any star that varies in brightness, sometimes by more than one magnitude, within a few minutes. The cause is thought to be the eruption of flares much larger than, but otherwise similar to, those observed on the Sun. Flare stars are sometimes called UV Ceti stars, from a prototype star in the constellation Cetus. Proxima Centauri, the closest star to the Sun, is a flare star. All known flare stars are red dwarfs; flares in intrinsically brighter stars are not presently detectable. In UV Ceti and a few others, radio flares have been observed often corresponding to the optical outbursts.