Accretion disc

An accretion disc (or accretion disk) is a structure formed by diffuse material in orbital motion around a central body. The central body is typically either a young star, a protostar, a white dwarf, a neutron star, or a black hole. Instabilities within the disc redistribute angular momentum, causing material in the disc to spiral inward towards the central body. Gravitational energy released in that process is transformed into heat and emitted at the disk surface 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.
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A disc-like structure of dust and gas which forms around the massive object at the centre of a region undergoing gravitational collapse. It is caused because the regions of collapse are not stationary and move through space. Specifically they are in orbit around the centre of the galaxy. Thus, they have angular momentum and as they fall into the potential well of the massive object, instead of falling straight onto the central body, they spiral downwards. This causes the material to build up in a disc around the object. The more mass contained by the object the deeper its potential well and the faster the material in the accretion disc will swirl around it. Thus, the more heating the disc will suffer and the more energy it will release in the form of electromagnetic radiation. Accretion discs are, primarily, found around protostars and black holes.