Animation of a cataclysmic variable star system. It contains two stars, a white dwarf and a main sequence secondary star. The stars are close enough that material, mostly hydrogen, from the secondary is drawn by gravity towards the white dwarf, where it forms an accretion disc, with a hot spot where material meets the disc. Hot disc material emits ultraviolet and X_ray radiation. If enough hydrogen accretes on the star it can become hot enough to ignite a fusion reaction, which causes an outburst that greatly increases the brightness of the system for a short time. The process then begins again. If enough material accretes to raise the mass of the white dwarf above the Chandrasekhar limit, the runaway reaction can destroy the dwarf in a type Ia supernova.