Accretion disk
An accretion disk is a structure (often a circumstellar disk) formed by diffused material in orbital motion around a massive central body. The central body is typically a star. Gravity causes material in the disk to spiral inward towards the central body. Gravitational and frictional forces compress and raise the temperature of the material causing the emission of electromagnetic radiation. The frequency range of that radiation depends on the central object's mass. Accretion disks of young stars and protostars radiate in the infrared; those around neutron stars and black holes in the X-ray part of the spectrum. The study of oscillation modes in accretion disks is referred to as diskoseismology.
Manifestations
Accretion disks are a ubiquitous phenomenon in astrophysics; active galactic nuclei, protoplanetary disks, and gamma ray bursts all involve accretion disks. These disks very often give rise to astrophysical jets coming from the vicinity of the central object. Jets are an efficient way for the star-disk system to shed angular momentum without losing too much mass.
