Astronomers using NASA's Hubble Space Telescope have reconstructed 500 million years of star formation in the Andromeda galaxy, finding a long decline that became markedly steeper during the past 40 million years. Phys.org reported the result within the past 24 hours, following publication of the research in The Astrophysical Journal and a NASA account of the study.
The team combined the Panchromatic Hubble Andromeda Treasury and its southern counterpart, surveys that together cover about two-thirds of Andromeda's disk. Researchers measured roughly 200 million individual stars and divided the images into thousands of parcels about 300 light-years wide. Because massive young stars appear blue and die quickly while lower-mass stars remain redder and live longer, each parcel preserves evidence of when stars formed.
NASA said Andromeda formed stars at about one solar mass per year 500 million years ago. That rate had fallen by half around 40 million years ago and is now about one-fifth of a solar mass annually. The paper calculated a mean rate of 0.445 solar masses per year over the last 100 million years in the surveyed footprint and 0.285 over the last 20 million years, showing that commonly used ultraviolet indicators can miss a rapidly changing trend.
Much of the recent activity, and therefore much of the decline, lies in a star-forming ring about 32,000 light-years from Andromeda's center. The region nearest the satellite galaxy M32 shows an especially pronounced reduction beginning roughly 60 million years ago. The researchers said M32 is the leading suspect for influencing that area, but its three-dimensional position remains uncertain and the data do not establish that an encounter caused the slowdown.
Andromeda lies about 2.5 million light-years away and offers the nearest large spiral comparison with the Milky Way. The team interprets the decline as the late stage of a natural wind-down after a burst of star formation around two billion years ago, rather than proof that the galaxy has run out of raw material. Further Hubble and ground-based analysis is planned, while an approved Nancy Grace Roman Space Telescope program is expected to image Andromeda's full disk and halo and measure hundreds of millions more stars.
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