When a star like the Sun exhausts the hydrogen in its core, it begins to die. The core contracts and heats up, and the outer layers expand enormously. The star swells into a red giant, growing hundreds of times its original size and cooling at the surface. It glows red because cooler stars emit more red light.

The Sun will become a red giant in about 5 billion years. It will expand past the orbit of Mercury and Venus, and possibly swallow Earth. The planet will be scorched and possibly destroyed. Even if Earth survives, its oceans will boil away and its atmosphere will be stripped. The Sun will then shed its outer layers and become a white dwarf.

Red giants are not all the same. Some are modest, like the Sun will be. Others are red supergiants, much larger and more luminous. Betelgeuse, in the constellation Orion, is a red supergiant. It is so large that if it replaced the Sun, its surface would extend past Jupiter's orbit. It will eventually explode as a supernova.

Red giants are important for the universe. They produce heavy elements through fusion and shed them into space, enriching the interstellar medium. The carbon in your body and the iron in your blood were forged in red giants and supernovae.