accretion disk

An accretion disk is material around a star or other massive astronomical body distributed in the shape of a disk centered on the body, spiraling in toward the body, attracted by gravity. Accretion disks are associated with interacting binary stars, with protostars (protoplanetary disks often include accretion) and with black holes (black hole accretion disk or BHAD), including supermassive black holes.

Disks form because material drawn toward a body by gravity generally began with some motion sideways to the body, and thus doesn't fall straight into it. Material consisting of interacting objects, such as a gas, tend to form general patterns of movement, and swirl around the body together; pressure arresting immediate accretion and its own gravity drawing it into a flat, rotating disk, orbiting the body. Accretion occurs, but slowly, as the material accreting at the inner edge leaves a pressure deficit, drawing further disk material toward the body. Once such a disk forms, any additional material arriving from elsewhere is often drawn into the disk pattern, replenishing it.

Characteristics such as the accreting body's temperature and electromagnetic radiation affect the form of the disk, and models such as the alpha disk model and advection dominated accretion flow (ADAF) model have been developed to describe different scenarios.