The Morgan–Keenan (or MK) system
The Morgan–Keenan (or MK) system is used in modern astronomy to classify stars according to their spectral type and luminosity class.
The spectral type of a star depends upon its effective temperature, which is the temperature that an idealised black body, with the same surface area as the star, would need to have in order to produce the same total energy output. The MK system uses the letters O, B, A, F, G, K, and M to designate a star’s spectral type, by subdividing the range of possible stellar temperatures, from the coolest, M, to the hottest, O. Each letter is subdivided into numbers, with 0 being the hottest and 9 the coolest.
O · B · A · F · G · K · M
A common mnemonic for remembering: “Oh, Be A Fine Guy/Girl: Kiss Me!”
Background Image: Star formation region Lupus 3
Credit: ESO/R. Colombari
Surface Temperature: ≥ 30,000 K | Main-sequence mass: ≥ 16☉
O-type stars are very hot and extremely luminous, with most of their radiated output in the ultraviolet range. These are the rarest of all main-sequence stars. About 1 in 3,000,000 (0.00003%) of the main-sequence stars in the solar neighborhood are O-type stars. Some of the most massive stars lie within this spectral class. O-type stars frequently have complicated surroundings that make measurement of their spectra difficult.
O-type Stars: S Monocerotis, O9V – 10 Lacertae
Surface Temperature: 10,000–30,000 K | Main-sequence mass: 2.1–16 M☉
B-type stars are very luminous and blue. Their spectra have neutral helium lines, which are most prominent at the B2 subclass, and moderate hydrogen lines. As O- and B-type stars are so energetic, they only live for a relatively short time. Thus, due to the low probability of kinematic interaction during their lifetime, they are unable to stray far from the area in which they formed, apart from runaway stars.
B-type Stars: Alnilam, Eta Aurigae, Rigel
Surface Temperature: 7,500–10,000 K | Main-sequence mass: 1.4–2.1 M☉
A-type stars are among the more common naked eye stars, and are white or bluish-white. They have strong hydrogen lines and also lines of ionized metals. About 1 in 160 (0.625%) of the main-sequence stars in the solar neighborhood are A-type stars, which includes 9 stars within 15 parsecs.
A-type Stars: Vega, Eta Leonis, Sirius A, Deneb
Surface Temperature: 6,000–7,500 K | Main-sequence mass: 1.04–1.4 M☉
F-type stars have strengthening spectral lines. Their spectra are characterized by the weaker hydrogen lines and ionized metals. Their color is white. About 1 in 33 (3.03%) of the main-sequence stars in the solar neighborhood are F-type stars, including 12 stars within 10 pc.
F-type Stars: Zeta Leonis, Alpha Leporis, Ursae Majoris
Surface Temperature: 5,200–6,000 K | Main-sequence mass: 0.8–1.04 M☉
G-type stars, including the Sun, have even weaker hydrogen lines than F, but along with the ionized metals, they have neutral metals. Class G main-sequence stars make up about 7.5%, nearly one in thirteen, of the main-sequence stars in the solar neighborhood.
G-type Stars: Sun, Beta Aquilae, Beta Aquarii, Kappa1 Ceti
Surface Temperature: 3,700–5,200 K | Main-sequence mass: 0.45–0.8 M☉
K-type stars are orangish stars that are slightly cooler than the Sun. They make up about 12% of the main-sequence stars in the solar neighborhood. There are also giant K-type stars. They have extremely weak hydrogen lines, if those are present at all, and mostly neutral metals. Mainstream theories would thus suggest such stars have the optimal chances of heavily evolved life developing on orbiting planets.
K-type Stars: Sigma Draconis, Pollux, Epsilon Cygni, Gamma Draconis
Surface Temperature: 2,400–3,700 K | Main-sequence mass: 0.08–0.45 M☉
Class M stars are by far the most common. About 76% of the main-sequence stars in the solar neighborhood are class M stars. However, class M main-sequence stars (red dwarfs) have such low luminosities that none are bright enough to be seen with the unaided eye. Although most class M stars are red dwarfs, most of the largest-ever supergiant stars in the Milky Way are M stars.
M-type Stars: Beta Andromedae, Chi Pegasi, Betelgeuse, Mu Cephei
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