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What causes a red giant to become so large?

Answer 1

When a star uses up all its hydrogen, helium is then fused into carbon.

Because of its immense gravity, a "main sequence" star like our sun uses its abundant supply of hydrogen to fuse it into helium. The energy released from this fusion prevents the star from collapsing in on itself.

When the star's supply of hydrogen runs out, all that's left is helium, which causes it to shrink and become denser. As a result, the temperature rises, allowing the helium to start fusing with carbon. This new fusion releases enormous amounts of energy, which causes the star to explode into a red giant that is many times larger than it was before.

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Answer 2

Levi Anderson

Depletion of hydrogen in the core causes a red giant to enlarge, causing its outer layers to expand and become more luminosous.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.