Which force is the reason stars are spherical?

Stars become nearly spherical because gravity pulls matter inward equally from all directions. As a star forms, collapsing gas heats up until pressure from thermal motion and radiation balances gravity. This state is called hydrostatic equilibrium. If gravity were much stronger locally, the star would collapse further; if pressure were stronger, it would expand. The spherical shape is a direct consequence of gravity acting on a fluid-like plasma over long periods.