Adwarf nova (pl.novae), orU Geminorum variable, is one of several types ofcataclysmic variable star, consisting of a closebinary star system in which one of the components is awhite dwarf thataccretes matter from its companion. Dwarf novae are dimmer and repeat more often than "classical" novae.[1]
The first one to be observed wasU Geminorum in 1855; however, the mechanism was not known until 1974, whenBrian Warner showed that the nova is due to the increase of the luminosity of the accretion disk.[2] They are similar to classicalnovae in that the white dwarf is involved in periodic outbursts, but the mechanisms are different.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 criticaltemperature that causes a change inviscosity, 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.[3][4]
Dwarf novae are distinct from classical novae in other ways; theirluminosity is lower, and they are typically recurrent on a scale from days to decades.[3] The luminosity of the outburst increases with the recurrence interval as well as the orbital period; recent research with theHubble Space Telescope suggests that the latter relationship could make dwarf novae usefulstandard candles for measuring cosmic distances.[3][4]
There are three subtypes of U Geminorum star (UG):[5]
In addition to the large outbursts, some dwarf novae show periodic brightening known as “superhumps”. They are caused by deformations of theaccretion disk when its rotation is in resonance with the orbital period of the binary.
| Formation | |
|---|---|
| Fate | |
| In binary systems | |
| Properties | |
| Related | |
| Pulsating |
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|---|---|---|---|---|---|---|---|---|---|
| Eruptive |
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| Cataclysmic | |||||||||
| Rotating |
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| Eclipsing | |||||||||
