Red Giant Star

A red giant is a star that has exhausted the primary supply of hydrogen fuel at its core and is now using another element such as helium as the fuel for its energy-producing thermonuclear fusion reactions. Hydrogen fusion continues outside the core and causes the star to expand dramatically, making it a giant. Expansion also cools the star's surface, which makes it appear red. Red giant stars are near the end of their lives, and die either in a supernova explosion, or more quietly as a planetary nebula. Both fates involve the expulsion of the star's outer layers, which leave behind the small, exposed core.