Ablue supergiant (BSG) is a hot, luminousstar, often referred to as an OBsupergiant. They are usually considered to be those withluminosity classI andspectral class B9 or earlier,[1] although sometimes A-class supergiants are also deemed blue supergiants.[2][3][4]
Blue supergiants are found towards the top left of theHertzsprung–Russell diagram, above and to the right of the main sequence. By analogy to thered giant branch forlow-mass stars, this region is also called theblue giant branch.[2] They are larger than theSun but smaller than ared supergiant, with surface temperatures of 10,000–50,000 K andluminosities from about 10,000 to a million times that of the Sun. They are most often an evolutionary phase between high-mass, hydrogen-fusingmain-sequence stars and helium-fusing red supergiants,[4][5][6] although new research suggests they could be the result ofstellar mergers.[7][8]
The majority of supergiants are also blue (B-type) supergiants; blue supergiants from classes O9.5 to B2 are even more common than their main sequence counterparts.[9] More post-main-sequence blue supergiants are observed than what is expected from theoretical models, which expect blue supergiants to be short-lived. This results in theblue supergiant problem, although unusual stellar interiors (such as hotter blue supergiants having oversized hydrogen-fusing cores and cooler ones having undersized helium-fusing cores) may explain this.[10]
It was once believed that blue supergiants originated from a "feeding" with theinterstellar medium when stars passed through interstellar dust clouds,[11][8] although the current consensus is that blue supergiants are evolved high-mass stars, a natural consequence of stellar evolution, larger and more luminous thanmain-sequence stars. O-type and early B-type stars with initial masses around 10–300 M☉evolve away from the main sequence in just a few million years as their hydrogen is consumed andheavy elements (with atomic numbers of 26 (Fe) and less) start to appear near the surface of the star. These stars usually become blue supergiants, although it is possible that some of them (particularly the more massive ones) evolve directly toWolf–Rayet stars.[12] Expansion into the supergiant stage occurs when hydrogen in the core of the star is depleted and hydrogen shell burning starts, but it may also be caused as heavy elements are dredged up to the surface by convection and mass loss due to radiation pressure increases.[13]
Blue supergiants are newly evolved from the main sequence, have extremely high luminosities, high mass loss rates, and are generally unstable. Many of them becomeluminous blue variables (LBVs) with episodes of extreme mass loss. Lower mass blue supergiants continue to expand until they become red supergiants. In the process they must spend some time asyellow supergiants oryellow hypergiants, but this expansion occurs in just a few thousand years and so these stars are rare. Higher mass red supergiants blow away their outer atmospheres and evolve back to blue supergiants, and possibly onwards to Wolf–Rayet stars.[14][15] Depending on the exact mass and composition of a red supergiant, it can execute a number of blue loops before either exploding as atype II supernova or finally dumping enough of its outer layers to become a blue supergiant again, less luminous than the first time but more unstable.[16] If such a star can pass through the yellow evolutionary void it is expected that it becomes one of the lower luminosity LBVs.[17]
The most massive blue supergiants are too luminous to retain an extensive atmosphere and they never expand into a red supergiant. The dividing line is approximately 40 M☉, although the coolest and largest red supergiants develop from stars with initial masses of 15–25 M☉. It is not clear whether more massive blue supergiants can lose enough mass to evolve safely into old age as a Wolf Rayet star and finally a white dwarf, or they reach the Wolf Rayet stage and explode assupernovae, or they explode as supernovae while blue supergiants.[12]
Supernova progenitors are most commonly red supergiants and it was believed that only red supergiants could explode as supernovae.SN 1987A, however, forced astronomers to re-examine this theory, as its progenitor,Sanduleak -69° 202, was a B3 blue supergiant.[18] Now it is known from observation that almost any class of evolved high-mass star, including blue and yellow supergiants, can explode as a supernova although theory still struggles to explain how in detail.[19] While most supernovae are of the relatively homogeneous type II-P and are produced by red supergiants, blue supergiants are observed to produce supernovae with a wide range of luminosities, durations, and spectral types, sometimes sub-luminous like SN 1987A, sometimes super-luminous such as many type IIn supernovae.[20][21][22]
Because of their extreme masses they have relatively short lifespans and are mainly observed in young cosmic structures such asopen clusters, the arms ofspiral galaxies, and inirregular galaxies. They are rarely observed in spiral galaxy cores,elliptical galaxies, orglobular clusters, most of which are believed to be composed of older stars, although the core of the Milky Way has recently been found to be home to several massive open clusters and associated young hot stars.[23]
The best known example isRigel, the brightest star in the constellation ofOrion. Its mass is about 20 times that of the Sun, and itsluminosity is around 117,000 times greater. Despite their rarity and their short lives they are heavily represented among the stars visible to the naked eye; their immense brightness is more than enough to compensate for their scarcity.[citation needed]
Blue supergiants have fast stellar winds and the most luminous, calledhypergiants, have spectra dominated by emission lines that indicate strong continuum driven mass loss. Blue supergiants show varying quantities of heavy elements in their spectra, depending on their age and the efficiency with which the products ofnucleosynthesis in the core are convected up to the surface. Quickly rotating supergiants can be highly mixed and show high proportions of helium and even heavier elements while still burning hydrogen at the core; these stars show spectra very similar to a Wolf Rayet star.[citation needed]
While the stellar wind from a red supergiant is dense and slow, the wind from a blue supergiant is fast but sparse. When a red supergiant becomes a blue supergiant, the faster wind it produces impacts the already emitted slow wind and causes the outflowing material to condense into a thin shell. In some cases, several concentric faint shells can be seen from successive episodes of mass loss, either previous blue loops from the red supergiant stage, or eruptions such as LBV outbursts.[25]
^Massey, P.; Puls, J.; Pauldrach, A. W. A.; Bresolin, F.; Kudritzki, R. P.; Simon, T. (2005). "The Physical Properties and Effective Temperature Scale of O-Type Stars as a Function of Metallicity. II. Analysis of 20 More Magellanic Cloud Stars and Results from the Complete Sample".The Astrophysical Journal.627 (1):477–519.arXiv:astro-ph/0503464.Bibcode:2005ApJ...627..477M.doi:10.1086/430417.S2CID18172086.
^abGeorges Meynet; Cyril Georgy; Raphael Hirschi; Andre Maeder; Phil Massey; Norbert Przybilla; Fernanda Nieva (2011). "Red Supergiants, Luminous Blue Variables and Wolf-Rayet stars: The single massive star perspective".Bulletin de la Société Royale des Sciences de Liège.80 (39):266–278.arXiv:1101.5873.Bibcode:2011BSRSL..80..266M.
^Eggenberger, P.; Meynet, G.; Maeder, A. (2009). "Modelling massive stars with mass loss".Communications in Asteroseismology.158: 87.Bibcode:2009CoAst.158...87E.
^Kleiser, I.; Poznanski, D.; Kasen, D.; et al. (2011). "The Peculiar Type II Supernova 2000cb".Bulletin of the American Astronomical Society.43: 33726.Bibcode:2011AAS...21733726K.
^Saio, H.; Georgy, C.; Meynet, G. (2013). "Strange-Mode Instability for Micro-Variations in Luminous Blue Variables".Progress in Physics of the Sun and Stars: A New Era in Helio- and Asteroseismology. Proceedings of a Fujihara Seminar held 25–29 November. Astronomical Society of the Pacific Conference Series. Vol. 479. p. 47.arXiv:1305.4728.Bibcode:2013ASPC..479...47S.
^Przybilla, N.; Butler, K.; Becker, S. R.; Kudritzki, R. P. (2005-09-22). "Quantitative spectroscopy of BA-type supergiants".Astronomy & Astrophysics.445 (3):1099–1126.arXiv:astro-ph/0509669.doi:10.1051/0004-6361:20053832.
