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flare stars(UV Ceti stars) Intrinsically faint cool red dwarf stars that undergo intense outbursts of energy from localized areas of the surface, causing transient but appreciable increases in the brightness of the star. The brightness can change by two magnitudes or more in several seconds, decreasing to its normal minimum in about 10 to 60 minutes; the output usually peaks in the near-ultraviolet. There are also radio and X-ray flares, not always coincident with optical flares. Flare stars are of spectral type M or sometimes K with spectral emission lines of hydrogen and ionized calcium, i.e. they are Me stars. They have unusually strong magnetic fields (typically 10–4 tesla), suggesting a similar mechanism to solar flares. Most flare stars are either young (found in associations) or are a component of a close binary star: the fast rotation due to youth or to tidal effects, respectively, is probably responsible for the strong magnetism. The nearby binary UV Ceti (M6e V) is a typical flare star.
stars that flare up for periods ranging from several minutes to several hours; the intervals between outbursts vary from several hours to several dozens of hours. During the most intense outbursts, the apparent brightness of the stars increases dozens of times. The outbursts are accompanied by changes in the stellar spectra and by radiation in the radio frequency range.
There are two types of flare stars. The first type includes some of the red dwarfs, which are widely distributed throughout the galaxy. A typical representative of this type is UV Ceti, which is 35,000 times weaker than the sun, 30 times smaller in mass, and 17 times smaller in diameter. Because of their poor luminosity, this type of flare star can be seen only in the immediate vicinity of the sun. The second type is found in groupings of young stars. These are also dwarfs, but they are larger and brighter than the flare stars of the first type. The outbursts are caused by the sudden transition of a yet unknown form of energy into heat and radiation energy, which apparently occurs in the external layers of the stellar atmosphere. In this respect the outbursts are related to solar flares, but they generate thousands of times more luminous energy and hundreds of thousands of times more radio-frequency radiation than the solar flares.
REFERENCEGershberg, R. E. Vspyshki krasnykh karlikovykh zvezd. Moscow, 1970.
P. F. CHUGAINOV