Classification systems
Kappa mechanism
A classification of stellar spectra according to luminosity for a given spectral type. The luminosity class is an indication of a star's surface gravity. It is shown by a Roman numeral as follows: I (supergiants), II (bright giants), III (normal giants), IV (subgiants), and V (dwarf stars, or main-sequence stars). Luminosity classes VI (subdwarfs) and VII (white dwarfs) are rarely used. Subclasses a, b, and c are especially used for supergiants, while the most luminous hypergiants are assigned luminosity class Ia-0.
Luminosity classification
Luminosity class