Why do red giants form?

The star cools in the red giant phase as this outer shell of hydrogen begins to fuse into helium, making it both extremely bright and massive. The energy production is nearly the same but now distributed over a larger area.

A star enters a phase of its life in which only the core survives and the outer layers are discarded, and this process varies depending on the type of star. This phase begins when all of the hydrogen in the outer shell is also consumed.

In the later phases of a star's life cycle, red giants are created when the star expands due to the fusion of helium following the depletion of hydrogen fuel in the core.

Red giants form as a result of the late-stage evolution of low to intermediate-mass stars, typically ranging from about 0.5 to 10 solar masses. As a star exhausts its core hydrogen fuel, it undergoes gravitational collapse, increasing the pressure and temperature in its core. This leads to the initiation of hydrogen fusion in a shell surrounding the core, causing the outer layers of the star to expand and cool, turning the star into a red giant. This expansion occurs because the energy generated by the fusion reactions in the shell is transferred outward, causing the outer layers to expand and the surface temperature to decrease, resulting in a reddish appearance.