The process whereby the mass of an astronomical body grows as a result of the accumulation of matter. Accretion is most important in systems where matter is falling in under the influence of gravity; it allows a massive body to gain more mass from other matter in its surroundings. If the infalling matter has significant[[ angular momentum]] an [[acretion disc]] is formed. The [[luminosity]] of various types of astronomical object, including [[X-ray binary]] and [[active galactic nuclei]], is ultimately the result of accretion. [[../Chapters/Topic 3 Mapping the Universe]] [[Docent/S284 Astronomy]]