Supergiant

Supergiant

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 or Ia+.

Supergiants have masses of 10 or more times the sun and luminosities from 30,000 to over a million times the sun (L_☉). They vary greatly in radius, usually from 30 to 500, or even in excess of 1,000 solar radii (R_☉). 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. Radiation pressure limits the largest cool supergiants to around 1,500 R_☉ and the most massive hot supergiants to around a million L_☉ (M_V around -9). Stars near and occasionally beyond these limits become unstable, pulsate, and experience rapid mass loss.

O type main sequence stars and the most massive of the B type blue-white stars become supergiants. Because of their extreme masses they have short lifespans of 30 million years down to a few hundred thousand years. 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 40 Solar-masses first evolve from the main sequence to become blue supergiants and then progress to become red supergiants. Red supergiants perform "blue loops" as they initiate burning of heaver and heavier elements in their cores, expelling their atmospheres to become smaller and hotter, then expanding again until they either explode as a supernova or finally lose their atmosphere to become a blue supergiant. In contrast stars with more massive than about 40 M_☉ remain as blue supergiants while some very massive rotating stars evolve directly from the main sequence to Wolf-Rayet stars without a supergiant phase. The modelling of supergiants is still an active area of research and is made more difficult by issues such as stellar mass loss which can vary greatly in binary systems, rotating stars, or stars with different metallicity from the sun.

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, while the less common type Ib/c supernovae are produced by hotter Wolf-Rayet stars that have completely lost more of their hydrogen atmosphere. The progenitor for the unusual Supernova 1987A was a blue supergiant, thought to have already passed through the red supergiant phase of its life, and much research is now focused on how blue supergiants can explode as a supernova.