dwarf novaeA small group of intrinsically faint stars that are characterized by sudden increases in brightness occurring at intervals of a few weeks or months, the maximum brightness lasting only a few days. The change in brightness (i.e. amplitude) is usually between 2–5 magnitudes. The first to be discovered, U Geminorum, is typical of the majority, which are therefore classified as U Geminorum stars . This subgroup displays a fairly smooth decline in brightness from the maximum, unlike the much smaller subgroup of Z Camelopardalis stars that can undergo standstills, i.e. periods of nearly constant intermediate brightness, before dropping to minimum brightness. Both the occurrence of the standstill and its duration – a few days to many months – are quite unpredictable. There are also periods of erratic light variations. SU Ursae Majoris stars , another subgroup, differ from U Gem stars by occasionally having particularly long outbursts – superoutbursts – that are brighter than normal outbursts.
Dwarf novae are a class of cataclysmic variables, i.e. close binary stars in which the primary is a white dwarf. The secondary is a cooler main-sequence star, spectral type K or G. The components have similar masses (about 0.7 to 1.2 solar masses); the orbital periods are between about 3 to 15 hours. There are two popular models for dwarf novae. In the mass-transfer instability model, the secondary is undergoing irregular expansion and episodically fills its Roche lobe (see equipotential surfaces. Hydrogen-rich gas then streams from the secondary and takes up a disk-shaped orbit around the primary, ultimately leading to an outburst. In the more popular disk instability model, the outbursts are caused by episodically recurring thermal instabilities in the accretion disk (see mass transfer), which is continually being fed by mass transfer from the Roche-lobe filling secondary. The outburst itself does not involve an explosion, and no significant amount of mass is ejected. The gas in the disk spirals down on to the white dwarf, where it may eventually cause a nova explosion.