Question

We believe the sun will undergo two "red giant" phases. What physical processes cause these two phases. Why and how does the first red giant phase end, and why and how does the second red giant phase end?

Answer 1

The sun has fusion reaction taking place at its surface. The hydrogen reacts and turns into helium through nuclear fusion. Once the fusions ends, gravitational force takes over and starts pulling the star towards it core. Temperature rises along with compression and becomes such that helium fuses into carbon. Then helium starts burning either in a flash or gradually.This heating up results in the hydrogen shell around the core to heat up. When it becomes hot enough hydrogen fusion starts again. In such situation the star burns in the red part of the spectrum which is why it is known as the red giant. This is the first phase.

This red giant phase takes place for few thousand to billion years. After a time, the helium core runs out and fusion stops. This will shrink the star and make helium shell reach the core. In such a situation, helium ignites and the outer layers of the star are blown off creating planetary nebulae. This shells are larger but fainter than the original star. The core keeps on collapsing. This is the phase two of red giant.

Eventually, the sun shall contract and cool down and become a white dwarf at the end of its life. This will happen as the mass of the sun is not enough to create a supernova.