An M-type star is within the M-class, a spectral class indicating stars with and very weak hydrogen absorption lines and molecular lines (particularly titanium oxide), a red color, and a surface temperature in the 2400-3700 K range. The spectral energy distribution peaks in the near infrared. These include some red dwarves (M-dwarf main sequence stars) and red giants (a type of post-main-sequence star), and some hotter brown dwarfs. (The terms red giant also includes spectral types cooler than M class, and sometimes the terms red dwarf and even m dwarf are use similarly.) Some M dwarfs are Proxima Centauri, AD Leonis, AU Microscopii, Barnard's Star, G239-25, Kapteyn's Star, Lacaille 9352, Lalande 21185, LHS 1140, Luyten 726-8, Ross 154, Ross 248, Scholz's Star, TRAPPIST-1, Wolf 359, and Gliese 581. Some characteristics of main-sequence (MS) M dwarfs:
|1-10 trillion years||main-sequence lifetime|
M-class spectral types, with mass, radius and luminosity of main-sequence M dwarfs as a fraction of the solar values:
|class||temp(K)||MS solar masses||MS radius(solar)||MS luminosity(solar)|
A Roman numeral V suffix (e.g., M7V) indicates specifically a main sequence (dwarf, as in non-giant) star.
An example M-class red giant is Betelgeuse (an especially large one, i.e., a supergiant). Stars of hotter spectral classes (e.g., the Sun) eventually spend time in a red giant phase following their time on the main sequence. Their luminosity rises and the internal heat puffs them up creating a very large surface with a low temperature. The red giant phase can last up to a billion years, the least massive of such stars having the longest phase.