Red Supergiants

The Orion region showing the red supergiant Betelgeuse
By Rogelio Bernal Andreo CC BY-SA 3.0

Red supergiants have the largest radius of all known stars. They have low surface below 4,100 K. This is very cool for a star and makes them to shine with a red colour. The star Betelgeuse in the constellation of Orion is a red supergiant.

Red supergiants evolve from large main sequence stars. These are stars which contain more than 8 times the mass of our Sun. Some stars are born with more than 200 times the mass of the Sun! 

Like all stars, massive stars release energy by fusing elements together. However, massive stars use up their supply of hydrogen quite quickly, within 5 - 20 million years. The star then begins fusing the helium in its core. This creates heavier elements, like carbon. Hydrogen fusion continues in a shell of hydrogen around the helium core.

These changes release a lot of energy which causes the star to expand. As the star gets bigger, the heat is spread over a larger area, so the surface cools down and glows with a redder colour. These enormous, cool stars are red supergiants.

Supergiants burn all the helium in their cores within a few million years. They then start to burn the carbon. This continues with heavier and heavier elements until the star contains a core of iron. At this point, fusion stops and the star collapses under its own gravity. This creates a supernova explosion.