• Star which exhibits a sudden rapid increase of brightness, short of the massive brightening associated with a supernova. The Latin word nova (meaning ‘new’) implies a new star, but in fact novae are old stars prone to sudden brightening, not new-born ones. Most novae are thought to be double stars in which material is transferred from one star to another, probably from a main sequence star to a white dwarf. Most novae are seen to erupt only once, and in these cases, thermonuclear burning, set off when a critical mass of material accumulates in a gas layer around the white dwarf, is probably responsible. These stars may become novae at long intervals. Dwarf novae brighten at intervals from days to years, with greater brightening after longer intervals, and the brightening is probably due to energy being released periodically as it builds up in a gas disk around the white dwarf. There are also recurrent novae, which seem to involve red giants as the companion to the dwarf, and which may erupt via complex processes involving magnetic fields building material up in energetic columns above the surface of the white dwarf, from which it is released in occasional enormous bursts. A nova can become 5–15 or even more magnitudes brighter during an eruption.