Accretion discAn accretion disk is a structure formed by material falling into a gravitational source. Conservation of angular momentum requires that, as a large cloud of material collapses inward, any small rotation it may have will increase. Centrifugal force causes the rotating cloud to collapse into a disk, and tidal effects will tend to align this disk's rotation with the rotation of the gravitational source in the center. Friction between the particles of the disk generates heat and saps orbital momentum, causing material in the disk to spiral inward until it impacts on the central body.
The most spectacular accretion disks found in nature are those of black holes. As matter spirals into a black hole, the intense gravitational gradient gives rise to intense frictional heating; the accretion disk of a black hole is hot enough to emit x-rays just outside of the event horizon. Matter is crushed by the black hole's tremendous tidal forces just before the matter crosses the event horizon and is lost to the black hole. An accretion disk can be detected by observation because the matter is crushed and stretched with such violence that X-ray radiation is emitted from the disk. Often, in binary systems with one black hole, observations show matter being pulled from the visible star into the black hole's accretion disk. The largest and most voracious black holes known are those which form the cores of quasars, whose accretion disks emit more radiation than entire galaxies of stars.