A Class M star is the dimmest type of main sequence star beside 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 5,030 F or 2,776.7 C (3,860 F to 6,200 F), and a lifespan from 250,000,000,000 to well over 1,000,000,000,000 years. M-class stars are estimated to have core temperatures of 7,500,000°K (13,000,000°F).
Although they have very long lifespans of over a trillion years, red dwarfs' low mass and luminosity (and their habitable zones of about 0.1 AU) mean that many planets will end up frozen or tidally locked to their parent star; these are generally not good conditions for intelligent life. It may be possible or even common for simple life to develop on these planets, however.
Even though almost all of the class M stars are red dwarfs, most giants and some supergiants are also class M. For example, Betelgeuse, Antares, Mu Cephei, VY Canis Majoris, Westerlund 1-26 and the current largest known star, UY Scuti These stars have low or intermediate mass (roughly 0.3–8 solar masses (M☉) for giants, 10-40 M☉ for supergiants). These stars have a large radius, basically ranging from 8-850 R☉ for giants, while supergiants, 100-2,600 R☉.
More massive M-class stars, which are generally half the size of the Sun, could live for more than 100 billion years, and may theoretically expand to red subgiants no more than 5,000,000 - 10,000,000 km in diameter. The smaller red dwarfs could live for trillions of years. When they exhaust their fuel, they will shrink, but will increase in temperature, since the star's core has smaller amount of space to give heat. The blue dwarf will soon collapse under pressure and gravity, and a white dwarf will be left. Quadrillions of years later, the white dwarf will be a black dwarf, a completely dead star.
Since the heaviest red dwarfs have a significantly different life cycle, are twice as large and hot, and are almost ten times as heavy and bright than the lightest red dwarfs, there is a minor proposal that Class M stars should be split into two different classes. However, this proposal hasn't been heavily considered, although it may possibly be soon when both the public community and the scientific community realize the major differences between the M0 and M9 stars.
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