Shell of gas from a nova, artwork. Expanding shell of gas (red and blue) following a nova explosion. A nova is thought to be the periodic explosion that occurs in a cataclysmic variable binary star system. This is when a binary star system consists of a red dwarf star (centre) orbiting close to a white dwarf star (blue, lower right edge of red dwarf). Material falls from the red dwarf star onto the white dwarf star, accumulating until it eventually explodes as a nova. Novae, which recur regularly, are seen as a massive increase in the brightness of the star system. A red dwarf star is a small star, while a white dwarf star is a dense, collapsed star formed from a small or medium star.