Cataclysmic variable star
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Cataclysmic variables (also U Geminorum Stars) are a class of binary stars containing a white dwarf and a companion star. The companion star is usually a red dwarf, although in some cases it is another white dwarf or a slightly evolved star (subgiant). Several hundred cataclysmic variables are known.
From the observational viewpoint, cataclysmic variables are relatively easy to discover. They are usually quite blue objects, whereas the majority of stars are red. The variability of these systems is usually quite rapid and strong. Strong ultraviolet or even X-ray emission and peculiar emission lines are other typical properties.
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 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 tenth of the disk material falls onto the white dwarf.
During the accretion process, mass is accumulating on the white dwarf surface. Usually the donor star is rich in hydrogen. Eventually the density and temperature at the bottom of the accumulated hydrogen layer rise high enough to ignite nuclear fusion reactions. The reactions burn the bulk of the hydrogen layer to helium in a short time. This is seen as a nova outburst. The outer parts of the hydrogen layer and some of the fusion products are ejected to interstellar space. 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 disrupts the white dwarf.
Cataclysmic variables are subdivided into several smaller groups, often presented by a bright prototype star characteristic of the class. The prototype stars include SS Cygni, U Geminorum, Z Camelopardalis, SU Ursae Majoris, AM Herculis, DQ Herculis, VY Sculptoris, SW Sextantis.
In some cases the magnetic field of the white dwarf disrupts the inner accretion disk or even prevents disk formation. Magnetic systems often show strong and variable polarisation in their optical light, and are therefore sometimes called intermediate polars (in case of a disrupted disk) or polars (in case of prevented disk formation). Another naming convention, often used in variable star classification, is naming the class after a well-known prototype star. Intermediate polars and polars are sometimes referred to as DQ Herculis stars and AM Herculis stars, respectively.
