Learn more physics!
How long does a star remain in the Red Giant stage and what occurs to the star during the Red Giant stage?
Pioneer High School, Ann Arbor Michigan US
When a star becomes a red giant, most of the hydrogen fuel in the core
of the star has been used. The helium shell around the core contracts
and heats up. The temperature eventually gets high enough that the
hydrogen shell outside the helium starts to undergo fusion. That makes
the outside of the star expand enormously. As it expands, it cools off
very quickly. The star spends about 100 million years as a red giant.
That is actually very short compared to the 10 billion years a star
like the sun spends in it's normal stages.
(published on 10/22/2007)
Follow-up on this answer.