A dwarf nova (pl.

novae A nova ( novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", Latin for "new") that slowly fades over weeks or months. All observed novae involve white ...

), or U Geminorum variable, is one of several types of

cataclysmic variable star In astronomy, cataclysmic variable stars (CVs) are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state. They were initially called novae (), since those with an outburst brightness visible t ...

, consisting of a close

binary star A binary star or binary star system is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars us ...

system in which one of the components is a

white dwarf A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...

that accretes matter from its companion. Dwarf novae are dimmer and repeat more often than "classical" novae.

Overview

The first one to be observed was U Geminorum in 1855; however, the mechanism was not known until 1974, when Brian Warner showed that the nova is due to the increase of the luminosity of the accretion disk. They are similar to classical

nova A nova ( novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", Latin for "new") that slowly fades over weeks or months. All observed novae involve white ...

e 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 critical

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

that causes a change in

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

, 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. Dwarf novae are distinct from classical novae in other ways; their

luminosity Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electroma ...

is lower, and they are typically recurrent on a scale from days to decades. The luminosity of the outburst increases with the recurrence interval as well as the orbital period; recent research with the

Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...

suggests that the latter relationship could make dwarf novae useful

standard candle The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A ''direct'' distance measurement of an astronomical object is possible ...

s for measuring cosmic distances. There are three subtypes of U Geminorum star (UG): *

SS Cygni SS Cygni is a variable star in the northern constellation Cygnus (constellation), Cygnus (the Swan). It was discovered in 1896 by Louisa D. Wells, a computer working under Edward Charles Pickering, Edward Pickering at Harvard College Ob ...

stars (UGSS), which increase in brightness by 2–6

mag Mag, MAG, Mags or mags may refer to: Arts, entertainment and media * ''MAG'' (video game), released in 2010 * ''Mág'' (film), a 1988 Czech film * ''Mag'' (Slovenian magazine), published from 1995 to 2010 * '' The Mag'', a British music magazin ...

in V in 1–2 days, and return to their original brightnesses in several subsequent days. *

SU Ursae Majoris SU Ursae Majoris, or SU UMa, is a close binary star in the northern circumpolar constellation of Ursa Major. It is a periodic cataclysmic variable that varies in magnitude from a peak of 10.8 down to a base of 14.96. The distance to thi ...

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 ER Ursae Majoris is a variable star in the northern circumpolar constellation of Ursa Major, abbreviated ER UMa. It is a prototype system for a subclass of SU Ursae Majoris dwarf novae. The system ranges in brightness from a peak apparen ...

stars and

WZ Sagittae WZ Sagittae (''WZ Sge'') is a dwarf nova cataclysmic star system in the constellation Sagitta. It consists of a white dwarf primary being orbited by a low mass companion. The white dwarf is about 0.85 solar masses while the companion is ...

stars (UGWZ). *

Z Camelopardalis Z Camelopardalis (Z Cam) is a cataclysmic variable star system in the northern constellation of Camelopardalis. It has an apparent visual magnitude which varies between 9.8 and 14.5. This system is the prototype star for the fa ...

stars (UGZ), which temporarily "halt" at a particular brightness below their peak; a behavior termed a "standstill". They are interpreted as occupying the border between the classes of dwarf nova and the more stable nova-like variables. In addition to the large outbursts, some dwarf novae show periodic brightening known as “

superhump In astronomy, a superhump is a periodic brightness variation in a cataclysmic variable star system, with a period within a few percent of the orbital period of the system. History Superhumps were first seen in SU Ursae Majoris (SU UMa) stars, a su ...

s”. They are caused by deformations of the

accretion disk An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is most frequently a star. Friction, uneven irradiance, magnetohydrodynamic effects, and ...

when its rotation is in resonance with the orbital period of the binary.

References

External links

* * * * {{Authority control Astronomical events Binary stars