Accretion disk

A rotating disk of matter that spirals inward under gravity and releases energy as it settles onto a central object. Friction and magnetic stresses transport angular momentum outward, allowing matter to move inward.

Angular momentum transport: The outward redistribution of angular momentum that allows matter in a rotating system to move inward. Disk material cannot accrete efficiently until stresses carry its angular momentum outward.
Angular momentum: A conserved physical quantity measuring rotational motion about an axis or point. Its conservation keeps infalling material orbiting rather than plunging directly into the central object.
Protoplanetary disk: A disk of gas and dust surrounding a young star, from which planets can form. Dust grains and gas in these disks accrete onto the star while assembling planetary systems.
Bondi accretion: Spherically symmetric accretion of gas onto an object from a surrounding medium. Unlike a rotationally supported disk, the idealized Bondi flow falls inward from all directions.
Disk wind: An outflow of gas launched from the surface or outer regions of an accretion disk. Winds can remove mass and angular momentum, changing the rate of accretion.
Magnetorotational instability: An instability in differentially rotating, weakly magnetized fluids that amplifies turbulence and magnetic stresses. It provides a major mechanism for transporting angular momentum through ionized disks.
Keplerian orbit: An orbit governed primarily by the gravity of a central mass, following Kepler’s laws. Many disks rotate approximately at the orbital speed set by the central object's gravity.
X-ray binary: A binary star system in which a compact object accretes matter from a companion and emits X-rays. Its bright accretion disk converts gravitational energy into observable X-ray radiation.
Advection-dominated accretion flow: A hot, optically thin accretion flow in which much of the dissipated heat moves inward with the gas. It contrasts with thin disks, which radiate most of their dissipated energy locally.
Accretion disk instability: A change in disk structure that can trigger strong variations in accretion rate and luminosity. Thermal and ionization instabilities help explain recurring outbursts in some accreting systems.

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