||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