An M-type star is of the M-class, a spectral class for cool stars including red dwarves (M-dwarf main sequence stars), red giants (which are beyond their main sequence), and some hotter brown dwarfs. The common element is the temperature, which is less than (other) main sequence stars. They have very weak hydrogen absorption lines.
Some red dwarfs are Proxima Centauri, Barnard's Star, Lacaille 9352, Lalande 21185, Ross 154, Ross 248, Wolf 359, and Gliese 581.
Some characteristics of main-sequence M dwarfs:
|1-10 trillion years||main-sequence lifetime|
Subclasses, with mass, radius and luminosity of main-sequence M dwarfs as a fraction of the solar values:
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. The red giant phase lasts on the order of a billion years. Their size gives them their brighter absolute magnitude despite their low temperature.