Our closest large neighbor – the spiral galaxy Andromeda, visible to the naked eye in dark skies – is gradually losing its stellar activity. A new study based on data from the Hubble Space Telescope has shown that the process of new star formation within it has sharply slowed down over the last half-billion years, and in the last 40 million years, the decline has become even steeper.
Astronomers combined the results of two large-scale Hubble surveys – Panchromatic Hubble Andromeda Treasury and Panchromatic Hubble Andromeda Southern Treasury. Together, they covered about two-thirds of the galaxy's disk with incredible detail. As a result, scientists measured the properties of approximately 200 million individual stars – a true fossil record of the galaxy. The color and brightness of stars can determine when they were born: massive blue stars live short lives, while red dwarfs live much longer. By dividing the images into thousands of patches about 300 light-years in size, researchers reconstructed the star formation history in each of them.
The picture turned out quite clear. About two billion years ago, Andromeda experienced a powerful surge in star birth – likely due to interaction or merger with another galaxy. After that, activity began to gradually decrease. 500 million years ago, the galaxy was still forming about one solar mass of stars per year. By 40 million years ago, this figure had halved, and now it is only about one-fifth of the Sun's mass per year. The fading is particularly noticeable in a ring of active star formation approximately 32,000 light-years from the center.
Interestingly, this does not look like simple fuel burnout. Rather, the galaxy is simply resting after a previous stormy phase, like a marathon runner after a distance. Scientists also checked for a possible connection with the nearby satellite M32, which projects at a distance of about 16,000 light-years from Andromeda's disk. In the areas near M32, star formation decreased earlier – about 60 million years ago. This makes the satellite the prime suspect for local influence, although the precise three-dimensional position of M32 is still unclear, and there is no direct evidence of interaction yet.
The visualization, created from Hubble data, clearly shows this fading: bright colors, which marked zones of active star birth, have gradually dimmed over the last half-billion years.
Researchers emphasize the value of Hubble's archival data: Andromeda is close enough (only 2.5 million light-years) to study its stars individually, and at the same time provides insight into the typical evolution of galaxies like our Milky Way. In the future, data from ground-based observatories will be included in the analysis, and NASA's Roman telescope, which will be able to cover the entire galaxy, promises even more prospects.
For now, Andromeda's story reminds us that even huge galaxies have their life cycles – periods of turbulent youth and quieter maturity. And Hubble continues to help us read this ancient chronicle.
