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I read some about red giants and so far I understand red giants become exhausted of burning hydrogen in the core, so then start hydrogen burning at shell and may or may not be burning helium in the core, in the meantime.

I know when stars start burning heavier atoms the nuclear energy against the gravitation becomes weaker and stars start to collapse. But why red giants get bigger while burning helium or just hydrogen burning shell ? If it was gaining energy it would not become red, what makes it to expand then ?

aQuestion
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The He fusion process needs more heat. The heat convects through the rest of the star and gets HOT. The hot gas expands more against the force of gravity.

JDługosz
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  • So you say red giants always burn helium and that's why they expand right? I do not think it is sufficient, it may just be hydrogen shell burning. So what causes the heat now? – aQuestion Jan 26 '15 at 20:24
  • Some reaction is taking place in the core that operates at a higher temperature and produces energy to sustain it. – JDługosz Jan 26 '15 at 21:09