Many of the stars in this reference are dim, red (class-M) dwarfs that exhibit unusually violent flare activity. Since these flare stars seem to be so common, a stand-alone treatment of them is in order.

Flares do occur on our own sun every so often, but the amount of energy released in a solar flare is small compared to the total amount of energy the sun produces. However, imagine if you will a flare the size of a solar flare occuring on a red dwarf star (such as Proxima Centauri), which is more than ten thousand times dimmer than our sun. A solar-flare-sized flare occurring on Proxima Centauri would emit about as much light as Proxima Centauri itself. From our standpoint here on the Earth, it would appear that, during the flare, Proxima Centauri doubled in brightness.

Flares on these so-called flare stars occur sporadically, with successive flares spaced anywhere from an hour to a few days apart. It only takes a few minutes for a flare to reach peak brightness, and in fact more than one flare can occur at a time.

More importantly, flares on such dim dwarfs may emit up to 10 000 times as many X-rays as a comparably sized flare on our own sun. They would be lethal to any life forms otherwise developing on planets near the flare star, so life around such stars is unlikely. (Even if life could spring up despite the flares, flare stars are red dwarfs anyway, meaning that they have very narrow comfort zones due to their dimness and may be too red in color for autotrophic life (plants) to perform photosynthesis.) Some flare stars have also been observed emitting radio bursts simultaneously with the flares.

Note that since flare stars are variable stars, they will usually have a variable star designation such as UV Ceti or V645 Centauri.

As more and more red dwarfs are observed in detail, more and more of them are being classified as flare stars. It may turn out that most red dwarfs are flare stars, and that red dwarfs without violent flare activity are the exception rather than the rule.

Some of the flare stars included in this compendium are: UV Ceti, Alpha Centauri Proxima, Wolf 359, and FL Virginis.