Classification of stars is based primarily on their temperatures. The Harvard spectral classification scheme assigns each star a spectral type which is further divided into 10 sub-classes depending on the absorption features present in the spectrum. For example, our Sun has a temperature of about 5,700 Kelvin and is classified as a G2 star.

However, this classification scheme does not completely describe the star as it cannot distinguish between stars with the same temperature but different luminosities. In other words, it cannot distinguish between main sequence (dwarf) stars, giant stars and supergiant stars.

For this reason, the Morgan-Keenan luminosity class (MK or MKK) was established. Originally containing roman numerals between I (supergiant star) and V (main sequence), these days, class I stars have been subdivided into Ia-O, Ia and Ib, and classes VI (sub-dwarf) and D (white dwarf) have been added.

To completely describe the star, the MK luminosity class is appended to the original Harvard classification for the star. For example, our Sun is a main sequence G2 star, therefore its full classification is G2V.

The following table summarises the MK Luminosity Classes:

See also: Hertzsprung-Russell Diagram.