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Supergiant stars have an exceptionally high mass.
Supergiant stars can have masses from 10 to 70 solar masses. Supergiant stars are some of the brightness stars from 30,000 up to hundreds of thousands times the solar luminosity. Supergiant stars vary greatly in radii, usually from 30 to 500, or even in excess of 1000 solar radii. The Stefan-Boltzmann law dictates that the relatively cool surfaces of red supergiant stars radiate much less energy per unit area than those of blue supergiant stars; thus, for a given luminosity red supergiant stars are larger than their blue supergiant stars counterparts.
Supergiant stars are among the most massive stars in the universe. In the Hertzsprung-Russell diagram supergiant stars occupy the top region of the diagram. In the Yerkes spectral classification supergiant stars are class Ia (most luminous supergiant stars) or Ib (less luminous supergiant stars). Supergiant stars typically have bolometric absolute magnitudes between -5 and -12. The most luminous supergiant stars are often classified as hypergiant stars of class 0.
Because of their extreme masses supergiant stars have short lifespans of 30 million years down to a few hundred thousand years (by the equation M - 2.5 × 1010 where M = mass in sols). Supergiant stars are mainly observed in young galactic structures such as open clusters, the arms of spiral galaxies, and in irregular galaxies. Supergiant stars are less abundant in spiral galaxy bulges, and are rarely observed in elliptical galaxies, or globular clusters, which are believed to be composed of old stars.
Supergiant stars occur in every spectral class from young blue class O supergiants stars to highly evolved red class M supergiants. Rigel, the brightest star in the constellation Orion is a typical blue-white supergiant, whereas Betelgeuse and Antares are red supergiants.
The modelling of supergiant stars is still an active area of research and is made more difficult by issues such as stellar mass loss. Rather than modelling individual stars, the latest trend has been to model clusters of stars and then compare the distribution of the resulting models with the observed supergiant distributions in galaxies like the Magellanic Clouds.
The first stars in the universe are thought to have been considerably brighter and more massive than the stars in the modern universe. These stars were part of the theorized population III of stars. Their existence is necessary to explain observations of elements other than hydrogen and helium in quasars.
Most type II supernova progenitors are thought to be red supergiants. However, the progenitor for Supernova 1987A was a blue supergiant. It is believed that it was a red supergiant before losing its outer layers to its strong stellar wind.
Currently, the largest known stars in terms of physical size, not mass, brightness or luminosity, are the supergiants VY Canis Majoris, VV Cephei, V354 Cephei, KW Sagittarii, KY Cygni, and µ Cephei (the Garnet Star).
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