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Cataclysmic variable star

Cataclysmic variables 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 hundreds of cataclysmic variables are known.

From the observational viewpoint, cataclysmic variables are relatively easy to discover. They are usually quite blue objects, as 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 wide 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 disk around the white dwarf. Strong UV and in 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 the entire disk falls onto the white dwarf.

During the accretion proces, 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. It is possible that after several nova outbursts the mass of the white dwarf exceeds the Chandrasekhar limit and it implodes to a neutron star. This is known as accretion induced collapse To an outside observer, this would be seen as a supernova.

Cataclysmic variables are subdivided into several smaller groups, often presented by a bright prototype star characteristic of the class. Some of the protype stars are SS Cygni, U Geminorum, Z Camelopardalis, SU Ursae Majoris, AM Herculis, DQ Herculis, VY Sculptoris, SW Sextantis etc.

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 protype star. Intermediate polars and polars are sometimes referred to as DQ Herculis stars and AM Herculis stars, respectively.

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