Luminosity class is a classification of stellar spectra according to luminosity for a given spectral type; it was introduced as part of the Morgan-Keenan classification. The luminosity class broadly indicates whether a star is a dwarf (that is, a main sequence star), a giant, or a supergiant, since luminosity is directly related to surface area. Luminosity class is expressed as a Roman numeral, from I to V, and appears after the spectral type; for example, Tau Ceti, with spectral type G8, is listed as a G8V object.

 

Luminosity class	Description
Ia-0	Hypergiant (extreme supergiant)
Ia	Bright supergiant
Iab	Normal supergiant
Ib	Subluminous supergiant
II	Bright giants
III	Normal giants
IV	Subgiant
V	Main-sequence (dwarf) star