A Class M star is the dimmest type of main sequence star besides brown dwarfs. They are typically colored red or red-orange. They are by far the most common type of luminous star, making up roughly 76% of all main sequence stars.
A typical Class M star has a mass of 0.20 (0.08 to 0.45) solar masses (M☉), a radius of 0.3 (≤0.7) solar radii, a luminosity of 0.01 (≤0.08) solar luminosities, a surface temperature of 3,000 K (2,400 K to 3,700 K), and a lifespan of 1 to 10 trillion years.
Although they have very long lifespans, red dwarfs' low mass and luminosity (and their habitable zones of about 0.1 AU) means that many planets will end up frozen or tidally locked to their parent star; these are generally not good conditions for intelligent life. However, it may be possible or even common for simple life to develop on these planets.
Even though almost all of the class M stars are red dwarfs, most giants, supergiants and hypergiants, including leading candidates for largest known stars, are also class M. Examples of Class M supergiants and hypergiants include Antares A, Betelgeuse, Mu Cephei, Westerlund 1-26, VV Cephei A, VY Canis Majoris, and UY Scuti.
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