What happens when a star becomes a red giant?

The answer

When a Sun-like star exhausts the hydrogen in its core, the core contracts and heats up, causing the outer layers to expand enormously and cool, turning the star red. Our Sun will swell to about 256 times its current radius, reaching approximately Earth's current orbit. Its surface temperature will drop from 5,500°C to about 3,000°C, but its luminosity will increase by a factor of 2,000 due to the vastly larger surface area. The red giant phase lasts only about a billion years - a brief twilight in a star's life.