A red dwarf (or M dwarf) is a small and relatively cool star on the main sequence, of M spectral type. Red dwarfs range in mass from a low of 0.075 to about 0.50 solar mass and have a surface temperature of less than 4,000 K. Sometimes K-type main-sequence stars, with masses between 0.50-0.8 solar mass, are also included.
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 be easily 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 fifty of the sixty 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 helium buildup at the core, thereby prolonging the period of fusion. Red dwarfs therefore develop very slowly, maintaining a constant luminosity and spectral type for trillions of years, until their fuel is depleted. Because of the comparatively short age of the universe, no red dwarfs exist at advanced stages of evolution.
Class M:: By far the most common class. At least 80% of the main sequence stars in the solar neighborhood are red dwarfs such as Proxima Centauri. M is also host to most giants and some supergiants such as Antares and Betelgeuse , as well as Mira variables. The late-M group holds hotter brown dwarfs that are above the L spectrum. This is usually in the range of M6.5 to M9.5