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Blue supergiant
Hot, luminous star with a spectral type of A9 or earlier / From Wikipedia, the free encyclopedia
A blue supergiant (BSG) is a hot, luminous star, often referred to as an OB supergiant. They are usually considered to be those with luminosity class I and spectral class B9 or earlier,[1] although sometimes A-class supergiants are also deemed blue supergiants.[2][3][4]
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Blue supergiants are found towards the top left of the Hertzsprung–Russell diagram, above and to the right of the main sequence. By analogy to the red giant branch for low-mass stars, this region is also called the blue giant branch.[2] They are larger than the Sun but smaller than a red supergiant, with surface temperatures of 10,000–50,000 K and luminosities from about 10,000 to a million times that of the Sun. They are most often an evolutionary phase between high-mass, hydrogen-fusing main-sequence stars and helium-fusing red supergiants,[4][5][6] although new research suggests they could be the result of stellar 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 the blue 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]