A supergiant, with the exception of hypergiants, is the brightest, largest kind of star. Supergiants have luminosities of 10,000 to 100,000 solar luminosities and radii of 20 to several hundred solar radii (about the size of Jupiter's orbit). The two commonest types are red supergiants, exemplified by Betelgeuse and Antares, and blue supergiants, exemplified by Rigel. When a star of at least 15 solar masses exhausts the hydrogen in its core, it first swells to become a red giant. But when it reaches the stage of helium-to-carbon burning, by the triple-alpha process, it expands to an even larger volume. This much brighter, but still reddened star is a red supergiant. Through a vigorous stellar wind, red supergiants steadily lose their extended atmospheres and turn into smaller but much hotter blue supergiants. A blue supergiant may then develop a fresh distended envelope and revert to the red supergiant phase. Both types, red and blue, can explode as supernovae. This came as something of a surprise to astronomers, since stellar evolution theory had long taught that supernovae always come from the red variety. However, the great Supernova 1987A was found to have had a blue supergiant precursor.