Sometimes a star is in orbit around a white dwarf star. If it is large enough and close enough to the white dwarf, the dwarf may suck gasses from its companion's atmosphere. Over a period of time, a great deal of gas may collect on the white dwarf. As this gas is compacted by the white dwarf's gravity, it will eventually undergo nuclear fusion, resulting in a very bright outburst of light, known as a "nova". In some cases, the white dwarf may gather so much gas that the explosion completely destroys it, in what is called a "supernova". Such an event may also result in runaway stars, as the larger star no longer has a heavy companion which keeps it in orbit.