Binary brightness

White dwarf stars may form binary systems with other stars (see above) which have typical orbital periods of 1-10 hours. These systems are known as cataclysmic variables as the second star loses material to the dense white dwarf by accretion (the growth of an astronomical body by the addition of mass through gravitational attraction), releasing X-ray radiation. The second star is usually a late main sequence star or a red giant and the accretion process leads to an abrupt increase in brightness of the system over a time period of a day followed by a reduction in luminosity that can last from weeks to several months – a process referred to as a type Ia supernova.