A structure that forms around a compact object (e.g. a white dwarf, neutron star, or black hole) when matter flows towards it. Accretion disks are found in interacting binary stars, and are assumed to exist in active galactic nuclei and quasars. In a binary, mass lost from the secondary star can form a disk of gas around the compact object. The disk may have a hot spot (1) where the stream of material hits its outer edge. Material is fed from the inner edge of the disk through a boundary layer (which may radiate as much energy as the disk itself) on to the compact object. When the compact object has an extremely strong magnetic field, as in an AM Herculis star, the material may form an accretion column over each magnetic pole, rather than a disk. The gravitational energy that is released can cause high ultraviolet or X-ray luminosities and may accelerate jets of material from the disk to very high speeds.