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Accretion DiscAn accretion disc (or 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 disc, and tidal effects will tend to align this disc's rotation with the rotation of the gravitational source in the middle. Viscosity within the disc generates heat and saps orbital momentum, causing material in the disc to spiral inward until it impacts in an accretion shock on the central body if the body is a star, or slips toward the event horizon if the central body is a black hole. The most spectacular accretion discs found in nature are those of active galactic nuclei and quasars, which are believed to be massive black holes at the center of galaxies. As matter spirals into a black hole, the intense gravitational gradient gives rise to intense frictional heating; the accretion disc of a black hole is hot enough to emit x-rays just outside of the event horizon. Often, in binary systems with one black hole, observations show matter being pulled from the visible star when it exceeds its roche limit and falling into the black hole's accretion disc. The largest and most voracious black holes known are those which form the cores of quasars, whose accretion discs emit more radiation than entire galaxies of stars. Protoplanetary discs are referred to as accretion disks when viewed as material falling into the central protostar. See also References - Accretion power in astrophysics, Frank, King, Raine, Cambridge University Press, 1985
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