Accretion ( Latin accretio "growth", "increase") is the term in astronomy for a process in which a cosmic object collects matter due to its gravitation or tidal forces (see Roche limit ).
The collecting central object is called the accretor or gravitating object. It can be an ordinary star. Especially young stars such as B. Protostars or T-Tauri stars are known to collect material from the cloud of matter from which they were formed. Another important group of accretors are the compact objects . These include the white dwarfs , neutron stars , and black holes as they also occur in X-ray binary stars .
A number of radiation processes are associated with accretion. The matter in the accretion flow becomes hot due to friction and therefore emits heat radiation . Brake radiation becomes relevant in a less dense accretion flow . Non-thermal forms of radiation such as cyclotron radiation and synchrotron radiation also play a role if strong magnetic fields and fast electrons occur in the accreted material. In general one speaks of radiation transport . In very hot matter reservoirs, the Comptonization of low-energy radiation becomes essential. This explains the hard spectra of X-rays down to a few 100 k eV . Whether and how pronounced the mentioned radiation processes occur depends on the accretor. In general, the accretion fluxes in compact objects become very hot and fast and thereby ionize the accreted material. With normal stars or protostars, the accretion flow is cold and slow. Therefore, atomic and molecular physics play a major role here. Accretion is the most efficient process for converting gravitational energy into radiant energy. It is much more efficient than fusion in stars, because up to 42% of the mass can be converted into radiation energy during accretion. In contrast, thermonuclear fusion in the interior of stars has an efficiency of only 0.7%. Accretion to a compact object therefore creates the greatest known luminosity in the universe.
- Accretion disk
- Pulsed Accretion
- Gravitational energy as an energy source for the observed processes.
- ↑ accretion. In: Andreas Müller - Lexicon of Astrophysics. August 11, 2007, accessed February 5, 2019 .