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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 comprise 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. They emit little light, sometimes as little as 1/10,000th that of the sun. Due to the slow rate at which they burn hydrogen, red dwarfs have an enormous estimated lifespan; estimates range from tens of billions up to trillions of years. Red dwarfs never initiate helium fusion and so cannot become red giants; the stars slowly contract and heat up until all the hydrogen is consumed. 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 remain on the main sequence while older stars have moved off the main sequence allows one to date star clusters by finding the mass at which the stars turnoff the main sequence. In addition, the fact that no red dwarfs have evolved off the main sequence have been observed is evidence that the universe has a finite age.

One mystery which has not been solved as of 2004 is the lack of red dwarf stars with no metalFor alternative meanings see metal (disambiguation). In chemistry, a metal is an element that readily forms cations and has metallic bonds, and it is sometimes said that it is similar to a cation in a cloud of electrons. The metals are one of the three grs (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 are not. The conventional explanation is that without heavy elements, low mass stars cannot form and the first stars were extremely high mass population III stars which died quickly and produced the metals necessary for low mass stars to form later.

Red dwarf stars are believed to be the most common star type in the universe. Proxima CentauriThe red dwarf Proxima Centauri (also Alpha Centauri C , part of the Alpha Centauri star system, is the nearest star to Earth, other than the Sun. As the name suggests, it is located in the constellation of Centaurus. Based on the parallax of 772. 42 milli, the nearest star to the Sun is a red dwarf, (Type M5, magnitude 11.0) as are twenty of the next thirty nearest.

See also

• Cataclysmic variable starCataclysmic variables are a class of binary stars containing a white dwarf and a companion star. The companion star is usually a red dwarf, although in some cases it is another white dwarf or a slightly evolved star (subgiant). Several hundreds of catacly
• Hertzsprung-Russell diagram
• Red giant
• Yerkes luminosity classification
• Stellar evolutionIn astronomy, stellar evolution is the sequence of changes that a star undergoes during its " lifetime", the millions or billions of years during which it emits light and heat. Over the course of that time, the star will change radically. Stellar evolutio
• White dwarfA white dwarf is a star with a color like most other stars, but with low absolute brightness. Such stars were discovered in the 19th century; the first ones found were white. The color of a star is a measure of the surface temperature: white stars are lik