accretion

Accretion is the process by which material is gathered onto a central body under gravity, typically involving the inward flow of gas or dust and the redistribution of angular momentum. In many astrophysical contexts, accretion occurs through a rotating disk: gas and solids lose angular momentum and spiral inward, heating the disk and emitting radiation as a byproduct of accretion energy.

In stars and compact objects, accretion often involves gas drawn from a surrounding reservoir or a companion star. Bondi accretion describes spherical inflow onto a point mass, while in many systems the inflowing material forms an accretion disk due to angular momentum. Viscous or magnetohydrodynamic stresses transport angular momentum outward, allowing matter to move inward. Accretion can drive a wide range of phenomena, from protostellar growth to active galactic nuclei.

Planet formation proceeds in part by accretion: microscopic dust grains collide and stick, growing into kilometer-sized planetesimals and eventually protoplanets. Once a sufficiently large core forms, it can accrete gas from the protoplanetary disk, forming a gas giant if the disk remains available long enough. Accretion also occurs in binary star systems, where material is transferred from one star to its companion, often via Roche lobe overflow and accretion disks.

Around compact objects such as white dwarfs, neutron stars, and black holes, accretion disks can emit strongly across the electromagnetic spectrum, especially in X-rays. The luminosity is limited by the Eddington rate for a given mass, and the efficiency depends on the accretion geometry and the central object’s properties. Accretion is a unifying mechanism for growth and energy release across many scales in the universe.