Luminosity Class

in astronomy, one of the parameters of the two-dimensional spectral classification of stars; the luminosity class characterizes the sequence to which the star belongs on the Hertzsprung-Russell diagram. Five luminosity classes are generally accepted: I—supergiants (Ia—brightest, lb—less luminous), II—bright giants, III—giants, IV—subgiants, and V—main-sequence stars.

Figure 1. Relation between absolute stellar magnitude and spectral class

As a supplement to the one-dimensional spectral classification of stars by temperature, the luminosity class makes it possible to classify spectra with respect to the physical state of the stellar atmosphere. The two-dimensional spectral classification based on this principle and proposed in the USA (the MK system) is represented on the diagram in Figure 1. The diagram makes it possible to find the absolute magnitudes of stars from their spectra and their luminosity classes. Since stars actually do not fall strictly into linear sequences but form bands (as a result of the differences in chemical composition and other parameters), new systems of two-dimensional and three-dimensional spectral classification have been proposed. For instance, there is a French system that takes into account the peculiarities of the continuous and ultraviolet spectra of stars.