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

Supergiants have the largest radius of all known stars. They evolve from large main-sequence stars. Supergiants can contain 8 - 200 times the mass of the Sun! They also shine brightly. One supergiant can be as bright as a million Suns.

The colour of supergiants can be red or blue. Red supergiants have low surface temperatures, below 4,100 K. This is very cool for a star and makes them shine with a red colour. The star Betelgeuse in the constellation of Orion is a red supergiant. Blue supergiants are much hotter. They can be 12,000 to 15,000 K which makes them the hottest stars in the Universe. The star Rigel in Orion is a blue supergiant.

Supergiants use up the hydrogen and 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.