View source for Dwarf nova

{{Short description|Cataclysmic variable star, consisting of a close binary star system}}
[[File:HTCas-LB1-2010Nov12.jpg|250px|thumb|right|Dwarf nova HT Cas seen in outburst ([[apparent magnitude|mag]] ~13.4) on November 2, 2010]]
A '''dwarf nova''' (pl. [[wiktionary:nova|novae]]), or '''U Geminorum variable''', is one of several types of [[cataclysmic variable star]], consisting of a close [[binary star]] system in which one of the components is a [[white dwarf]] that [[accretion disk|accretes]] matter from its companion. Dwarf novae are dimmer and repeat more often than "classical" novae.<ref name="GCVS">{{cite web |url=http://www.sai.msu.su/groups/cluster/gcvs/gcvs/iii/vartype.txt |title=GCVS Variability Types and Distribution Statistics of Designated Variable Stars According to their Types of Variability |access-date=2013-02-08 |df=dmy-all |author1=Samus, N.N. |author2=Durlevich, O.V. |date=February 12, 2009}}</ref>

==Overview==
The first one to be observed was [[U Geminorum]] in 1855; however, the mechanism was not known until 1974, when [[Brian Warner (astronomer)|Brian Warner]] showed that the nova is due to the increase of the luminosity of the accretion disk.<ref name="Warner1974">{{cite journal|title=Observations of Rapid Blue Variables – XIV: Z C HAMAELEONTIS|first=Brian|last=Warner|doi=10.1093/mnras/168.1.235|journal=Monthly Notices of the Royal Astronomical Society|volume=168|issue=1|date=July 1974|pages=235–247|doi-access=free}}</ref> They are similar to classical [[nova]]e in that the white dwarf is involved in periodic outbursts, but the mechanisms are different. [[nova|Classical novae]] result from the fusion and detonation of accreted hydrogen on the primary's surface. Current theory suggests that dwarf novae result from instability in the accretion disk, when gas in the disk reaches a critical [[temperature]] that causes a change in [[viscosity]], resulting in a temporary increase in mass flow through the disc, which heats the whole disc and hence increases its luminosity. The mass transfer from the donor star is less than this increased flow through the disc, so the disc will eventually drop back below the critical temperature and revert to a cooler, duller mode.<ref name="CV intro">{{cite web |url=http://home.mindspring.com/~mikesimonsen/cvnet/id1.html |publisher=Cataclysmic Variable Network |title=Introduction to CVs |access-date=2006-04-17 |df=dmy-all |website=mindspring.com |editor=Simonsen, Mike |url-status=dead |archive-url=https://web.archive.org/web/20080226032415/http://home.mindspring.com/~mikesimonsen/cvnet/id1.html |archive-date=2008-02-26}}</ref><ref name="S&T">{{cite magazine |title=Calibrating Dwarf Novae |magazine=Sky & Telescope |date=September 2003 |page=20}}</ref>

Dwarf novae are distinct from classical novae in other ways; their [[luminosity]] is lower, and they are typically recurrent on a scale from days to decades.<ref name="CV intro"/> The luminosity of the outburst increases with the recurrence interval as well as the orbital period; recent research with the [[Hubble Space Telescope]] suggests that the latter relationship could make dwarf novae useful [[standard candle]]s for measuring cosmic distances.<ref name="CV intro"/><ref name="S&T"/>

There are three subtypes of U Geminorum star (UG):<ref name="Darling UG">{{cite web |url=http://www.daviddarling.info/encyclopedia/U/U_Geminorum_star.html |title=U Geminorum star |website=Daviddarling.info |date=2007-02-01 |df=dmy-all |access-date=2013-02-09 |author=Darling, David}}</ref>
* [[SS Cygni]] stars (UGSS), which increase in brightness by 2–6&nbsp;[[apparent magnitude|mag]] in [[V band|V]] in 1–2&nbsp;days, and return to their original brightnesses in several subsequent days.
* [[SU Ursae Majoris]] stars (UGSU), which have brighter and longer "supermaxima" outbursts, or "super-outbursts," in addition to normal outbursts. Varieties of SU Ursae Majoris star include [[ER Ursae Majoris]] stars and [[WZ Sagittae]] stars (UGWZ).<ref name="Darling SUUM">{{cite web |url=http://www.daviddarling.info/encyclopedia/S/SU_Ursae_Majoris_star.html |title=SU Ursae Majoris star |work=Daviddarling.info |date=2007-02-01 |df=dmy-all |access-date=2013-02-09 |author=Darling, David}}</ref>
* [[Z Camelopardalis]] stars (UGZ), which temporarily "halt" at a particular brightness below their peak; a behavior termed a "standstill".<ref>{{cite journal | title=The standstill luminosity in Z Cam systems | last1=Stehle | first1=R. | last2=King | first2=A. | last3=Rudge | first3=C. | journal=Monthly Notices of the Royal Astronomical Society | volume=323 | issue=3 | pages=584–586 | date=May 2001 | doi=10.1046/j.1365-8711.2001.04223.x | doi-access=free | arxiv=astro-ph/0012379 | bibcode=2001MNRAS.323..584S | s2cid=14478251 }}</ref> They are interpreted as occupying the border between the classes of dwarf nova and the more stable [[nova-like variable]]s.<ref>{{cite journal | title=Dwarf-Nova Outbursts | last=Osaki | first=Yoji | journal=Publications of the Astronomical Society of the Pacific | volume=108 | page=39 | date=January 1996 | doi=10.1086/133689 | bibcode=1996PASP..108...39O | doi-access=free }}</ref>

In addition to the large outbursts, some dwarf novae show periodic brightening known as “[[superhump]]s”. They are caused by deformations of the [[accretion disk]] when its rotation is in resonance with the orbital period of the binary.

{{gallery|width=280
|File:U Gem aavso.png|AAVSO [[light curve]] of [[U Geminorum]] (SS Cygni type)
|File:HT-Cas LC Chart.jpg|Light curve of eclipsing dwarf nova [[HT Cassiopeiae]] during outburst, showing [[eclipse]]s and [[superhump]]s (SU Ursae Majoris type)
|File:Z Camelopardalis light curve.png|Light curve of [[Z Camelopardalis]] (Z Camelopardalis type)
}}

==References==
{{Reflist|25em}}

==External links==
{{Portal|Astronomy}}
* {{cite web |url=https://spaceflightnow.com/news/n0305/30distance/ |website=Spaceflight Now |title=New Method of Estimated Dwarf Novae Distances |date=30 May 2003 |access-date=17 April 2006}}
* {{cite web |url=https://www.aavso.org/vsots_suuma |publisher=[[American Association of Variable Star Observers]] |title=SU Ursae Majoris}}
* {{cite web |url=http://www.eso.org/sci/publications/messenger/archive/no.80-jun95/messenger-no80-43-45.pdf |title=Amateur Astronomers and Dwarf Novae |publisher=[[European Southern Observatory]]}}
* {{cite web |url=https://sites.google.com/site/aavsocvsection/ |publisher=Cataclysmic Variable Network |via=Google |title=Activity at a glance (list of recently detected dwarf nova outbursts)}}

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[[Category:Dwarf novae| ]]
[[Category:Astronomical events]]
[[Category:Binary stars]]