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.