M-type star

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 monoxide), a red color, and a surface temperature in the 2400-3700 K range. The spectral energy distribution (SED) peaks in the near infrared. These include some main sequence stars (M dwarfs aka red dwarfs), some 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, Gliese 436, Gliese 581, Kapteyn's Star, K2-18, Lacaille 9352, Lalande 21185, LHS 1140, LHS 3844, Luyten 726-8, Proxima Centauri, Ross 154, Ross 248, Scholz's Star, Teegarden's Star, TOI 700, TRAPPIST-1, and Wolf 359. Some characteristics of main-sequence (MS) M-type stars:

The vast majority of main sequence stars are in the M-class, including most of the stars in the solar system's neighborhood: our closest neighbor is an M dwarf. However, with their low luminosity and with much of their EMR not optical, we can see no M-dwarf without a telescope, and their research is a challenge. The M-class covers the coolest hydrogen burning stars, but encompasses a wide range of main-sequence masses (roughly an order-of-magnitude), radii, luminosities, and expected lifetimes. (In contrast, the range of G-type main-sequence masses range over roughly 25%.) Their lives are so long that all existing M dwarfs are near the beginning of their main sequence and their evolution is so slow that their age differences currently have little effect on their characteristics, which now basically depend only on their mass and metallicity (and any binary-star interaction). M-class spectral types, with mass, radius and luminosity of main-sequence M-type stars as a fraction of the solar values:

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). MS 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.