1. Gravity: This force pulls all the matter in the star inward towards its center. It is the force that initially caused the star to form from a cloud of gas and dust.

2. Nuclear Fusion: This process occurs in the core of the star and releases immense amounts of energy. This energy exerts an outward pressure that counteracts gravity.

These two forces are in a delicate balance. If gravity were stronger, the star would collapse. If nuclear fusion were stronger, the star would expand and possibly explode. The balance between these forces determines the star's size, temperature, and lifespan.