Here's how it works:

* Nuclear Fusion: In the core of a main sequence star, hydrogen atoms are fused together to form helium, releasing immense amounts of energy. This energy pushes outward, creating outward pressure.

* Gravity: The star's own mass exerts a strong inward pull, creating inward pressure.

* Equilibrium: The outward pressure from nuclear fusion perfectly balances the inward pressure from gravity. This balance is what keeps the star stable and prevents it from collapsing or exploding.

This delicate balance is what defines a main sequence star. If the outward pressure were to dominate, the star would expand. If the inward pressure were to dominate, the star would collapse.

Think of it like a tug-of-war between the star's own energy and its own gravity. The two forces are locked in a perpetual stalemate, keeping the star stable and shining for billions of years.