Problem 3

Given information

A red giant star is a large bright star formed when the hydrogen in the cores of regular stars is used up. 

Step-by-step explanation

A red giant star is a cool star with a reddish colour. There is a fusion around the core-shell. On fusing He and Be we get heavier nuclei.

The nuclear reactions that take place in the core cannot continue and the core begins to contract due to its own gravity. This brings the hydrogen in the shell into a zone where the temperature and pressure are conducive for fusion reaction leading to expansion of this layer.