An M star is a cool, red star, of spectral type M, with a surface temperature of less than 3,600°C. Molecular absorption bands are prominent in the spectrum, with bands of titanium oxide becoming dominant at the lower end of the temperature range.
Main sequence M stars, known as red dwarfs, have a mass of less than 0.5 the mass of the Sun and a luminosity of less than 0.08 the luminosity of the Sun); examples include the nearby Proxima Centauri and Barnard's Star.
M-type giant stars, known as red giants, occur in the mass range 1.2 to 1.3 solar masses and may have luminosities exceeding 300 solar luminosities. The largest stars of all are M-type supergiants, such as Betelgeuse and Antares, of mass of 13 to 25 solar masses and luminosity of 40,000 to 500,000 solar luminositie.
An Me star is an M star with emission lines in its spectrum. Some Mira variables are of this type, as also are some M-type dwarfs known as dMe stars.