A flare star is a type of variable star that will suddenly and unpredictably have a massive increase in its brightness across the electromagnetic spectrum for a few minutes. These are similar to solar flares but far more energetic and intense in relation to the star's normal output. For example, Luyten 726-8 emits as much as 75 times its normal brightness during a flare.
Virtually all known flare stars are M class or late K class red dwarfs, but it is also possible for brown dwarfs to also be capable of such flaring. The frequency of these flares vary from less than an hour to several days, but only need a few minutes to reach maximum brightness. The decay back to normal, quiescent levels of brightness takes several more minutes to hours.
It is believed the cause of the massive flares is the sudden release of magnetic energy in the photosphere of the star. A star's magnetic field varies in strength over time. As solar material and magnetic fields are coupled together, one effect of this coupling is excess energy being transferred to the plasma of the star when the magnetic field drops in strength. This heats up the solar plasma very quickly, which causes it to radiate intensely as a spike in brightness, or a flare. In addition to increased luminosity, high energy particles are released such as x-rays and gamma rays.
The first known flare stars are V1396 Cygni and AT Microscopii, discovered as such in 1924. The most famous flare star is Luyten 726-8, otherwise known by its flare star name UV Ceti. Today most flare stars that fall under the same category are considered UV Ceti type variable stars, and use the UV abbreviation for their variable star name, although sometimes the V abbreviation is used.
It is questioned if life as we know it could survive on a hypothetical planet orbiting a flare star. This is due to the fact any world that would be warm enough to have liquid water on its surface would have to be in a very narrow habitable zone very close to its parent star, as virtually all flare stars are red dwarfs. However when flare stars suddenly flare, they release up to 10,000 times their normal x-ray output for the duration of the flare. Such a large amount of x-ray radiation bathing a planet in such a close orbit would be lethal to any life such as found on Earth.