Red Dwarf

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 (the upper limit for a brown dwarf) to about 50% of the Sun 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 galaxy, at least in the neighborhood of the Sun, but due to their low luminosity, individual red dwarfs cannot easily be observed. From Earth, not one is visible to the naked eye.[1] 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.

Stellar models indicate that red dwarfs with less than 35% of the Sun's mass are fully convective.[2] 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

Red dwarfs are very low mass stars.[3] Consequently they have relatively low temperatures in their cores and energy is generated at a slow rate through nuclear fusion of hydrogen into helium by the proton-proton (PP) chain mechanism. Hence these stars emit little light, sometimes as little as 1⁄10,000 that of the Sun. Even the largest red dwarfs (for example HD 179930, HIP 12961 and Lacaille 8760) have only about 10% of the Sun's luminosity.[4] In general, red dwarfs with less than 35% of the Sun's mass transport energy from the core to the surface by convection. Convection occurs because of opacity of the interior, which has a high density compared to the temperature. As a result, energy transfer by radiation is decreased, and instead convection is the main form of energy transport to the surface of the star. Above this mass, the red dwarfs will have a region around their core where convection does not occur.[5]