Blue supergiant

A blue supergiant is an incredibly hot and luminous star. These massive stars are often called OB supergiants. They belong to luminosity class I and typically have a spectral class of B9 or earlier. Sometimes, scientists even include A-class supergiants in this group. On the Hertzsprung–Russell diagram, they sit at the top left. This specific region is known as the blue giant branch.

These stars are much larger than our Sun. Their surface temperatures are extremely high, ranging from 10,000 to 50,000 K. Their brightness, or luminosity, is also massive. A single blue supergiant can be 10,000 to 1,000,000 times more luminous than the Sun. They represent a specific phase in the life of a high-mass star. Most are an evolutionary step between hydrogen-fusing stars and red supergiants. However, new research suggests some might form from stellar mergers.

The transition to a supergiant involves complex physical changes. It begins when the hydrogen in the star's core is depleted. This depletion triggers hydrogen shell burning around the core. As this happens, the star expands significantly. Other factors can also cause this expansion. For example, convection can dredge up heavy elements to the surface. Radiation pressure can also cause the star to lose mass.

Stars follow different paths depending on their initial mass. O-type and early B-type stars evolve away from the main sequence quickly. This happens in just a few million years as hydrogen is consumed. During this time, heavy elements appear near the surface. Some massive stars may skip the supergiant stage and become Wolf–Rayet stars. Lower mass blue supergiants will eventually expand into red supergiants. They may pass through a yellow supergiant stage first. This expansion is very fast, lasting only a few thousand years.

High-mass stars have very short lifespans because they use fuel so quickly. Because of this, we find them in young cosmic structures. They are common in open clusters and the arms of spiral galaxies. You will rarely find them in old globular clusters or elliptical galaxies. One famous example is Rigel, located in the constellation Orion. Rigel is about 20 times the mass of the Sun. It is also 117,000 times more luminous than our Sun.

Historically, our understanding of these stars has changed. Scientists once thought blue supergiants grew by feeding on interstellar dust clouds. We now know they are simply evolved high-mass stars. Another major shift involved the study of supernovae. It was once believed that only red supergiants could explode. However, the supernova SN 1987A changed everything. Its progenitor, Sanduleak -69° 20, was a B3 blue supergiant.

Today, we know that almost any evolved high-mass star can explode. This includes blue and yellow supergiants. Blue supergiants produce supernovae with many different characteristics. Some are sub-luminous, like SN 1987A. Others are super-luminous, such as type IIn supernovae. These stars also feature fast stellar winds. While a red supergiant has a dense and slow wind, a blue supergiant has a fast but sparse wind. When a star evolves from a red to a blue supergiant, these winds collide. This collision can cause material to condense into thin shells in space.