A red giant star is a late stage in the life cycle of a star that has exhausted the hydrogen fuel in its core. As the core contracts under gravity, the outer layers expand and cool, giving the star its characteristic reddish hue. This phase occurs after a star has spent most of its life in the main sequence stage. Red giants can vary in size and brightness, often becoming thousands of times larger than the Sun. Eventually, they will shed their outer layers, creating a planetary nebula, while the core remains as a white dwarf.