Dwarf nova

photo: Artist's conception of a white dwarf star accreting hydrogen from a larger companion

A dwarf nova is a type of cataclysmic variable, consisting of a close binary star system in which one of the components is a white dwarf, which accretes matter from its companion. They are similar to classical novae 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, while current theory suggests that dwarf novae result from instability in the accretion disk, when gas in the disk reaches a critical temperature that causes a change in viscosity, resulting in a collapse onto the white dwarf that releases large amounts of gravitational potential energy.

Dwarf novae are distinct from classical novae in other ways; their luminosity 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 suggests that the latter relationship could make dwarf novae useful standard candles for measuring cosmic distances.

There are currently three known types of dwarf novae:

1. Z Camelopardalis stars, which temporarily "halt" at a particular brightness below their peak.
2. SU Ursae Majoris stars, which have "superoutbursts" which are brighter than the average.
3. U Geminorum stars, which include all dwarf novae outside the previous two categories.