What occurs in the third stage of a star's life cycle?

In the third stage of a star's life cycle, the star undergoes a critical balance between gravitational forces and the pressure from nuclear fusion occurring in its core. This stage is characterized by the star being in a stable phase, where the outward pressure from the nuclear reactions in the core counteracts the inward pull of gravity.

This equilibrium allows the star to maintain a stable size and luminosity for an extended period, known as the main sequence stage of a star, where it “burns” hydrogen into helium. This balance is vital for the stability of the star and is crucial for its longevity. When a star eventually exhausts its nuclear fuel, different processes occur depending on its mass, but during this third stage, the forces are ideally balanced.

This understanding is essential because when the forces become unbalanced, it can lead to significant transformations in the star, such as expansion to form a red giant or, in more massive stars, the eventual explosion as a supernova.