Accretion disc

node

The flattened, orbiting, inspiralling structure that forms when gas with angular momentum falls toward a compact object, and the source of essentially everything ever observed about black holes before 2015. Material cannot fall straight in: it must shed angular momentum first, and the mechanism is friction between adjacent annuli orbiting at different speeds. Shakura and Sunyaev parameterised that friction in 1973 with a single number, the alpha viscosity, without specifying its physical origin; the origin is now generally attributed to the magnetorotational instability, in which a weak magnetic field threading differentially rotating plasma becomes unstable and transports angular momentum outward. The disc heats as it drains, radiating from infrared to X-rays with the hottest material innermost, and terminates at the innermost stable circular orbit. Quasars are this process around supermassive holes; X-ray binaries are the same process around stellar-mass ones. Not a place on Earth.

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