Red Dwarf Stars

At the small, cool, and low luminosity extreme on the Hertzsprung-Russell Diagram are dim stars which are called red dwarfs or M-dwarfs. These low-luminosity stars are difficult to observe from the Earth. Their radiation is largely in the invisible, infrared portion of the electromagnetic spectrum. Since the 1970s, it has been realized that they are the most common type of star in the solar neighborhood. They probably account for 80 percent or more of the total number of stars in the universe. One of the reasons that there are so many of them is that they burn so slowly that virtually all of them that have been created are still burning, whereas very massive stars have comparatively very short lifetimes.

Flare Stars

A flare star is a variable star that can undergo unpredictable dramatic increases in brightness for a few minutes. It is believed that the flares on flare stars are analogous to solar flares in that they are due to the magnetic energy stored in the stars' atmospheres. Most flare stars are dim red dwarfs. The Sun's nearest neighbor Proxima Centauri is a flare star.

Also in the Sun's neighborhood is the flare star Wolf 359 (2.39 +/- .01 parsecs). It is a red dwarf of spectral class M6.5 and its flare activity involves the emission of X-rays with a relatively rapid flare rate. A mean magnetic field strengh about 2.2kG (0.2 Tesla) is indicated, but subject to sizable changes over time scales as short as six hours. By comparison, the Sun's average magnetic field is only about 1 Gauss, but it can rise as high as 3kG in active sunspot regions.

The Sun's second nearest neighbor, Barnard's Star, is also a flare star. Since it is considerably older than the Sun, 7-12 billion years in age, it was presumed to be quiescent. But an intense flare from it was observed in 1998.

Extreme cases of flare stars are referred to as "superflares".

Superflares

Extreme cases of flare stars can have explosions in the neighborhood of 10,000 times a typical flare of our Sun. Since such violent events are not predictable, we owe our present picture of them to the Kepler satellite, which has been able to monitor many solar-type stars over an extended period and with high accuracy. While Kepler's main mission was to discover planets around nearby solar-type stars, its capabilities also made it a very good detector for flare events. Wichmann, et al. report on the discovery of 11 superflare stars by Kepler with energies between 10³³ and 10³⁶ ergs. The largest known solar flare was the Carrington event of 1859 with an estimated energy of abou 10³² ergs. This is equal to 10²⁵ Joules or about a hundred million megatons of TNT!. Having a star superflare 10,000 times that amount is hard to comprehend.

Wichmann, et al. comment that the flares on low-mass main sequence stars are caused by solar-type magnetic activity, and that this activity depends chiefly on rotation. They observe that the most active stars are those that are fast rotators, either because they are young or because they are in close binary systems.