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. This stellar classification is the most commonly used.
Background Image: Star formation region Lupus 3
Credit: ESO/R. Colombari
Spectral types
The spectral type of a star depends upon its effective temperature. 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!”
K= Kelvin | Colors referred to Illuminant D65 | M☉= solar mass
O-type
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
B-type
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
A-type
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
F-type
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
G-type
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
K-type
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
M-type
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
Luminosity Classification
This designation of spectral type is then appended by roman numerals indicating the luminosity class of the star. This is determined by the width of certain absorption lines in the star’s spectrum, which vary with the density of the star’s atmosphere. This luminosity classification, therefore, distinguishes between different types of giant stars and main sequence stars (or dwarf) stars.
The different luminosity classes are as follows:
0 or Ia+ hypergiants or extremely luminous supergiants.
Ia – luminous supergiants
Iab – intermediate luminous supergiants
Ib – less luminous supergiants
II – bright giants
III – normal giant stars
IV – subgiants
V main-sequence stars, also known as dwarf stars
Some prefixes are also used, such as:
sd prefix – subdwarf prefix
D prefix – white dwarf prefix
Background Image: Star formation region Lupus 3
Credit: ESO/R. Colombari
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