Artist's conception of a cataclysmic variable system

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. A nova (pl novae or novas) is a Cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a White

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. A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. Mass transfer is the phrase commonly used in engineering for physical processes that involve molecular and convective transport of Atoms and Molecules 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. An accretion disc (or accretion disk) is a structure (often a Circumstellar disk) formed by diffuse material in orbital motion around a central body 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. Instability in systems is generally characterized by some of the Outputs or internal states growing without Bounds. A dwarf nova (pl novae) is a type of Cataclysmic variable, consisting of a close Binary star system in which one of the components is a The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus

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. The Chandrasekhar limit limits the mass of bodies made from Electron-degenerate matter, a dense form of matter which consists of nuclei immersed in a gas of Electrons Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 A Type Ia supernova is a sub-category of cataclysmic Variable

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. In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges 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

Catalogues of cataclysmic variables

There are over 1600 known CV systems ^[3], 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. Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but

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 ^[4]; 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. 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 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 ^[5]

References

External links

• The Catalog and Atlas of Cataclysmic Variables
• TPP/CVcat - a catalogue of Cataclysmic Variable Stars
• RKcat (Ritter and Kolb), 7th edition
• CVNet, a web site and community for CV enthusiasts and researchers - features announcements of new discoveries
• A Beginner's Guide to Cataclysmic Variables - features a very good categorisation of the different classes of stars
• Cataclysmic Variables, NASA's HEASARC page