If our Sun is an average-sized star, there are some true monsters out there. They are supergiant stars, and they come in two colours: red and blue. Supergiant stars are among the most massive and luminous stars in the universe. Supergiant stars are formed out of massive main-sequence stars that have run out of hydrogen in their cores.
This causes them to expand greatly, similarly to low-mass stars, however, they begin to fuse helium in their core not long after exhausting their hydrogen supplies. Supergiant stars vary in mass. They typically have at least eight times the mass of our Sun, and their luminosity is usually at least 1,000 times greater than our Sun, possibly even a million times greater. Their radius is no exception. It also varies greatly. Supergiant stars have a radius of at least 30 to even 1,000 times greater than our Sun. They are massive enough to begin helium-core burning before the core becomes degenerate, without a flash and the strong dredge-ups that lower-mass stars usually go through. Supergiant stars go on to successively ignite heavier elements, often all the way up to iron. They have very short lifespans, living from 30 million down to just a few hundred thousand years. Since they are so massive, they always end up exploding as supernovae. Currently, the largest known stars in terms of physical size, not mass or luminosity, are the supergiants VV Cephei, V354 Cephei, KW Sagitarii, KY Cygni, and the Garnet Star.
