Why are red giant stars so large even though they are running out of fuel?
Quick Answer
When a Sun-like star exhausts hydrogen in its core, the core contracts and heats up, while the outer layers expand enormously. That expansion can inflate the star to tens or even hundreds of times its original size.
The Full Story
A red giant forms after a star like the Sun uses up the hydrogen fuel in its core. The core then contracts under gravity and heats up, while hydrogen fusion continues in a shell around the core. That extra energy pushes the outer layers outward, making the star much larger and cooler at the surface. The Sun is expected to become a red giant in about 5 billion years and may expand out to roughly the orbit of Mercury.
Key Facts
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