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I understand that stars transition into the red giant phase by obeying the mirror principle, as written here:

Shell-burning stars obey something called the mirror principle, which is an observation gleaned from the models that when a shell contracts, the envelope of the star will expand and vice versa.

However, I read that this information was gleaned from observation. Could anyone explain the reason or the physics behind why this happens?

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  • $\begingroup$ What is the "mirror principle"? I googled for it and did not find anything. $\endgroup$ – user259412 Sep 8 at 11:25
  • $\begingroup$ I found it here: low-sky.github.io/stellarev $\endgroup$ – Vin Sep 8 at 12:32
  • $\begingroup$ "Shell-burning stars obey something called the mirror principle, which is an observation gleaned from the models that when a shell contracts, the envelope of the star will expand and vice versa." $\endgroup$ – Vin Sep 8 at 12:32
  • $\begingroup$ Thanks! I understood the post and clarified it. $\endgroup$ – user259412 Sep 8 at 12:56
  • $\begingroup$ Section 12.4 of ast.cam.ac.uk/~pettini/STARS/Lecture12.pdf addresses this, although a proper answer would need more of a dive into the physics, especially the virial theorem, which I have never understood properly. $\endgroup$ – Steve Linton Sep 8 at 14:44

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