Accretion disk

An accretion disk (or disc) is an astronomical term. It describes a disc of material being drawn in by gravity to a centre of mass, such as a star or black hole. As the material comes in, it spins round the central body and radiates energy.

The central body is typically either a young star, a protostar, a white dwarf, a neutron star, or a black hole. As the material moves in, gravitational energy released in that process is transformed into heat and rotational movement. 

The energy emitted at the disk surface is in form of electromagnetic radiation. The frequency range of that radiation depends on the central object. Accretion discs of young stars and protostars radiate in the infrared, those around neutron stars and black  holes in the X-ray part of the spectrum.

Other websites
 Professor John F. Hawley homepage 
 Nonradiative Black Hole Accretion 
 Accretion Discs on Scholarpedia
 Magnetic fields snare black holes' food - New Scientist

Category:Astrophysics
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Category:Black holes