The fusion of two smaller nuclei results in an increase in the binding energy of the product as long as the mass number is less than 56. For fusion to occur, the nuclei would have to collide at high speed i.e. with high kinetic energy to overcome the proton repulsion.Nuclear fusion occurs in the Sun due to high temperatures at the centre of the Sun of about 1.5 × 107 Kelvin. The atoms exist as 'plasma' as the electrons have been stripped off. The high temperatures and gravity cause collisions that overcome the repulsive force of the protons and bring the nucleons sufficiently close together so that the attractive strong nuclear force can act and bind the nucleons together.