Blue supergiant

Blue supergiants (BSGs) are hot luminous stars, referred to scientifically as OB supergiants. They have luminosity class I and spectral class B9 or earlier.

They are found towards the top left of the Hertzsprung–Russell diagram to the right of the main sequence. 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 the Sun.

Formation

Supergiants are evolved high-mass stars, larger and more luminous than main-sequence stars. O class and early B class stars with initial masses around 10-100 M_☉evolve away from the main sequence in just a few million years as their hydrogen is consumed and heavy elements start to appear near the surface of the star. These stars usually become blue supergiants, although it is possible that some of them evolve directly to Wolf–Rayet stars. 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 increase.