Artist's conception of a cataclysmic variable system . Here gas from a blue giant star is shown being stripped away into an accretion disk around its compact binary companion. Gas in the accretion disk swirls around, heats up, and eventually falls onto the compact star. Extreme conditions frequently occur on the surface of the compact star as gas falls in, many times causing detectable X-rays, gamma-rays, or even cataclysmic novae explosions. Studying the extreme conditions in these systems tells us about the inner properties of ordinary matter around us. STScI
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, with an orbital period generally between about 80 and 700 minutes. 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 burn 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 disrupts the white dwarf.
Classification of cataclysmic variables
Cataclysmic variables are subdivided into several smaller groups, often named after a bright prototype star characteristic of the class. In some cases the magnetic field of the white dwarf is strong enough to disrupt the inner accretion disk or even prevent disk formation altogether. Magnetic systems often show strong and variable polarization in their optical light, and are therefore sometimes called polars; these often exhibit small-amplitude brightness fluctuations at what's presumed to be the period of rotation of the white dwarf
|Dwarf novae, or U Geminorum stars, are cataclymic variables which are observed to brighten repeatedly, though by a smaller amount than classical novae.
|Z Camelopardalis stars
||Temporarily "halt" at a particular brightness below their peak
|SU Ursae Majoris stars
||Have "superoutbursts" which are brighter than the average
|SS Cygni stars
||Have outbursts of two distinct lengths
||These are stars where the magnetic field of the white dwarf has locked the binary into synchronous rotation, and matter streams down fairly constantly onto the white dwarf rather than forming a disc.
||Also called 'intermediate polars', these have a slightly weaker magnetic field than AM Herculis stars; there is an accretion disc, but substructure in it is created by the field.
||These are stars which occasionally drop in brightness by more than one magnitude, with very occasional dwarf-nova-type outbursts during the dim state. They may be a subclass of polars 
|AM Canum Venaticorum
||These are cataclysmic variables both of whose components are white dwarfs; the accretion disc is composed primarily of helium, and they are of interest as sources of gravitational waves
||These are like dwarf novae but have the accretion disc in a steady state, so don't show outbursts; the disc emits non-uniformly. They are usually also eclipsing variables, though this appears to be a selection artefact. .
Catalogues of cataclysmic variables
There are over 1600 known CV systems , though more are discovered each year whilst that catalogue was frozen as of 1 February 2006.
Discovery of cataclysmic variables
Cataclysmic variables are among the classes of astronomical objects most commonly found by amateurs, since a cataclysmic variable in outburst is bright enough to be detectable with very modest instruments, and the only celestial objects easily confused with them are bright asteroids whose movement from night to night is clear.
Around six novae are discovered each year, whilst models based on observations in other galaxies suggest that the rate of occurrence ought to be between 20 and 50 ; this discrepancy is due partly to obscuration by interstellar dust, and partly to a lack of observers in the southern hemisphere and to the difficulties of observing while the Sun is up and at Full Moon.
Verifying that an object is a cataclysmic variable is also fairly straightforward: they are usually quite blue objects, they exhibit rapid and strong variability, and they tend to have peculiar emission lines. They emit in the ultraviolet and X-ray ranges; they are expected also to emit gamma rays, from annihilation of positrons from proton-rich nuclei produced in the fusion explosion, but this has not yet been detected