M
Stellar classification

A red dwarf is a small and relatively cool star on the main sequence, either late K or M spectral type. Red dwarfs range in mass from a low of 0.075 solar masses (M☉) to about 0.50 M☉ and have a surface temperature of less than 4,000 K. Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun, but because of their low luminosity, individual red dwarfs cannot easily be observed. From Earth, not one is visible to the naked eye. Proxima Centauri, the nearest star to the Sun, is a red dwarf (Type M5, apparent magnitude 11.05), as are twenty of the next thirty nearest stars. According to some estimates, red dwarfs make up three-quarters of the stars in the Milky Way. Stellar models indicate that red dwarfs less than 0.35 M☉ are fully convective. Hence the helium produced by the thermonuclear fusion of hydrogen is constantly remixed throughout the star, avoiding a buildup at the core. Red dwarfs therefore develop very slowly, having a constant luminosity and spectral type for, in theory, some trillions of years, until their fuel is depleted. Because of the comparatively short age of the universe, no red dwarfs of advanced evolutionary stages exist.