Stars are classified according to their temperature through the Harvard spectral classification scheme. Unfortunately this scheme is not sufficient to completely describe the star, as it does not distinguish between stars that have the same temperature but different luminosities. i.e. it cannot distinguish between a main sequence (dwarf) star, a giant star star and a supergiant star.

To fully classify stellar spectra, a stellar luminosity class was also required. This was defined by William Morgan, Philip Childs Keenan and Edith Marie Kellman in 1943 and is known as the Morgan-Keenan luminosity class.