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Using the data from the most detailed Hubble Space Telescope image of the nearby Andromeda galaxy (shown above), astronomers have mapped out the galaxy’s stars in a way that shows the likely star formation rate over the past 500 million years, and discovered that the rate has been dropping steadily, with a sudden decline about 40 million years ago.

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.

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 research has also pinpointed the location of this on-going star formation, within a dusty ring about 32,000 light years from Andromeda’s center.

A side note for anyone wishing to star-gaze: Andromeda is actually the largest visible galaxy in the night sky, about six times the width of the full Moon. If you can get to a very dark-sky location, get your eyes very dark-adapted, and if you know where to look, you can actually see it with the naked eye. I did this once at a star party, helped by a bunch of amateur astronomers. The galaxy is very faint, and it helps to use binoculars to help locate it, but once identified its size on the sky truly is breath-taking. It is BIG.