Red supergiants are supergiant stars (luminosity class I) of spectral type K or M. They are the largest stars in the universe in terms of volume although they are not the most massive. Betelgeuse (α Orionis) and Antares (α Scorpii) are the best known examples of a red supergiant.
Red giants also may evolve into Blue supergiants in the future. One example is Rigel, which was suspected to once be a supergiant. Red supergiants may blow off their outer layers, forming back into a blue supergiants. Blue supergiants are smaller, therefore are hotter.