Inastronomy,cataclysmic variable stars (CVs) are stars which irregularly increase inbrightness by a large factor, then drop back down to a quiescent state. They were initially callednovae (from Latin 'new'), since those with an outburst brightness visible to thenaked eye and an invisible quiescent brightness appeared as new stars in the sky.
Cataclysmic variable stars arebinary stars that consist of two components; awhite-dwarf primary, and amass-transferring secondary. The stars are so close to each other that thegravity of the white dwarf distorts the secondary, and the white dwarfaccretes matter from the companion. Therefore, the secondary is often referred to as thedonor star, and it is usually less massive than the primary.[1] The infalling matter, which is usually rich inhydrogen, forms in most cases anaccretion disk around the white dwarf. StrongUV andX-ray emission is often detected from the accretion disc, powered by the loss ofgravitational potential energy from the infalling material.[2] The shortest observed orbit in a hydrogen-rich system is 51 minutes inZTF J1813+4251.[3]
Material at the inner edge of the disc falls onto the surface of the white-dwarf primary. A classicalnova outburst occurs when thedensity andtemperature at the bottom of the accumulated hydrogen layer rise high enough to igniterunawayhydrogen fusion reactions, which rapidly convert the hydrogen layer tohelium. If the accretion process continues long enough to bring the white dwarf close to theChandrasekhar limit, then the increasing interior density may igniterunaway carbon fusion and trigger aType Ia supernova explosion, which would completely destroy the white dwarf.
The accretion disc may be prone to an instability leading todwarf-nova outbursts, when the outer portion of the disc changes from a cool, dull mode to a hotter, brighter mode for a time, before reverting to the cool mode. Dwarf novae can recur ontimescales of days to decades.
Cataclysmic variables are subdivided into several smaller groups, often named after a bright prototype star characteristic of the class. In some cases, themagnetic 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 variablepolarization in their optical light, and are therefore sometimes calledpolars; these often exhibit small-amplitude brightness fluctuations at what is presumed to be the white dwarf's period of rotation.
| Supernovae | These are classed as cataclysmic variables and have extremely large outbursts that destroy the progenitor star. Some result from white dwarfs in binary systems, but others are very massive stars. | ||||||
| (Classical)novae | These cataclysmic variables have very large outbursts, of 6 to 19 magnitudes, caused by thermonuclear fusion of material accreted onto the white dwarf. | ||||||
| Recurrent novae | These have outbursts of about 4 to 9 magnitudes, repeating every 10 to 80 years.[4] Examples includeT Pyxidis andRS Ophiuchi. | ||||||
| Dwarf novae | Dwarf novae, orU Geminorum stars, are cataclysmic variables which are observed to brighten repeatedly, though by a smaller amount than classical novae.
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| Luminous red novae | These are stellar mergers that become very red after outburst. | ||||||
| Polars |
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| VY Sculptoris | 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.[5] | ||||||
| AM Canum Venaticorum | These are cataclysmic variables whose components are both white dwarfs; the accretion disc is composed primarily of helium, and they are of interest as sources ofgravitational waves. | ||||||
| SW Sextantis | These are like dwarf novae but have the accretion disc in a steady state, so do not show outbursts; the disc emits non-uniformly. They are usually alsoeclipsing variables, though this appears to be aselection artefact.[6] | ||||||
| Z Andromedae (symbiotic variables) | These are close binaries with a large cool component losing mass to a hotter compact component and accretion disc. |
There are over 1600 known CV systems.[7] The catalog was frozen as of 1 February 2006 though more are discovered each year.
Cataclysmic variables are among the classes of astronomical objects most commonly found by amateurs, since a cataclysmic variable in its outburst phase is bright enough to be detectable with very modest instruments, and the only celestial objects easily confused with them are brightasteroids whose movement from night to night is clear.
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 peculiaremission lines. They emit in theultraviolet andX-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.[8]
Around sixgalactic novae (i.e. inthe Milky Way) 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 per year;[9] this discrepancy is due partly toobscuration 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 atfull moon.
Some cataclysmic variables experience periodic brightenings caused by deformations of theaccretion disk when its rotation is in resonance with the orbital period of the binary.
| Formation | |
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| Fate | |
| In binary systems | |
| Properties | |
| Related | |
| Pulsating |
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| Eruptive |
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| Cataclysmic | |||||||||
| Rotating |
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| Eclipsing | |||||||||
