Accretion (Latin Accretio " growth ", " increase " ) is in astronomy the name for a process in which a cosmic object matter due to its gravitational and tidal forces ( see Roche limit ) collects.
The aufsammelnde central object is called accretor or gravitating object. These are either ordinary stars or compact objects. These include the white dwarfs, neutron stars, stellar and supermassive black holes. The flow of matter is called in the accretion accretion.
The particles, such as atoms in molecular clouds, dust in a protoplanetary system or gas in a binary star system developed produce planetesimals, precursors of planets.
In young and old stars (eg T Tauri stars or X-ray binaries ) are often formed by the angular momentum of the system of accretion disks that affect the observable physical effects of these objects and are used for example as an explanation for bipolar currents.