A classification of stars according to their luminosity, which can vary widely for a given spectral type. Luminosity class indicates, for example, whether a star is a supergiant, a giant, or a dwarf. The luminosity class is assigned by examination of the star’s spectrum, looking at luminosity-sensitive lines or line ratios. The luminosity classes generally used are:

• Ia–0 Extreme supergiants (sometimes called hypergiants)

• Ia Luminous supergiants

• Iab Normal supergiants

• Ib Underluminous supergiants

• II Bright giants

• III  Giants

• IV Subgiants

• V  Dwarfs (main-sequence stars)

The accompanying diagram shows the luminosity classes on a Hertzsprung–Russell diagram. See also spectral classification.

luminosity class:

A plot of the absolute magnitude of stars against their spectral type shows different luminosity classes.