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Flare stars, also known as variable stars or UV Ceti variables, are red dwarf stars that experience large fluctuations in surface temperature and solar flux over a course of hours or even minutes. Flare stars are highly unpredictable, but amazing to watch, because most stars, like those on the main sequence, experience variability in their solar output only over time spans of millions of years.
Flare stars are caused by the equivalent of solar flares, except they are much larger in size relative to the star. Because red dwarf stars have a mass only a few tenths of the Sun, they have less gravity and flares are able to eject material more energetically from the star's surface.
When a flare star goes off, the surface temperature can increase from 3000 K to 10,000 K in a course of mere minutes. These happen during magnetic reconnection events. The material of which a star is made, plasma, responds to magnetic fields generated by the star. Typically, flux patterns of plasma are aligned with the magnetic fields. Occasionally, magnetic fields collapse erratically to a lower energy state, dramatically rearranging their configuration. The plasma rushes into the new configuration, colliding with itself and heating up thousands of degrees in the process. Because red dwarf stars are usually relatively cool, the huge flare-up in heat and energy is very noticeable. Some solar flares on flare stars are so huge that they project 20% off the surface of the star relative to its diameter.
The closest star to the Earth besides the Sun, Proxima Centauri, is a flare star. Because red dwarfs are inherently very faint, we can only observe those less than 50 or so light years from the Earth, so all flare stars are similarly close. The most famous is UV Ceti, discovered in 1948.