Supergiants are among the most massive stars. They occupy the top region of the Hertzsprung-Russell diagram. In the Yerkes spectral classification, supergiants are class Ia (most luminous supergiants) or Ib (less luminous supergiants). They typically have bolometric absolute magnitudes between -5 and -12. The most luminous supergiants are often classified as hypergiants of class 0.
Supergiants can have masses from 10 to 70 solar masses and brightness from 30,000 up to hundreds of thousands times the solar luminosity. They vary greatly in radius, usually from 30 to 500, or even in excess of 1,000 solar radii. The Stefan-Boltzmann law dictates that the relatively cool surfaces of red supergiants radiate much less energy per unit area than those of blue supergiants; thus, for a given luminosity red supergiants are larger than their blue counterparts.
Because of their extreme masses they have short lifespans of 30 million years down to a few hundred thousand years (by the calculation M - 2.5 × 1010 where M = mass in sols).[1] They are mainly observed in young galactic structures such as open clusters, the arms of spiral galaxies, and in irregular galaxies. They 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.
Supergiants 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. In theory, hydrogen-fusing dwarf stars of 10 to around 60 Solar-masses first evolve as spectral type O to become blue supergiants and then progress to become red supergiants (type M). Stars with 30 to 60 Solar-masses then "loop back" from swollen and cooler, red supergiant phase back into much hotter but smaller blue supergiants; in contrast, those starting with more 60 Solar-masses remain as blue supergiants.
The modelling of supergiants 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 supergiants in terms of physical size, not mass, brightness or luminosity, are the supergiants KY Cygni, and μ Cephei (the Garnet Star).
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