An accretion disk is the flattened, swirling structure of gas and dust that forms as material spirals inward toward a compact object such as a black hole, neutron star, or young star. Friction within the disk heats the infalling matter to millions of degrees, making the innermost regions blaze across the electromagnetic spectrum. Quasars, the most luminous steady objects in the universe, are powered by accretion disks around supermassive black holes. The same physics, at gentler scale, builds planets inside the disks around newborn stars.