Timothy R. Bedding

A star trembles like a bell after being struck. These tremors, tiny fluctuations in brightness and surface, can be caught by a telescope. From the frequencies of these 'stellar notes,' astronomers infer what's happening inside: how old the star is, how it's structured.

31 Aquilae is an aging star that has puffed up into a subgiant. It once resembled the Sun, but has since cooled and grown larger. It rotates very slowly — one full turn takes many days.

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%.

A red giant is a star that has swelled up, cooled on the outside, and become hundreds of times larger than the Sun. Inside, it's far from calm: layers of gas oscillate like a giant bell, causing its brightness to flicker slightly. Astronomers pick up these oscillations to probe what's happening deep within.