M

Stars classified under the Morgan-Keenan (MK) system use the letters O, B, A, F, G, K, and M, a sequence used to identify the hottest (O type) to the coolest (M type) stars

Class—M is a stellar classification for the coolest stars, based on their spectral characteristics. Class M—stars are by far the most common. About 76% of the main-sequence stars in the solar neighborhood are class M stars.^[1] However, class M main-sequence stars (red dwarfs) have such low luminosities that none are bright enough to be seen with the unaided eye, unless under exceptional conditions. The brightest known M-class main-sequence star is M0V Lacaille 8760, with magnitude 6.6 (the limiting magnitude for typical naked-eye visibility under good conditions is typically quoted as 6.5), and it is extremely unlikely that any brighter examples will be found.

Although most class M stars are red dwarfs, most giants and some supergiants such as VY Canis Majoris, Antares and Betelgeuse are also class M. Furthermore, the larger, hotter brown dwarfs are late class M, usually in the range of M6.5 to M9.5.

The spectrum of a class M star contains lines from oxide molecules (in the visible spectrum, especially TiO) and all neutral metals, but absorption lines of hydrogen are usually absent. TiO bands can be strong in class M stars, usually dominating their visible spectrum by about M5. Vanadium(II) oxide bands become present by late M.

Spectral standards:^[2]

• M0IIIa – Beta Andromedae
• M2III – Chi Pegasi
• M1-M2Ia-Iab – Betelgeuse
• M2Ia – Mu Cephei

See also

• Stellar classification

References