A flare star is a red dwarf that displays sudden and unpredictable changes in light output lasting for a few minutes to a few hours. The flares are believed to occur in the star's chromosphere and to be similar in nature to solar flares of our own Sun but much more energetic. They occur irregularly, follow no predictable pattern, and can dramatically increase the star's brightness over a broad wavelength range from X-rays to radio waves. The nearest star to the Sun, Proxima Centauri, is a flare star, as is one of the components of the nearby binary star system, Luyten 726-8, also known as UV Ceti. Indeed, it may be that the majority of these red dwarfs are flare stars.

Flare stars have frequently been encountered in Star Trek: The Next Generation. In the episode "Up The Long Ladder", the flare star Bringloid entered a period of intense activity, forcing the re-location of the Bringloid V colony. Another known example of a flare star located within Federation space is Wolf 359 - the site of the epic first battle between the Federation and the Borg in the episode "The Best of Both Worlds"

The intense emission of X-rays during flare activity on red dwarfs raises the question of whether life would possible on worlds orbiting such stars. There are potential obstacles, in any case, to life around red dwarfs, including the relatively narrow habitable zones of such stars and the redness of the light which might be an obstacle to photosynthesis. However, the fact that flares on red dwarfs may unleash up to 10,000 times as many X-rays as a typical flare from our Sun poses a special problem to the survival of life as we know it.

Wolf 359, in the constellation Leo. Also known as
CN Leonis, it is one of the faintest known stars
and a flare star with casual eruptions