The Yerkes or Morgan-Keenan (MK) classification system is almost universal.
Supergiants have the lowest surface gravities and hence are the largest and brightest at a particular temperature. The luminosity differences between stars are most apparent at low temperatures, where giant stars are much brighter than main-sequence stars. Larger stars are more luminous at a given temperature and can now be grouped into bands of differing luminosity. This system uses certain diagnostic spectral lines to estimate the surface gravity of a star, hence determining its size relative to its mass. Stars are classified as supergiants on the basis of their spectral luminosity class. Betelgeuse and Antares A are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous.
Red supergiants ( RSGs) are stars with a supergiant luminosity class ( Yerkes class I) of spectral type K or M.
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