The path on an HR diagram after a star leaves the main sequence stage is known as post-main sequence evolution. In the graph, it consists of 2 parts: sub-giant branch and red giant branch. The near-horizontal is the former one, and the near-vertical is the latter one. Subgiants are near-horizontal because as it expands, it loses temperature so the luminosity remain approx. constant. However, in the red giant stage, it suddenly pops up growing extremely large with the temperature decreases only slightly. I have two questions upon this: 1. What is the trigger event of a star changing from subgiant to red giant? (About electron Degeneracy?) 2. Why does red giants expand so much, but the temperature remains constant? (just as subgiants temperature should decrease along expansion) Thank you!