A star of 15 solar masses exhausts its hydrogen in about one-thousandth the lifetime of our sun. It proceeds through the red giant phase, but when itreaches the triple-alpha process of nuclear fusion, it continues to burn for a time and expands to an even larger volume. The much brighter, but still reddened star is called a red supergiant. Betelgeuse, at the shoulder of Orion, is the best-known example. Absolute luminosities may reach -10 magnitude compared to +5 for our sun.
Some of these supergiants are unstable and form the very importantCepheid variables. In their finalstages, supergiants may explode intosupernovae. The collapse of these massive stars may produce a neutron star or a black hole.