accretion

Accretion is the drawing of matter into an astronomical body by gravity, e.g., a star drawing matter, such as a binary star drawing matter from its companion. Black holes also accrete matter, and in both cases, the accretion can produce observable phenomena. Accretion by developing planets is considered a key part of planet formation. Accreting material often forms a disk (accretion disk) consisting of material orbiting the body as it gradually approaches, formed by the material's angular momentum in relation to the accreting body. An accreting body's accretion rate (mass per unit time) is generally of interest.

Accretion can produce heat, transformed from the potential energy due to the gravitational force between the accreting matter and the body to which it is falling, and the rate in which energy is so-transformed is termed the accretion luminosity. A maximum accretion rate (Eddington accretion rate) can be calculated by assuming that black-body radiation from the accretion-generated heat creates radiation pressure against the fall of the accreting matter, and assuming the phenomena is spherically symmetric and steady-state. Higher accretion (super-Eddington accretion) can occur if not all these assumptions apply, and may be relevant to some apparently very bright sources.

Runaway accretion (aka accretion runaway) refers to situations where some usual mechanism for regulating accretion doesn't occur or is nullified and the accretion rate increases until some other mechanism limits it. In some cases, the limiting factor could be the eventual absence of material near enough to accrete. These concepts are used regarding a number of phenomena including planet formation (e.g., gas giants), star formation, and some stages in the evolution of stars.

Pebble accretion, associated with planet-formation theories, consists of a planetesimal accreting objects on the order of a centimeter to a meter in size (pebbles) within a protoplanetary disk.

The term accretion has some uses in meteorology, one use being the formation of raindrops and other forms of precipitation; this may include water molecules joining an existing drop, somewhat analogous to the astrophysical usage, but the process is not due to their gravitational attraction.