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Red Dwarf stars are relatively cool stars in the universe.
Red Dwarf star is a small and relatively cool star, according to the Hertzsprung-Russell diagram. A Red Dwarf star is of the main sequence, either late K or M spectral type. Red Dwarf stars 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. Red dwarfs fuse Hydrogen to helium via the proton-proton (PP) chain. Due to the low temperatures in the core, fusion proceeds slowly. Thus red dwarfs have an enormous estimated lifespan; from tens of billions up to trillions of years depending upon mass. Consequently they emit little light, sometimes as little as 1/10,000th that of the sun. In general red dwarfs transport energy from the core to the surface via convection. 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. In any event, there has not been sufficient time since the Big Bang for red dwarfs to evolve off the main sequence.
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. This provides a lower, stellar, age limit to the universe and also allows formation timescales to be placed upon the structures within the Milky Way galaxy. Namely the Galactic halo and Galactic disk.
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). The Big Bang model predicts the first generation of stars should have only hydrogen, helium, and lithium. If such stars included red dwarfs, they should still be observable today, but as yet none have been identified. One explanation is that without heavy elements, low mass stars cannot form. Alternatively as they are dim and could be few in number, we simply may not have observed them yet.
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 ºC, despite how dim the star is. In 2006 a planet similar in size to Earth was found orbiting a red dwarf; it lies 390 million km (2.6 AU) from the star and its surface temperature is -220 ºC (56 K).
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