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 of 1,000,000,000,000 years.

Although they have very long lifespans, 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.

Stellar Classes
Φ · Ψ · Ω · Q · DE · GR · σ ς · Θ · N · D · BS · Y · T · L · M · C · S · QS · K · G · F · A · B · LBV · β · O · W · N · X · n0 · Exotic (Π · Σ · Γ · Δ · μ) · δ · I · TŻO · BL · ·

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