Explain why a star similar in mass to the Sun remains stable during its main sequence stage, and describe what happens to the star once its hydrogen fuel runs out.

During the main sequence the inward pull of gravity is exactly balanced by the outward pressure produced by nuclear fusion in the core. The fusion of hydrogen nuclei into helium releases a large amount of energy, which creates the pressure that supports the star against collapse.

When the core hydrogen is exhausted the fusion rate drops, so the outward pressure can no longer balance gravity. The core then contracts and heats up, while the outer layers expand and cool, turning the star into a red giant. In the red‑giant phase heavier elements are produced by further fusion reactions.

Once the core can no longer sustain fusion, the star stops burning and the outer layers are shed. The remaining core collapses and cools to become a white dwarf.