Blue supergiant
Hot, luminous stars often evolving from high-mass main-sequence stars.
Wikipedia / Wikimedia Commons
A blue supergiant is a massive, intensely hot and bright star, sometimes called an OB supergiant. Astronomers typically classify them as having luminosity class I and a spectral type of B9 or hotter, though some A-class supergiants are also included. On the Hertzsprung–Russell diagram, they sit in the upper-left region, above and to the right of the main sequence—an area sometimes called the blue giant branch, by analogy with the red giant branch for lower-mass stars. These stars are bigger than the Sun but smaller than red supergiants, with surface temperatures ranging from 10,000 to 50,000 Kelvin and luminosities between 10,000 and one million times that of the Sun. They usually represent a transitional phase between high-mass main-sequence stars that fuse hydrogen and red supergiants that fuse helium, though recent studies suggest some may form through stellar mergers.
Most supergiants are actually blue (B-type) supergiants, and those in classes O9.5 to B2 are even more common than their main-sequence counterparts. Observations show more post-main-sequence blue supergiants than theoretical models predict, given their expected short lifetimes. This discrepancy is known as the blue supergiant problem. Unusual internal structures—such as hotter blue supergiants having oversized hydrogen-fusing cores and cooler ones having undersized helium-fusing cores—may help explain it.
Formation
It was once thought that blue supergiants formed by "feeding" on interstellar material as stars passed through dust clouds, but the current view is that they are evolved high-mass stars, a natural outcome of stellar evolution. They are larger and more luminous than main-sequence stars. O-type and early B-type stars with initial masses between about 10 and 300 solar masses leave the main sequence within a few million years as their hydrogen is consumed and heavier elements (up to iron) appear near the surface. These stars typically become blue supergiants, though some of the most massive ones may evolve directly into Wolf–Rayet stars. Expansion into the supergiant phase occurs when the core's hydrogen is depleted and hydrogen shell burning begins, but it can also be triggered by convection dredging heavy elements to the surface and by increased mass loss from radiation pressure.
Blue supergiants have only recently left the main sequence, so they are extremely luminous, have hi
- field
- Astronomy
- known_for
- Hot, luminous stars; supernova progenitors; examples include Rigel and Deneb
- temperature_range
- 10,000–50,000 K
- luminosity_range
- 10,000 to 1,000,000 times the Sun
- mass_range
- 10–300 M☉
Lore & Background
Blue supergiants are evolved high-mass stars, a natural consequence of stellar evolution, larger and more luminous than main-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 and heavy elements start to appear near the surface. These stars usually become blue supergiants, though some may evolve directly to Wolf–Rayet stars. Expansion into the supergiant stage occurs when hydrogen in the core is depleted and hydrogen shell burning starts, or as heavy elements are dredged up to the surface by convection and mass loss due to radiation pressure increases.
Blue supergiants are newly evolved from the main sequence, have extremely high luminosities, high mass loss rates, and are generally unstable. Many become luminous blue variables (LBVs) with episodes of extreme mass loss. Lower mass blue supergiants continue to expand until they become red supergiants, passing through a yellow supergiant or yellow hypergiant phase in just a few thousand years. Higher mass red supergiants can blow away their outer atmospheres and evolve back to blue supergiants, possibly onwards to Wolf–Rayet stars. The most massive blue supergiants are too luminous to retain an extensive atmosphere and never expand into a red supergiant.
Reader's Guide
Blue supergiants are significant because they represent a critical, short-lived phase in the evolution of high-mass stars. They are heavily represented among stars visible to the naked eye despite their rarity and short lives, with the best known example being Rigel. Their study has forced a re-examination of supernova progenitor theory, as SN 1987A's progenitor was a B3 blue supergiant, contradicting the earlier belief that only red supergiants could explode as supernovae. Now it is known that almost any class of evolved high-mass star, including blue and yellow supergiants, can explode as a supernova, producing a wide range of luminosities, durations, and spectral types. The blue supergiant problem—more post-main-sequence blue supergiants observed than theoretical models predict—remains an area of active research, with possible explanations involving unusual stellar interiors such as oversized hydrogen-fusing cores in hotter blue supergiants and undersized helium-fusing cores in cooler ones. Their fast stellar winds and role in creating concentric faint shells through interactions with slower winds from previous red supergiant phases contribute to the complex dynamics of stellar mass loss and the interstellar medium.
Did You Know?
- Blue supergiants have surface temperatures of 10,000–50,000 K and luminosities from about 10,000 to a million times that of the Sun.
- The best known example is Rigel, with a mass about 20 times that of the Sun and luminosity around 117,000 times greater.
- SN 1987A forced astronomers to re-examine supernova theory because its progenitor was a B3 blue supergiant.
- Many blue supergiant stars are Alpha Cygni variables.
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