Why do stars expand into red giants after leaving the main sequence?

When a star exhausts the hydrogen fuel in its core, it leaves the main sequence and begins to evolve into a red giant. This transition is driven by changes in the star's internal structure as it attempts to maintain equilibrium.

As hydrogen fusion ceases in the core, the core loses the outward pressure that previously countered the inward pull of gravity. Consequently, the core contracts and becomes hotter and denser. This contraction releases gravitational energy, which heats the surrounding shell of hydrogen, initiating hydrogen fusion in that shell.12

The increased energy production from this shell causes the star's outer layers to expand significantly. As these outer layers move further away from the energy source, they cool down, resulting in the characteristic red color and large radius of a red giant. In lower-mass stars, this process continues until the core becomes hot enough to ignite helium fusion, while the outer envelope remains extended and cool.12