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I am a bit confused when do stars become red giants? Is it just after they have finished core H burning and the core contracts creating high temperatures which result in core He burning to occur which creates outward pressures pushing the outer lowers apart or something different? Also does core He burning and shell H burning (by burning I mean fusion) occur at the same time and if not which comes first?? Or am i totally wrong with all these points?

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Is it just after they have finished core H burning and the core contracts creating high temperatures which result in core He burning...?

It is after the core finishes H burning, but He burning is not required. Hydrogen shell burning is sufficient to make it a red giant. Helium burning would make it a Horizontal Branch Star. See good explanation here: http://abyss.uoregon.edu/~js/ast122/lectures/lec16.html

Also does core He burning and shell H burning (by burning I mean fusion) occur at the same time and if not which comes first??

Shell H burning starts first, but can continue after He burning starts.

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  • $\begingroup$ What is the core doing when H-shell burning is taking place initially?? $\endgroup$ – user43487 Apr 9 '14 at 14:50
  • $\begingroup$ It's not about being required or not, He burning does happen around the core. That is why I mentioned it. $\endgroup$ – user42733 Apr 9 '14 at 14:54
  • $\begingroup$ @Joseph it is a helium core with no fusion taking place $\endgroup$ – DavePhD Apr 9 '14 at 14:54
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    $\begingroup$ @Joseph Once the core finishes hydrogen burning, contracting does start immediately. The shell hydrogen then begins fusion at high temperature and the star is then a red giant. Later, helium burning may begin in which case the star is a Horizontal Branch Star. $\endgroup$ – DavePhD Apr 9 '14 at 15:05
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    $\begingroup$ @Joseph Yes, including radiation pressure. $\endgroup$ – DavePhD Apr 9 '14 at 15:29
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Red giants are caused as outward forces become greater than gravity can pull inwards. This could be caused by stars loosing mass as the reactions take place or through more energetic reactions starting.

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