A U Geminorum-type variable star, or dwarf nova (pl.

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

) is one of several types of cataclysmic variable star, consisting of a close

binary star A binary star 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 using a telescope as separate stars, in wh ...

system in which one of the components is a

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...

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

Overview

The first one to be observed was U Geminorum in 1855; however, the mechanism was not known till 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 (plural novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", which is Latin for "new") that slowly fades over weeks or months. Causes of the dramati ...

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 expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

that causes a change in

viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

, 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 power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a st ...

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 (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...

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 o ...

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 (the Swan). It is the prototype of the subclass of dwarf novae that show only normal eruptions. It typically rises from 12th magnitude to 8th magnitude for 1–2 days e ...

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

mag Mag, MAG or mags may refer to: Arts, entertainment and media * ''MAG'' (video game), 2010 * ''Mag'' (Slovenian magazine), 1995–2010 * '' The Mag'', a British music magazine Businesses and organisations * MacKenzie Art Gallery, in Regina, Sask ...

in V in 1–2 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 WZ Sagittae (''WZ Sge'') is a cataclysmic variable, cataclysmic dwarf nova star system in the constellation Sagitta (constellation), Sagitta. It consists of a white dwarf primary being orbited by a low mass companion. The white dwarf is ...

stars (UGWZ). * Z Camelopardalis stars (UGZ), which temporarily "halt" at a particular brightness below their peak. In addition to the large outbursts, some dwarf novae show periodic brightening known as “ superhumps”. They are caused by deformations of the accretion disk when its rotation is in resonance with the orbital period of the binary.

References

External links

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