The night sky looks still, but many stars tremble like a drum after a strike. For seven straight years, the TESS satellite stared at the same patches of sky and caught this trembling in 19,151 red giants — old, bloated stars. Previously, far fewer of these 'singing' giants were known, and now the list has grown by 80%.
Each star pulsates in its own rhythm: the frequency of the strongest 'beat' and the spacing between neighboring beats depend on mass and size. The team measured these two numbers with an accuracy of 1.5%. Then a neural network split the stars into two groups: those still growing by burning hydrogen in a shell, and those that have already started burning helium in their cores, like a stove that has switched from wood to coal. For 10,298 stars, chemical composition data were also available, allowing the team to calculate mass and radius with accuracies of 7.5% and 2.8%—no worse than the famous Kepler telescope achieved in four years.
Such a large and homogeneous catalog is a gift for 'galactic archaeology.' The pulsations of old stars can reveal how the Milky Way grew and changed over time. Just as tree rings tell a tree's history, here the rhythm of stellar trembling tells the galaxy's story.
Red giants pulsate so slowly that a single 'beat' lasts from a few minutes to several hours. For comparison, our Sun oscillates with a period of about five minutes, but its trembling is thousands of times weaker.