Luminosity Class

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Related to Stellar classification: Spectral class

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.


References in periodicals archive ?
In September 1910, at an international meeting of astronomers held in Pasadena, California, Pickering led an afternoon session to discuss stellar classification.
Finally, at the first General Assembly of the International Astronomical Union in May 9, 1922, the IAU passed a resolution to formally adopt Cannon's stellar classification system.
The L spectral type follows M in the stellar spectral sequence and represents the first addition to the stellar classification scheme in more than 50 years (S&T: November 1998, page 26).
The students learn the basics of spectroscopy and stellar classification from astronomers at Harvard Observatory, search through the half million plates in Harvard's collection, browse many of Cannon's personal papers, and then see how astronomers today use the HD classifications in research.
Regarding the stellar classifications OBAFGKM, when I was studying astronomy at the University of Texas at Austin in the mid-1950s it was OBAFGKMRNS--or "Oh Be A Fine Girl Kiss Me Right Now Smack.
Of course, given that the hot O stars are the rarest of the major stellar classifications we see, it should not be too surprising that a 2nd-magnitude member of this class is going to rank high.