The Character of M Dwarfs

M dwarfs dominate the stellar population, accounting for three of every four stars, the nearest of which is Proxima Centauri, the closest destination beyond our Solar System. These cool stars span large ranges in luminosities (one ten-thousandth to 6% L⊙) and temperatures (2,100–3,900 K) and have spectra dominated by absorption bands of titanium oxide (TiO) and, for the latest spectral types, vanadium oxide (VO). They have masses that span 0.075 to 0.61 M⊙, a factor of eight, which is comparable with a spread in masses for dwarf types mid-B through K. Unlike these more massive stars, in the age of the Universe no M dwarfs have evolved in any significant way. M dwarf systems are multiple roughly one-quarter of the time, with the closest binaries found in orbits that have been circularized via tides for orbital periods of about one week. Unlike any other type of main sequence star, there is a gap in the distribution of M dwarfs near masses of 0.35 M⊙ that pinpoints the separation of partially and fully convective stars, yet both types of M dwarfs are often active, showing both Hα in emission and flares. Many planets are found orbiting M dwarfs, and most of them are terrestrial or neptunian in size, rather than jovian, yet much more work remains to be done to characterize the exoplanet population. Overall, the Solar Neighborhood is dominated by M dwarfs that are likely orbited by many small, as yet unseen, planets—some of which may harbor life very near to that in our Solar System. ▪ M dwarfs account for three of every four stars. ▪ M dwarf counts increase all the way to the end of the main sequence. ▪ M dwarfs are partially radiative at high masses and fully convective at low masses.