Red dwarfs are the coolest main-sequence stars, with a spectral type of M and a surface temperature of about 2,000–3,500 K. Because these stars are so cool, spectral lines of molecules such as titanium oxide, which would be disassociated in hotter stars, are quite prominent. Red dwarfs are also the dimmest stars, with luminosities between about 0.0001 and 0.1 times that of the Sun.

Smaller stars have longer lifetimes than larger stars.