The biggest red giants are the largest stars in the universe, swollen to diameters of a billion kilometres or more by changes in their cores as they near the end of their lives. As they swell in size and brighten to hundreds of thousands of times solar luminosity, their surfaces cool to a distinctive red colour. But many scientists say these stars are supergiants rather than true hypergiants.
Yellow supergiants seem to be a rare intermediate stage, though again they get their name from their size and brightness rather than their mass. They seem to be red supergiants that have shed large amounts of their outer gas as they head towards a supernova explosion. In the photo of the ‘Fried Egg Nebula’, rings of ejected material can be seen surrounding the central star.
Blue hypergiants are the real heavyweights of the universe – tens or even hundreds of times more massive than the Sun, and millions of times more luminous. Their powerful gravity limits their size, so their surfaces are intensely hot. The young star cluster NGC 3603, shown here, contains one binary system whose stars contain a staggering 90 and 120 solar masses of material.