The Morgan-Keenan spectral classification
The following illustration represents star classes with the colors very close to those actually perceived by the human eye. The relative sizes are for main sequence or "dwarf" stars.
 Class O
Main article: O-type main sequence star
Class O stars are very hot and very luminous, being bluish in color; in fact, most of their output is in the ultraviolet range. These are the rarest of all main sequence stars. About 1 in 3,000,000 of the main sequence stars in the solar neighborhood are Class O stars.[nb 1] Some of the most massive stars lie within this spectral class. Type-O stars are so hot as to have complicated surroundings which make measurement of their spectra difficult.
Spectrum of an O5 V star
O-stars shine with a power over a million times our Sun's output. These stars have dominant lines of absorption and sometimes emission for He II lines, prominent ionized (Si IV, O III, N III, and C III) and neutral helium lines, strengthening from O5 to O9, and prominent hydrogen Balmer lines, although not as strong as in later types. Because they are so massive, class O stars have very hot cores, thus burn through their hydrogen fuel very quickly, and so are the first stars to leave the main sequence. Recent observations by the Spitzer Space Telescope indicate that planetary formation does not occur around other stars in the vicinity of an O class star due to the photoevaporation effect.
When the MKK classification scheme was first described in 1943, the only subtypes of class O used were O5 to O9.5. The MKK scheme was extended to O4 in 1978, and new classification schemes have subsequently been introduced which add types O2, O3 and O3.5. O3 stars are the hottest stars.
Examples: Zeta Orionis, Zeta Puppis, Lambda Orionis, Delta Orionis, Theta¹ Orionis C, HD 93129A
 Class B
Main article: B-type main sequence star
The Pleiades open star cluster with many bright B stars
Propper motion of stars specral classes B and A in -/+ 200 000 years
3D viewing (for red-green or red-blue glasses) of propper motion
Class B stars are extremely luminous and blue. Their spectra have neutral helium, which are most prominent at the B2 subclass, and moderate hydrogen lines. Ionized metal lines include Mg II and Si II. As O and B stars are so powerful, they only live for a very short time, and thus they do not stray far from the area in which they were formed.
These stars tend to cluster together in what are called OB associations, which are associated with giant molecular clouds. The Orion OB1 association occupies a large portion of a spiral arm of our galaxy and contains many of the brighter stars of the constellation Orion. About 1 in 800 of the main sequence stars in the solar neighborhood are Class B stars.[nb 1].
Examples: Rigel, Spica, the brighter Pleiades, VV Cephei B, Algol A
 Class A
Main article: A-type main sequence star
Class A Vega (left) compared to the Sun (right).
Class A stars are amongst the more common naked eye stars, and are white or bluish-white. They have strong hydrogen lines, at a maximum by A0, and also lines of ionized metals (Fe II, Mg II, Si II) at a maximum at A5. The presence of Ca II lines is notably strengthening by this point. About 1 in 160 of the main sequence stars in the solar neighborhood are Class A stars.[nb 1]
Examples: Sirius, Deneb, Altair, Vega
 Class F
Main article: F-type main sequence star
Class F stars have strengthening H and K lines of Ca II. Neutral metals (Fe I, Cr I) beginning to gain on ionized metal lines by late F. Their spectra are characterized by the weaker hydrogen lines and ionized metals. Their color is white. About 1 in 33 of the main sequence stars in the solar neighborhood are Class F stars.[nb 1]
Examples: Alrakis, Canopus, Procyon
 Class G
Main article: G-type main sequence star
The most important class G star to humanity: our Sun. The dark area visible in the lower left is a large sunspot.
The movement of stars of spectral class G around the apex (left) and antapex (right) in -/+ 200 000 years
The movement of stars of spectral class G for 3D glasses (red-green or red-blue).
Class G stars are probably the best known, if only for the reason that our Sun is of this class. About 1 in 13 of the main sequence stars in the solar neighborhood are Class G stars.[nb 1]
Most notable are the H and K lines of Ca II, which are most prominent at G2. They have even weaker hydrogen lines than F, but along with the ionized metals, they have neutral metals. There is a prominent spike in the G band of CH molecules. G is host to the "Yellow Evolutionary Void". Supergiant stars often swing between O or B (blue) and K or M (red). While they do this, they do not stay for long in the G classification as this is an extremely unstable place for a supergiant to be.
Examples: Sun, Alpha Centauri A, Capella, Tau Ceti
 Class K
Main article: K-type main sequence star
Class K are orangish stars which are slightly cooler than our Sun. Some K stars are giants and supergiants, such as Arcturus, while orange dwarfs, like Alpha Centauri B, are main sequence stars. They have extremely weak hydrogen lines, if they are present at all, and mostly neutral metals (Mn I, Fe I, Si I). By late K, molecular bands of titanium oxide become present. About 1 in 8 of the main sequence stars in the solar neighborhood are Class K stars.[nb 1] There is a suggestion that K Spectrum stars are very well suited for life.
Examples: Alpha Centauri B, Epsilon Eridani, Arcturus, Aldebaran, Algol B
 Class M
Main articles: Red giant and Red dwarf
Betelgeuse is a red supergiant, one of the largest stars known. Image from the Hubble Space Telescope.
Class M is by far the most common class. About 76% of the main sequence stars in the solar neighborhood are Class M stars.[nb 1][nb 2]
Although most Class M stars are red dwarfs, the class also hosts most giants and some supergiants such as Antares and Betelgeuse, as well as Mira variables. The late-M group holds hotter brown dwarfs that are above the L spectrum. This is usually in the range of M6.5 to M9.5. The spectrum of an M star shows lines belonging to molecules and all neutral metals but hydrogen lines are usually absent. Titanium oxide can be strong in M stars, usually dominating by about M5. Vanadium oxide bands become present by late M.
Examples: Betelgeuse, Antares (supergiants)
Examples: Proxima Centauri, Barnard's star, Gliese 581 (red dwarf)
Examples: LEHPM 2-59 , SSSPM J1930-4311 (subdwarf)
Example: APMPM J0559-2903 (extreme subdwarf)
Examples: Teide 1 (field brown dwarf), GSC 08047-00232 B  (companion brown dwarf)