Editing Cataclysmic variable star

{{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.

== Classification ==
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 (waves)|polarization]] in their optical light, and are therefore sometimes called [[polar (cataclysmic variable)|polars]]; these often exhibit small-amplitude brightness fluctuations at what is presumed to be the white dwarf's period of rotation.

{| class="wikitable"
| [[Supernova]]e
| 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) [[nova]]e
| 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.<ref>{{cite book|page=[https://archive.org/details/cataclysmiccosmi00mobb/page/n70 59]|title=Cataclysmic Cosmic Events and How to Observe Them|url=https://archive.org/details/cataclysmiccosmi00mobb|url-access=limited|first=Martin|last=Mobberley|location=New York|publisher=Springer|date=2009|isbn=978-0-387-79945-2}}</ref> Examples include [[T Pyxidis]] and [[RS Ophiuchi]].
|-
| [[Dwarf nova]]e
| Dwarf novae, or [[U Geminorum star]]s, are cataclysmic variables which are observed to brighten repeatedly, though by a smaller amount than classical novae.
{|
|-
| [[Z Camelopardalis star]]s
| Temporarily "halt" at a particular brightness below their peak
|-
| [[SU Ursae Majoris star]]s
| Have "[[superoutburst]]s" which are brighter than the average
|-
| [[SS Cygni star]]s
| Have outbursts of two distinct lengths
|}
|-
| [[Luminous red nova]]e
| These are stellar mergers that become very red after outburst.
|-
| [[polar (cataclysmic variable)|Polars]]
|
{|
|-
| [[AM Herculis variable|AM Herculis]] stars are binaries in which the magnetic field of the white dwarf has synchronized the latter's rotational period with the binary orbital period. Matter from the donor star is magnetically channeled onto the white dwarf rather than forming a disc.
|-
| [[DQ Herculis variable|DQ&nbsp;Herculis]], also called 'intermediate polars', have a slightly weaker magnetic field than [[AM Herculis]] stars; there is an accretion disc, but substructure in it is created by the field.
|}
|-
| [[VY Sculptoris variable|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.<ref>{{cite journal|first1=Jean-Marie|last1=Hameury|first2=Jean-Pierre|last2=Lasota|journal=[[Astronomy and Astrophysics]]|arxiv=astro-ph/0207084|title=VY Sculptoris stars as magnetic CVs|volume=394|issue=1|pages=231–239|date=October 4, 2002|doi=10.1051/0004-6361:20021136|bibcode=2002A&A...394..231H|s2cid=5498393}}</ref>
|-
| [[AM Canum Venaticorum variable|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]].
|-
| [[SW Sextantis variable|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 also [[Eclipsing variable#Eclipsing binaries|eclipsing variables]], though this appears to be a [[selection artefact]].<ref>{{cite web|title=Defining Characteristics of the SW Sextantis Stars|url=http://spider.ipac.caltech.edu/staff/hoard/research/swsex/characteristics.html|url-status=dead|archive-url=https://web.archive.org/web/20071119022732/http://spider.ipac.caltech.edu/staff/hoard/research/swsex/characteristics.html|archive-date=2007-11-19}}</ref>
|-
| [[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.<ref>{{cite web|url=http://archive.stsci.edu/prepds/cvcat/index.html|title=A Catalog and Atlas of Cataclysmic Variables|first1=Ronald|last1=Downes|author2=<!-- cited as such at the URL --> |display-authors=etal}}</ref> The catalog was frozen as of 1 February 2006 though more are discovered each year.

==Discovery==

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 bright [[asteroids]] 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 peculiar [[emission line]]s. 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.<ref>{{cite journal|first1=F|last1=Senziani|first2=G.K.|last2=Skinner|first3=P.|last3=Jean|first4=M.|last4=Hernanz|title=Detectability of gamma-ray emission from classical novae with Swift/BAT|date=2008|doi=10.1051/0004-6361:200809863|journal=[[Astronomy and Astrophysics]]|volume=485|issue=1|pages=223–231|arxiv=0804.4791|bibcode=2008A&A...485..223S|s2cid=16650963}}</ref>

Around six [[Galactic astronomy|galactic]] novae (i.e. in [[Milky Way|our own galaxy]]) 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;<ref>{{cite journal|first1=M. J.|last1=Darnley|first2=M. F.|last2=Bode|first3=E.|last3=Kerins|first4=A. M.|last4=Newsam|first5=J.|last5=An|first6=P.|last6=Baillon|first7=V.|last7=Belokurov|first8=S.|last8=Calchi Novati|first9=B. J.|last9=Carr | first10 = M. | last10 = Creze|first11=N. W.|last11=Evans|first12=Y.|last12=Giraud-Heraud|first13=A.|last13=Gould|first14=P.|last14=Hewett|first15=Ph.|last15=Jetzer|first16=J.|last16=Kaplan|first17=S.|last17=Paulin-Henriksson|first18=S. J.|last18=Smartt|first19=Y.|last19=Tsapras | first20 = M. | last20 = Weston|title=Classical novae from the POINT-AGAPE microlensing survey of M31 -- II. Rate and statistical characteristics of the nova population|year=2006|doi=10.1111/j.1365-2966.2006.10297.x|journal=[[Monthly Notices of the Royal Astronomical Society]]|volume=369|issue=1|pages=257–271|doi-access=free |arxiv=astro-ph/0509493|bibcode=2006MNRAS.369..257D|s2cid=85510790}}</ref> 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]].

==Superhumps==
{{main|Superhump}}
Some cataclysmic variables experience periodic brightenings caused by deformations of the [[accretion disk]] when its rotation is in resonance with the orbital period of the binary.

==References==
{{Reflist}}

== External links ==
*[http://archive.stsci.edu/prepds/cvcat/index.html A Catalog and Atlas of Cataclysmic Variables (Archival Edition)]
<!-- *[http://www.cvcat.net/ TPP/CVcat – a catalogue of Cataclysmic Variable Stars] -->
*[https://wwwmpa.mpa-garching.mpg.de/RKcat/ Catalogue of Cataclysmic Binaries, Low-Mass X-Ray Binaries and Related Objects (RKcat Edition 7.24, 31 Dec 2015 – The Final Edition)]
*[https://sites.google.com/site/aavsocvsection/ CVnet, a website and community for CV enthusiasts and researchers] – features announcements of new discoveries
*[http://www.mssl.ucl.ac.uk/www_astro/gal/cv_beginners.html A Beginner's Guide to Cataclysmic Variables] – features a very good categorisation of the different classes of stars
*[https://heasarc.gsfc.nasa.gov/docs/objects/cvs/cvstext.html Cataclysmic Variables], NASA's High Energy Astrophysics Science Archive Research Center (HEASARC) page

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[[Category:Semidetached binaries]]
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