An M-type star is within the M-class, a spectral class indicating stars with very weak hydrogen absorption lines and with 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 similarly used for stars cooler than the M class.) Some M dwarfs are AD Leonis, AU Microscopii, Barnard's Star, G239-25, Kapteyn's Star, Lacaille 9352, Lalande 21185, LHS 1140, Luyten 726-8, Proxima Centauri, Ross 154, Ross 248, Scholz's Star, Teegarden's Star, TRAPPIST-1, Wolf 359, and Gliese 581. Some characteristics of main-sequence (MS) M dwarfs:
|< 0.08||bolometric luminosity(solar)|
|70 billion-5 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:
|type||temp(K)||MS solar masses||MS radius(solar)||MS luminosity(solar)|
A Roman numeral V suffix (e.g., M7V) is a luminosity class indicating a main sequence (i.e., dwarf, meaning non-giant) star.
An example M-class red giant is Betelgeuse (an especially large/bright one, i.e., a supergiant). Stars of hotter spectral classes (e.g., the Sun) eventually spend time in a giant phase following their time on the main sequence, in many cases as a red giant. Their luminosity rises and the internal heat puffs them up, resulting in a surface so large that the heat at the surface is diluted, and the surface temperature is lower, often in the M-class range. Such a red giant phase can last up to a billion years, the least massive of such stars having the longest such phase.