The Death of a Star

As the Sun expands, its temperature will be slightly cooler, so it will be red, rather than yellow. It will be known as a red giant star. At this point, it will be using helium as fuel. When the helium is used up, the core will shrink to about the size of Earth and the remaining layers of gas will float off into space. The core will become a white dwarf star. A white dwarf has no fuel to convert to radiant energy, only the leftover thermal energy from its energy-producing days keeps the star hot for a long time. Over a period of several million years, a white dwarf cools down and becomes a cold object called a black dwarf.

Massive stars go out in a blaze of glory. When a massive star's core runs out of fuel, it starts shrinking in on itself until it can shrink no farther. Powerful shock waves from this sudden stop fan outward, and particles of matter spin off into space carrying huge amounts of energy with them. A gigantic explosion occurs that is millions or billions of times as bright as the star ever was. This explosion is known as a supernova. It hurls matter and energy far out into space. Usually, all that will be left behind is a ball of neutrons that is about 20 km (12 mi) across. This city-sized object is called a neutron star.

If the core was quite massive—more massive than three Suns—the core's own gravity will keep causing it to shrink in on itself until it becomes a black hole. A black hole is a point in space that has such a strong force of gravity that nothing within a certain distance of it can escape getting pulled into the black hole—not even light.