Cataclysmic Variable (CV) - Definition

Cataclysmic Variable (CV)

Binary star systems with one white dwarf star and one normal star, in close orbit about each other. Material from the normal star falls onto the white dwarf, creating a burst of X-rays.

Cataclysmic variable stars (CV) are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state. They were initially called novae, from the Latin 'new', since ones with an outburst brightness visible to the naked eye and a quiescent brightness invisible appeared as new stars in the sky.

They consist of two component stars; a white dwarf primary, and a mass transferring secondary. The stars are so close to each other that the gravity of the white dwarf distorts the secondary, and the white dwarf accretes matter from the companion. Therefore, the secondary is often referred to as the donor star. The infalling matter, which is usually rich in hydrogen, forms in most cases an accretion disc around the white dwarf. Strong UV and X-ray emission is often seen from the accretion disc. The accretion disk may be prone to an instability leading to dwarf nova outbursts, when a portion of the disk material falls onto the white dwarf; the cataclysmic outbursts occur when the density and temperature at the bottom of the accumulated hydrogen layer rise high enough to ignite nuclear fusion reactions, which rapidly convert the hydrogen layer to helium.

If the accretion process continues long enough to bring the white dwarf close to the Chandrasekhar limit, the increasing interior density can ignite runaway carbon fusion and trigger a Type Ia supernova explosion, which completely destroys the white dwarf.

Source: wikipedia, NASA