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An accretion disk(disc) is an area, usually orbiting a young star, that is full of debris collected from the surrounding space. Protostars, neutron stars, white dwarfs, or black holes are the main bodies that have accretion discs.
Gravity causes material in the disc to spiral toward the central body and compresses the material. The compression causes the material to emit electromagnetic radiation. The frequency range of that radiation depends on the central object. Young stars and protostars have a disc that will radiate in the infrared and the discs around neutron stars and black holes radiate x-rays.
An accretion disk is fairly common in astrophysics: gamma ray bursts, active galactic nuclei, and protoplanetary discs all have them. These discs often cause jets coming from the vicinity of the central object. These jets are an easy way for the system to release angular momentum without losing much mass.
The most spectacular discs surround active galactic nuclei and quasars(supermassive black holes). When matter spirals into a black hole, it causes intense heating from friction. So, the accretion disc of a black hole is hot enough to emit x-rays from just outside of the event horizon. The luminosity of a quasar is believed to be caused by gas being accreted by supermassive black holes. This process can convert up to 10 percent of its mass into energy. Compare that to the 0.5 percent for used by the nuclear fusion created in a bomb.
In the case of a close binary system, the more massive primary component will evolve faster than the companion star. When the primary has become a white dwarf, a neutron star, or a black hole and the less massive companion reaches the giant state and exceeds its Roche lobe(area around a star where material is gravitationally bound), a gas flow will develop between the primary and companion. The primary will be pulling gas from the companion star. Angular momentum conservation prevents a straight flow from one star to the other, so an accretion disc forms instead.
Accretion discs surrounding pre-main sequence stars(T Tauri or Herbig stars) are called protoplanetary discs since scientists think that they are the ancestors, or parents if you will, of planetary systems. The gas disc in this example comes from the molecular cloud from which the star has formed.
Each accretion disk, and the process of accretion, have contributed to the mass of many celestial bodies. None of the heavenly bodies would have ever formed without them.
We have written many articles about the accretion disk for Universe Today. Here’s an article about Accretion, and here’s an article about the forming planets around binary stars.
If you’d like more information on accretion disk, check out NASA Images Website for more illustrations, and here’s a link to the NASA’s Imaging the Universe Homepage.
We’ve also recorded an episode of Astronomy Cast just about Black Holes. Listen here, Episode 18: Black Holes, Big and Small.
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