Editing Cataclysmic variable star (section)

{{Short description|Stars with irregular large fluctuations in brightness}}
[[File:Cataclysmic variable.svg|thumb|A non-magnetic cataclysmic variable. A white dwarf accretes matter from its [[Roche lobe]]-filling companion.]]

In [[astronomy]], '''cataclysmic variable stars''' ('''CVs''') are stars which irregularly increase in [[Absolute magnitude|brightness]] by a large factor, then drop back down to a quiescent state. They were initially called [[nova]]e ({{ety|la||new}}), since those with an outburst brightness visible to the [[naked eye]] and an invisible quiescent brightness appeared as new stars in the sky.

Cataclysmic variable stars are [[binary star]]s that consist of two components; a [[white dwarf]] primary, and a [[mass transfer]]ring secondary. The stars are so close to each other that the [[gravity]] of the white dwarf distorts the secondary, and the white dwarf [[Accretion (astrophysics)|accretes]] matter from the companion. Therefore, the secondary is often referred to as the ''donor star'', and it is usually less massive than the primary.<ref>{{Cite web |last1=Lavalle |first1=Mimi |last2=University |first2=Louisiana State |title=Binary star V Sagittae to explode as very bright nova by century's end |url=https://phys.org/news/2020-01-binary-star-sagittae-bright-nova.html |access-date=2024-07-14 |website=phys.org |language=en}}</ref> The infalling matter, which is usually rich in [[hydrogen]], forms in most cases an [[accretion disk]] around the white dwarf. Strong [[UV]] and [[X-ray]] emission is often detected from the accretion disc, powered by the loss of [[gravitational energy|gravitational potential energy]] from the infalling material.<ref>{{Cite web |last=NASA |date=2017-12-11 |title=Introduction to Cataclysmic Variables (CVs) |url=https://heasarc.gsfc.nasa.gov/docs/objects/cvs/cvstext.html |access-date=2024-10-06 |website=heasarc.gsfc.nasa.gov |language=en}}</ref> The shortest currently observed orbit in a hydrogen-rich system is 51 minutes in [[ZTF J1813+4251]].<ref>{{Cite web |title=Astronomers Discover 'Cataclysmic' Star Pair, Confirming Decades-Old Prediction |url=https://www.cnet.com/science/space/astronomers-discover-cataclysmic-star-pair-confirming-decades-old-prediction/ |access-date=2023-11-06 |website=CNET |language=en}}</ref>

Material at the inner edge of disc falls onto the surface of the white dwarf primary. A classical [[nova]] outburst occurs when the [[density]] and [[temperature]] at the bottom of the accumulated hydrogen layer rise high enough to ignite [[thermal runaway|runaway]] [[nuclear fusion|hydrogen fusion]] reactions, which rapidly convert 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 may ignite [[Carbon detonation|runaway carbon fusion]] and trigger a [[Type Ia supernova]] explosion, which would completely destroy the white dwarf.

The accretion disc may be prone to an instability leading to [[dwarf 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 on a [[Orders of magnitude (time)|timescale]] of days to decades.