Artist's conception of a cataclysmic variable system

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 hundreds of cataclysmic variables are known. Artists Conception of a Cataclysmic Variable Starsystem. ... Artists Conception of a Cataclysmic Variable Starsystem. ... A binary star system consists of two stars both orbiting around their barycenter. ... White dwarf Sirius-B in x-rays A white dwarf is an astronomical object which is produced when a low to medium mass star dies. ... According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type. ...

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. Ultraviolet (UV) radiation is electromagnetic radiation of a wavelength shorter than that of the visible region, but longer than that of soft X-rays. ... In the NATO phonetic alphabet, X-ray represents the letter X. An X-ray picture (radiograph) taken by Röntgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz... A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies. ...

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 disk 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. An accretion disc (or accretion disk) is a structure formed by material falling into a gravitational source. ... Instability in systems is generally characterized by some of the outputs or internal states growing without bounds. ...

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. 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 in a process known as accretion induced collapse. To an outside observer, this would be seen as a Type Ia supernova. General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... Density (symbol: ρ - Greek: rho) is a measure of mass per unit of volume. ... Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ... The deuterium-tritium fusion reaction is considered the most promising for producing fusion power. ... General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Atomic mass 4. ... This article needs to be cleaned up to conform to a higher standard of quality. ... The interstellar medium (or ISM) is a term used in astronomy to describe the rarefied gas and dust that exists between the stars (or their immediate circumstellar environment) within a galaxy. ... The Chandrasekhar limit is the maximum mass of a white dwarf, and is approximately 3 × 1030 kg, around 1. ... Cross section of a neutron star A neutron star is a type of degenerate star composed mostly of densely packed neutrons, generally about 20 km in diameter and as massive as an average star. ... Remnant of Keplers Supernova, SN 1604. ...

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. Current flowing through a wire produces a magnetic field (M) around the wire. ... This article treats polarization in electrodynamics. ...

External links

• The Catalog and Atlas of Cataclysmic Variables
• TPP/CVcat - a catalogue of Cataclysmic Variable Stars
• RKcat (Ritter and Kolb), 7th edition