A star classified after the Morgan-Keenan spectral classification.

A solar classification originated on Pre Astro Earth and still used in older  catalogs, no longer official Union.

The Morgan–Keenan spectral classification

The Yerkes spectral classification, also called the MKK system from the authors' initials, is a system of stellar spectral classification introduced in 1943 by William Wilson Morgan, Philip C. Keenan, and Edith Kellman from Yerkes Observatory.[12] This two-dimensional (temperature and luminosity) classification scheme is based on spectral lines sensitive to stellar temperature and surface gravity which is related to luminosity (whilst the Harvard classification is based on surface temperature only). Later, in 1953, after some revisions of list of standard stars and classification criteria, the scheme was named MK (by William Wilson Morgan and Philip C. Keenan's initials).[13]

Denser stars with higher surface gravity exhibit greater pressure broadening of spectral lines. The gravity, and hence the pressure, on the surface of a giant star is much lower than for a dwarf star because the radius of the giant is much greater than a dwarf of similar mass. Therefore differences in the spectrum can be interpreted as luminosity effects and a luminosity class can be assigned purely from examination of the spectrum.

A number of different luminosity classes are distinguished:

0, Ia-0, Ia+ hypergiants or extremely luminous supergiants (later addition)), Example: Eta Carinae (spectrum-peculiar)

Ia (luminous supergiants), Example: Deneb (spectrum is A2 Ia)

Iab (intermediate luminous supergiants), Example: Betelgeuse (spectrum is M2 Iab)

Ib (less luminous supergiants), Example:

II bright giants, Example: β Scuti (HD 173764) (spectrum is G5 II)

III normal giants, Example: ρ Persei (spectrum is O7.5 III(n)((f)))

IV subgiants, Example: ε Reticuli (spectrum is K1–2 IV)

V main-sequence stars (dwarfs), Example: AD Leonis (spectrum M3.5e V)

VI subdwarfs, Example: SSSPM J1930-4311 (spectrum sdM7)

VII (uncommon) white dwarfs. White dwarfs are represented with a prescript wD or WD.

Marginal cases are allowed; for instance a star classified as Ia-0 would be a very luminous supergiant, verging on hypergiant. Examples are below. The spectral type of the star is not a factor.

Marginal symbols Example Explanation

- G2 I-II A star is between supergiant and bright giant.

+ O9.5 Ia+ A star is a hypergiant star.

/ F2 IV/V A star is either a subgiant or a dwarf star.  

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