Blue Supergiant - Formation

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 can occur when helium burning begins in the core of the star 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.

Blue supergiants newly evolved from the main sequence have extremely high luminosities, high mass loss rates, and are generally unstable. Many of them become luminous blue variables with episodes of extreme mass loss. Lower mass blue supergiants continue to expand until they become red supergiants. In the process they obviously must spend some time as yellow supergiants or yellow hypergiants, but this expansion is extremely rapid and only a handful of known stars are thought the be evolving for the first time redwards through the yellow evolutionary void. Red supergiants blow away their outer atmospheres and evolve bluewards before settling down and expanding again, a process known as a blue loop.

Depending on the exact mass and composition of a red supergiant, it will execute a number of blue loops before either exploding as a type 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. Many or all of these are again LBVs. The blue loops and any final passage back to a blue supergiant cause the star to become a yellow supergiant for a time. The majority of observed yellow supergiants have previously been red supergiants. These post-red supergiants are of much lower mass than before they become red supergiants, but they have similar luminosities to lower mass main sequence stars just becoming supergiants for the first time.

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 isn't clear whether more massive blue supergiants can lose enough mass to evolve safely into a comfortable old age as a Wolf Rayet star and finally a white dwarf, whether they reach the Wolf Rayet stage and explode as supernovae, or whether they explode as supernovae while blue supergiants.

Supernova progenitors are most commonly red supergiants and it used to be expected that only red supergiants exploded as supernovae. Supernova 1987A forced a rethink as the progenitor was a B3 blue supergiant. Now it is known from observation that almost any class of evolved high mass star, including blue supergiants, can explode as a supernova although theory still struggles to explain how in detail. 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.