dwarf nova

A type of cataclysmic variable star that exhibits sudden increases in brightness at intervals ranging from several days to a year. The dwarf novae are binary systems consisting of a main-sequence star and a white dwarf. Mass transfer takes place from the main-sequence star to the white dwarf via an accretion disc that builds up around it. Outbursts take place when hot regions form on the accretion disc. Dwarf novae are also known as U Geminorum stars. Several subtypes have been identified. SS Cygni stars increase in brightness by between 2 and 6 magnitudes in an outburst lasting several days. SU Ursae Majoris stars have occasional supermaxima, two magnitudes brighter and five times longer than normal. Z Camelopardalis stars sometimes undergo periods when the eruptions are suspended for weeks or years.