Red supergiants have the largest radius of all known stars. They have low surface temperatures (for stars!) of below 4,100 K. This causes 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 that 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 create energy through nuclear fusion, but these stars use up all of their supply of hydrogen after 5 - 20 million years. Their core then becomes made of helium, which starts to burn instead. A shell of hydrogen around the core also starts to fuse together. This creates lots of energy which causes the star to expand. As the star gets bigger it cools down becoming a red colour. These enormous, cool stars are known as supergiants.
Supergiants will burn all of the helium in their cores within a few million years. They will then start to burn carbon. This continues with heavier and heavier elements until the star contains a core of iron. At this stage the star can no longer fuse elements to produce energy so the star collapses under gravity and produces a supernova.