1. Gravity: This force pulls all the matter in the star towards its center, constantly trying to collapse the star. It is the force responsible for the immense pressure and temperature within the star.

2. Nuclear Fusion: This is the force that opposes gravity. It occurs in the core of the star, where hydrogen atoms fuse to form helium, releasing enormous amounts of energy. This outward pressure counteracts the inward pull of gravity, keeping the star stable.

The balance between these two forces determines a star's lifetime, size, and luminosity.