1. Electromagnetic Force: This force repels the positively charged nuclei of atoms, preventing them from fusing. Think of it like magnets pushing away from each other.

2. Strong Nuclear Force: This force, much stronger than the electromagnetic force at very short distances, overcomes the repulsion and binds the nuclei together, creating heavier elements.

These forces are in constant tension during fusion. The intense heat and pressure within the star's core provide the energy necessary to overcome the electromagnetic repulsion and allow the strong nuclear force to take over, resulting in fusion.