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 mass of less than one-half of that of the Sun (down to about 0.075 solar masses, which are brown dwarfs) and a surface temperature of less than 4,000 K.
Stellar models indicate that red dwarfs are fully convective so that the nuclear fuel hydrogen is constantly remixed to be burned to helium. They therefore develop very slowly, having a constant luminosity and spectral type for some hundred of billions of years, until their fuel is depleted. Because of the comparatively short age of universe, no red dwarfs of advanced evolutionary stages exist in the current era.
Description and characteristics
Red dwarfs are very low-mass stars with no more than 40% of the mass of the Sun. 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. Thus these stars emit little light, sometimes as little as 1/10,000th 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.
In general red dwarfs 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.
As red dwarfs are fully convective, helium does not accumulate at the core and, compared to larger stars such as the Sun, they can burn a larger proportion of their hydrogen before leaving the main sequence. As a result, red dwarfs have estimated lifespans longer than the estimated age of the universe, and stars with less than 0.8 solar masses have not had time to leave the main sequence. The lower the mass of a red dwarf, the longer the lifespan. It is believed that the lifespan of these stars exceeds the expected 10 billion year lifespan of our Sun by the third or fourth power of the ratio of their masses to the solar mass; thus a red dwarf with 0.1 solar mass may continue burning for 10 trillion years. As the proportion of hydrogen in a red dwarf is consumed, the rate of fusion declines and the core starts to contract. The gravitational energy generated by this size reduction is converted into heat, which is carried throughout the star by convection.
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