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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. The most spectacular accretion disks found in nature are those of active galactic nuclei and of quasars, which are believed to be massive black holes at the center of galaxies. As matter follows the tendex line into a black hole, the intense gravitational gradient gives rise to intense frictional heating; the accretion disk of a black hole is hot enough to emit X-rays just outside of the event horizon. The large luminosity of quasars is believed to be a result of gas being accreted by supermassive black holes (Lynden-Bell, 1969).〔 〕 Elliptical accretion disks formed at tidal disruption of stars can be typical in galactic nuclei and quasars (Gurzadyan and Ozernoy, 1979). Accretion process can convert about 10 percent to over 40 percent of the mass of an object into energy as compared to around 0.7 percent for nuclear fusion processes.〔http://www3.mpifr-bonn.mpg.de/staff/mmassi/lezione2WEdd.pdf〕 In close binary systems the more massive primary component evolves faster and has already become a white dwarf, a neutron star, or a black hole, when the less massive companion reaches the giant state and exceeds its Roche lobe. A gas flow then develops from the companion star to the primary. Angular momentum conservation prevents a straight flow from one star to the other and an accretion disk forms instead. Accretion disks surrounding T Tauri stars or Herbig stars are called protoplanetary disks because they are thought to be the progenitors of planetary systems. The accreted gas in this case comes from the molecular cloud out of which the star has formed rather than a companion star. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Accretion disk」の詳細全文を読む スポンサード リンク
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