Which two opposing forces work against each other to keep a star stable and spherical?

Gravity and nuclear fusion

The physical integrity and spherical shape of a star are maintained through a delicate, ongoing balance between two fundamental forces. Gravity exerts a constant inward pull that attempts to crush the star's mass into a tight, dense core, while the energy generated by nuclear fusion in the core exerts an equal and opposing outward pressure. This constant battle prevents the star from collapsing under its own weight or expanding uncontrollably, allowing it to remain stable for billions of years.