red dwarf

A small, cool star, 0·5–0·1 the mass and diameter of the Sun, and 100–100 000 times fainter than the Sun. Red dwarfs are probably the most abundant type of star, but are difficult to see because they are so dim. Two of the nearest stars to the Sun, Proxima Centauri and Barnard's Star, are red dwarfs.

According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type. They constitute the vast majority of stars and have a diameter and mass of less than one-third that of the Sun (down to 0.08 solar masses, which are brown dwarfs) and a surface temperature of less than 3,500 K. As red dwarfs are fully convective, all the hydrogen in the star is available for fusion, which further increases their lifespan. Red dwarfs never initiate helium fusion via the triple alpha process and so cannot evolve beyond the red giant phase.

The fact that red dwarfs and other low mass stars remain on the main sequence while more massive stars have moved off the main sequence allows one to date star clusters by finding the mass at which the stars turn off the main sequence.

One mystery which has not been solved as of 2006 is the lack of red dwarf stars with no metals (in astronomy a metal is any element other than hydrogen and helium). If such stars included red dwarfs, they should still be observable today, but as yet none have been identified.

Red dwarfs are the most common star type in the Galaxy, at least in the neighbourhood of the Sun. Proxima Centauri, the nearest star to the Sun, is a red dwarf (Type M5, magnitude 11.0), as are twenty of the next thirty nearest. However, due to their low luminosity, individual red dwarfs cannot easily be observed over the vast intergalactic distances that luminous stars can.

Exoplanets have been discovered orbiting red dwarfs in 2005, one as small as the size of Neptune, or seventeen earth masses. It orbits just 6 million kilometers (0.04 AU) from its star, and so is estimated to have a surface temperature of 150 °C, despite how dim the star is..