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.
Red giants are swollen, aging stars. Deep inside, an ocean of plasma churns: heat forces its way from the core to the surface, generating waves like a mighty surf. The star's magnetic field acts like an invisible seabed – it steers these waves, changing their height and speed.