Supergiant

Supergiant 

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.