Astronomers using Hubble Space Telescope data have reconstructed 500 million years of star formation in the Andromeda galaxy. Their results show a long decline that became steeper over the past 40 million years, according to research highlighted by NASA on July 27, 2026.
The finding does not mean that Andromeda has suddenly stopped making stars or is about to disappear. Star formation continues there, but at a lower rate than during the periods examined by the researchers.
Andromeda lies about 2.5 million light-years from Earth and is a large spiral galaxy comparable in size to the Milky Way. Its relative closeness allows astronomers to study individual stars while still examining much of a galaxy at once.
To build that wider view, the team combined observations from two Hubble surveys known as PHAT and PHAST. Together, they cover about two-thirds of Andromeda’s star-forming disk.
The combined observations allowed the researchers to measure roughly 200 million individual stars. For the detailed analysis, the images were divided into thousands of areas, each spanning about 300 light-years on a side.
Astronomers can use the brightness and colour of stars in each area to reconstruct when new stars formed. Hot, short-lived blue stars are especially useful signs of comparatively recent activity, while older populations preserve evidence from earlier periods.
The NASA summary estimates that Andromeda was forming stars at a rate equivalent to about one Sun’s mass each year 500 million years ago. That rate had fallen to roughly half as much by 40 million years ago and is now estimated at about one-fifth of a solar mass per year.
A solar mass is the mass of our Sun. In this context, it is a standard unit for describing how much gas and dust a galaxy converts into stars over time, rather than a count of newly formed stars.
The decline was not spread evenly across the galaxy. Much of Andromeda’s recent star formation has been concentrated in a ring located about 32,000 light-years from its centre.
The research indicates that falling activity within this ring accounts for much of the galaxy-wide slowdown. The paper similarly identifies Andromeda’s ring structures as the main drivers of the recent decline.
The researchers interpret the pattern as the late stage of a wind-down that has unfolded over billions of years. NASA’s account says the result is not explained simply by Andromeda running out of the raw material needed to form stars.
Earlier work found evidence of a much stronger burst of star formation about two billion years ago, possibly associated with an interaction or merger. The new measurements help show how activity has eased since that more energetic period.
One question is whether M32, a small companion galaxy near Andromeda, influenced part of the recent change. The sharp decline is particularly noticeable in the region closest to M32.
The study cannot establish that M32 caused the slowdown. Astronomers do not know its exact three-dimensional position relative to Andromeda, so the timing and nature of any past interaction remain uncertain.
The team plans to combine further analysis of the Hubble archive with observations from ground-based telescopes. A planned Nancy Grace Roman Space Telescope programme is also expected to examine Andromeda’s full disk and parts of its surrounding halo.
The wider value of the study lies in showing how individual stars can preserve a galaxy’s history. Andromeda gives astronomers an unusually detailed view of how a large spiral galaxy can gradually move from a more active period into a quieter one.
