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.
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.