Nova, relatively small, very hot variable star that suddenly (usually within a few days) increases up to thousands of times in brightness. It is thought that the increase in brightness is caused by violent explosions that eject some of the star's mass, which escapes the star's gravitational field. The decline to the previous luminosity generally takes months or years. Recurrent novas are stars that flare in this way at irregular periods of a few decades. Dwarf novas are subdwarf stars that go nova every few weeks or months.