Question

How and why do red giants become so large?

Answer 1

Stars like Sun is now in the main sequence..It is in equilibrium due to 2 forces.

Explanation:

Fusion reaction causes high temperature and pressure inside core pushes outwards.
Gravity pulls towards center.center. Whe n hydrogen is finished the mass reduces and helium burning causes the star to expand.Due to less mass there is not enough gravity to pull inwards..So star expands and become gas giant...