Editing Variable star (section)

===Cataclysmic or explosive variable stars===
{{Main|Cataclysmic variable star|Symbiotic variable star}}

====Supernovae====
{{main|Supernova}}
Supernovae are the most dramatic type of cataclysmic variable, being some of the most energetic events in the universe. A supernova can briefly emit as much energy as an entire [[galaxy]], brightening by more than 20 magnitudes (over one hundred million times brighter). The supernova explosion is caused by a white dwarf or a star core reaching a certain mass/density limit, the [[Chandrasekhar limit]], causing the object to collapse in a fraction of a second. This collapse "bounces" and causes the star to explode and emit this enormous energy quantity. The outer layers of these stars are blown away at speeds of many thousands of kilometers per second. The expelled matter may form nebulae called ''[[supernova remnant]]s''. A well-known example of such a nebula is the [[Crab Nebula]], left over from a supernova that was observed in [[China]] and elsewhere in 1054. The progenitor object may either disintegrate completely in the explosion, or, in the case of a massive star, the core can become a [[neutron star]] (generally a [[pulsar]]) or a [[black hole]].

Supernovae can result from the death of an extremely massive star, many times heavier than the Sun. At the end of the life of this massive star, a non-fusible iron core is formed from fusion ashes. This iron core is pushed towards the Chandrasekhar limit till it surpasses it and therefore collapses. One of the most studied supernovae of this type is [[SN 1987A]] in the [[Large Magellanic Cloud]].

A supernova may also result from mass transfer onto a [[white dwarf]] from a star companion in a double star system. The Chandrasekhar limit is surpassed from the infalling matter. The absolute luminosity of this latter type is related to properties of its light curve, so that these supernovae can be used to establish the distance to other galaxies.

====Luminous red nova====
[[Image:V838 Monocerotis expansion.jpg|upright=1.2|right|thumb|Images showing the expansion of the light echo of [[V838 Monocerotis]]]]
{{main|Luminous red nova}}
Luminous red novae are stellar explosions caused by the merger of two stars.  They are not related to classical [[novae]].  They have a characteristic red appearance and very slow decline following the initial outburst.

====Novae====
{{Main|Nova}}
[[Nova]]e are also the result of dramatic explosions, but unlike supernovae do not result in the destruction of the progenitor star. Also unlike supernovae, novae ignite from the sudden onset of thermonuclear fusion, which under certain high pressure conditions ([[degenerate matter]]) accelerates explosively. They form in close [[binary system (astronomy)|binary system]]s, one component being a white dwarf accreting matter from the other ordinary star component, and may recur over periods of decades to centuries or millennia. Novae are categorised as ''fast'', ''slow'' or ''very slow'', depending on the behaviour of their light curve. Several [[naked eye]] novae have been recorded, [[Nova Cygni 1975]] being the brightest in the recent history, reaching 2nd magnitude.

====Dwarf novae====
{{Main|Dwarf nova}}
Dwarf novae are double stars involving a [[white dwarf]] in which matter transfer between the component gives rise to regular outbursts. There are three types of dwarf nova:
* [[U Geminorum star]]s, which have outbursts lasting roughly 5–20 days followed by quiet periods of typically a few hundred days. During an outburst they brighten typically by 2–6 magnitudes. These stars are also known as [[SS Cygni variable]]s after the variable in [[Cygnus (constellation)|Cygnus]] which produces among the brightest and most frequent displays of this variable type.
* [[Z Camelopardalis star]]s, in which occasional plateaux of brightness called ''standstills'' are seen, part way between maximum and minimum brightness.
* [[SU Ursae Majoris star]]s, which undergo both frequent small outbursts, and rarer but larger ''[[superoutburst]]s''. These binary systems usually have orbital periods of under 2.5 hours.

====DQ Herculis variables====
{{Main|Intermediate polar}}
DQ Herculis systems are interacting binaries in which a low-mass star transfers mass to a highly magnetic white dwarf. The white dwarf spin period is significantly shorter than the binary orbital period and can sometimes be detected as a photometric periodicity. An accretion disk usually forms around the white dwarf, but its innermost regions are magnetically truncated by the white dwarf. Once captured by the white dwarf's magnetic field, the material from the inner disk travels along the magnetic field lines until it accretes. In extreme cases, the white dwarf's magnetism prevents the formation of an accretion disk.

====AM Herculis variables====
{{Main|Polar (cataclysmic variable star)}}
In these cataclysmic variables, the white dwarf's magnetic field is so strong that it synchronizes the white dwarf's spin period with the binary orbital period. Instead of forming an accretion disk, the accretion flow is channeled along the white dwarf's magnetic field lines until it impacts the white dwarf near a magnetic pole. Cyclotron radiation beamed from the accretion region can cause orbital variations of several magnitudes.

====Z Andromedae variables====
{{main|Z Andromedae variable}}
These symbiotic binary systems are composed of a red giant and a hot blue star enveloped in a cloud of gas and dust. They undergo nova-like outbursts with amplitudes of up to 4 magnitudes. The prototype for this class is [[Z Andromedae]].

====AM CVn variables====
{{Main|AM Canum Venaticorum star}}
AM CVn variables are symbiotic binaries where a white dwarf is accreting helium-rich material from either another white dwarf, a helium star, or an evolved main-sequence star. They undergo complex variations, or at times no variations, with ultrashort periods.