M Star

M star Any member of a class of orange-red stars, the spectra of which are defined by molecular absorption bands of metallic oxides, particularly titanium oxide. Vanadium oxide is present in addition to some hydrides and water. Hydrogen is very weak and disappears towards cooler subclasses. Neutral calcium is strong; its weakening with increasing luminosity helps to determine the MK class. Only classes L and T are cooler than M stars. Main-sequence subclasses range from 2000 K at M9.5 to 3900 K at M0; zero-age masses range from the hydrogen-fusing limit near 0.08 solar mass to 0.5; zero-age luminosities range from 2 3 1024 to half solar (most of the radiation emerging in the infrared). Lifetimes far exceed that of the Galaxy. Long lifetimes and high rates of low-mass star formation make M stars numerous. Ignoring classes L and T (whose populations are not yet assessed), class M contains 70% of all dwarf stars (none are visible to the naked eye).

The convective envelopes of cool dwarfs extend more deeply as mass decreases, and at 0.3 solar mass (class M5) they reach the stellar centres, forcing cool M stars below about 3000 K to be completely mixed. Despite slow rotation, the deep convective zones somehow produce stellar magnetic fields and Sun-like activity. A good fraction of M dwarfs (dMe stars) display chromospheric emission and powerful flares that can brighten the stars by one or more magnitudes and that radiate across the spectrum.

No giant or supergiant is cooler than class M. Having evolved from dwarfs of classes G to O, such stars are much more massive than M dwarfs. As a result of lower densities, giant surface temperatures are somewhat cooler than their dwarf counterparts. M giants divide into those with quiet helium cores that are climbing the giant branch for the first time, and asymptotic giant branch (AGB) stars with quiet carbon-oxygen cores that are climbing it for the second time. First-ascent M giants have early subtypes and can reach luminosities of 1000 times solar and radii of 100 times solar. Second-ascent giants can reach into later subtypes, and become brighter (over 10,000 solar) and larger, with the more massive rivalling the diameter of Mars' orbit.

The brighter AGB stars pulsate as Mira variables, changing by five or more visual magnitudes over periods that range from 100 to 1000 days. Much of the visual variation is caused by small temperature variations that send the radiation into the infrared and create more powerful obscuring TiO bands. Bolometric (true luminosity) variation is much less, at about a magnitude. The pulsations create running shock waves, which generate emission lines, rendering Miras a class of me star. When M giants dredge the by-products of nuclear fusion (including carbon) to their surfaces, they become s stars and then carbon stars. Pulsations, plus the large luminosities and radii, promote powerful winds and mass-loss rates of 1025 solar masses (or more) per year. Silicate dust condenses in the winds of M class Miras; carbon dust condenses in the winds of carbon stars. The winds of class M Miras create vast envelopes that can radiate powerful OH maser emission, whence they are known as OH/IR (infrared) stars.

Among lower luminosity giants are found 'semi-regular' (SR) variables. SRa stars (S Aquilae, R Ursae Minoris) are similar to Miras but exhibit smaller light variations and irregularities in their periods. SRb stars (R Lyrae, W Orionis) have periods that are less well defined. 'Lb' (irregular) M giants have no periods at all.

The most luminous M supergiants develop from main-sequence O stars of up to 60 solar masses, most fusing helium in their cores. Bolometric luminosities can reach 750,000 times that of the Sun. Radii can approach 2000 times solar, rivalling the diameter of Saturn's orbit (nearly 10 AU). Many M supergiants are irregular (Lc) variables. betelgeuse (M2 Iab), for example, varies with a range of about half a magnitude on a timescale of years. Some of the lesser M supergiants (SRc stars) display some semi-regular periodicity.

Prominent examples of M stars include Proxima Cen-tauri M5 V, Betelgeuse M2 Iab, Antares M1.5 Ib, Mu Cephei M2 Ia, VV Cephei and M2 Iaep 1 O8 Ve.

Mu Cephei See garnet star

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