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Accretion disk

An accretion disk is a structure (often a circumstellar disk) formed by diffuse material[a] in orbital motion around a massive central body. The central body is most frequently a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other forces induce instabilities causing orbiting material in the disk to spiral inward toward the central body. Gravitational and frictional forces compress and raise the temperature of the material, causing the emission of electromagnetic radiation. The frequency range of that radiation depends on the central object's mass. Accretion disks of young stars and protostars radiate in the infrared; those around neutron stars and black holes in the X-ray part of the spectrum. The study of oscillation modes in accretion disks is referred to as diskoseismology.[1][2]

Excretion disk[edit]

The opposite of an accretion disk is an excretion disk where instead of material accreting from a disk on to a central object, material is excreted from the center outward onto the disk. Excretion disks are formed when stars merge.[33]

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Professor John F. Hawley homepage

John F. Hawley and Steven A. Balbus, 2002 March 19, The Astrophysical Journal, 573:738-748, 2002 July 10

The Dynamical Structure of Nonradiative Black Hole Accretion Flows

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Accretion discs

Merali, Zeeya (21 June 2006). . New Scientist.

"Magnetic fields snare black holes' food"