026, in The Astrophysical Journal, combines two major Hubble surveys covering approximately two-thirds of Andromeda's disk and about 200 million stars. Rather than treating the galaxy as a single unresolved point of light, the researchers used the ages, locations and properties of individual stars to reconstruct its recent star-formation history.
The measured trend is substantial. About 500 million years ago, Andromeda formed stars at a rate of roughly one solar mass per year. By 40 million years ago, the rate had fallen by about half. The current estimate is approximately 0.2 solar masses per year. Most of the recent activity is concentrated in a ring about 32,000 light-years from the galaxy's center, and the weakening of star formation in that ring accounts for much of the overall decline.
These values are estimates derived from stellar populations, not direct readings from a galaxy-wide star-formation meter. Young, massive stars provide evidence of comparatively recent activity, while older stars preserve earlier stages of the galaxy's history. In that sense, the resolved stellar population acts as a record of when and where Andromeda made its stars.
The merger question
Andromeda is already known to have undergone a major burst of star formation about 2 billion years ago. That event may have produced around one-fifth of the galaxy's stars and is widely associated with a past merger, possibly involving the compact elliptical galaxy M32, which now orbits Andromeda.
The new analysis is consistent with Andromeda gradually settling after that earlier disturbance, but it does not by itself establish that M32 caused the recent slowdown. Regions nearest to M32 show a more recent decline in star formation than some other parts of the disk, making gravitational interaction a plausible influence. The evidence remains inconclusive, however, because galactic star formation can also be shaped by gas distribution, stellar feedback and the longer-term consequences of earlier mergers.
This interpretation differs from the idea that the galaxy has abruptly become inactive. Andromeda still forms stars, but at a much lower rate than it did in the recent past. The distinction matters because galaxy evolution is often controlled by changes in the availability and structure of cold gas, rather than by a simple on-or-off process.
What Roman may add
Hubble's archive will remain important, but NASA's Nancy Grace Roman Space Telescope is expected to provide a wider view of the system. Roman's field of view will be at least 100 times larger than Hubble's, allowing it to survey the full Andromeda galaxy and much of its surrounding halo. Its planned launch on a SpaceX Falcon Heavy rocket is scheduled for as soon as Aug. 30, 2026; that date is a target, not a guarantee of launch or subsequent science operations.
Roman will complement, rather than simply replace, Hubble. A broader field can connect the inner disk to the galaxy's outer regions and halo, while high-resolution observations remain essential for separating individual stars. Similar observations of nearby galaxies, including the detailed infrared view of dusty structures in a nearby active galaxy mapped by the Webb telescope, show why different wavelength ranges and instruments reveal different parts of galactic evolution.
Andromeda's eventual interaction with the Milky Way is another reason its history matters. A collision may occur in about 5 billion years, although the final trajectory remains uncertain. The new Hubble result does not predict the outcome of that encounter; it instead provides a clearer record of the galaxy's present state and recent decline in star formation.
In astronomical images, a galaxy's apparent brightness is not a direct measurement of its current star-formation rate. Star formation is inferred by comparing the light from stars of different ages and by modeling how dust, stellar evolution and past activity affect that light. Those models cannot recover every detail of Andromeda's history, but the large resolved sample makes the broad decline more robust than an estimate based on a single integrated brightness measurement.
