| accretion disc |
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A disc structure that forms around a spinning object, such as a star or black hole, when
material falls on to it from a close companion in a binary star system. As stars evolve they enter a giant phase, when their size increases dramatically. In a binary system, the gravitational pull of the companion star on the bloated envelope of the giant may be stronger than the force holding the giant star and its envelope together. Under these circumstances, material flows across from one star to the other. The presence of an obscuring accretion disc may cause the star to be variable, as with Beta Lyrae stars, and this may be detectable from features in the spectrum. If the companion is spinning rapidly, as will a collapsed star (a white dwarf, neutron star or black hole), a swirling accretion disc forms on to which the material falls. The kinetic energy of the falling matter is turned into heat, and X-rays are produced. A mechanism of this kind is thought to be responsible for the production of X-rays in objects such as Cygnus X-1. See also: Roche lobe, SS433. |