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supergiant(soo -per-jÿ-ănt) The largest and most luminous type of star, lying above both the main sequence and the giant region in the Hertzsprung–Russell diagram. They are grouped in luminosity classes Ia and Ib (see spectral types) and generally have absolute bolometric magnitudes between –5 and –12. Only the most massive stars can become supergiants and consequently they are very rare. They are so bright, however, that they stand out in external galaxies. There is an upper limit to the absolute bolometric magnitude of cool red supergiants (–9.7), so the brightest supergiants can be used as approximate distance indicators. Examples of supergiant stars are Rigel and Betelgeuse in Orion, Antares in Scorpius, and Cepheid variable stars. See also luminous blue variables.
in astronomy, a massive star of the highest luminosity. The absolute stellar magnitudes of supergiants reach values of - 7 and - 8. Supergiants include stars of various spectral classes. The diameters of cool (red) supergiants, such as Betelgeuse or the red component of VV Cephei, are hundreds or thousands of times greater than the diameter of the sun. The diameters of hot supergiants, such as Rigel, exceed the diameter of the sun by a factor of 20 to 30. The total fraction of supergiants among all the stars is small. Supergiants occur in stellar associations and in younger open clusters; they often are components of binary systems. Observations indicate an escape of matter from the surface of many supergiants as well as other signs of instability. All such phenomena make the supergiants particularly important for the development of a theory of stellar evolution.