Red supergiant

Red supergiants are stars with a supergiant luminosity class (Yerkes class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Antares are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars.

Stars are classified as supergiants on the basis of their spectral luminosity class. This system uses certain diagnostic spectral lines to estimate the surface gravity of a star, hence determining its size relative to its mass. Larger stars are more luminous at a given temperature and can now be grouped into bands of differing luminosity.

The luminosity differences between stars is most apparent at low temperatures, where giant stars are much brighter than main-sequence stars. Supergiant have the lowest surface gravities and hence are the largest and brightest at a particular temperature.

The Yerkes or Morgan-Keenan (MK) classification system is almost universal. It groups stars into five main luminosity groups designated by roman numerals:

Specific to supergiants, the luminosity class is further divided into normal supergiants of class Ib and bright supergiants of class Ia. The intermediate class Iab is also used. Exceptionally bright, low surface gravity, stars with strong indications of mass loss may be designated by luminosity class 0 (zero) although this is rarely seen. More often the designation Ia-0 will be used, and more commonly still Ia⁺. These hypergiant spectral classifications are very rarely applied to red supergiants, although the term hypergiant is sometimes used for the most extended and unstable red supergiants.

The "red" part of "red supergiant" refers to the cool temperature. Red supergiants are the coolest supergiants, M-type and at least some K-type stars although there is no precise cutoff. K-type supergiants are uncommon compared to M-type, because they are a short-lived transition stage and somewhat unstable. The K-type stars, especially early or hotter K types, are sometimes described as orange supergiants (e.g. Zeta Cephei), or even as yellow (e.g. yellow hypergiant HR 5171A).

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