A star of spectral type M, with a very cool surface (temperature below 3900 K), appearing reddish in colour and emitting most of its radiation in the infrared. M-type dwarfs, popularly known as red dwarfs, lie near the bottom end of the main sequence. They have masses under 0.5 solar masses, and luminosities up to 0.08 times the Sun's, too faint for even the closest, Proxima Centauri and Barnard’s Star, to be seen with the naked eye. They have potential lifetimes longer than the present age of the Universe. Many are flare stars. M-type giants, however, are of 1.2–1.3 solar masses and luminosities of 300 or more times the Sun's. M-type supergiants, such as Betelgeuse and Antares, are of 13–25 solar masses and luminosities 40 000–500 000 times the Sun's. The low temperatures of M-type supergiants are a consequence of their inflated physical size, as large as the orbit of Jupiter; they also tend to vary in size and brightness. M-type spectra are completely dominated by broad molecular absorption bands, particularly titanium oxide (TiO), but neutral metal lines are also present. See also Dme Star; Me Star.