If light nuclei are forced together, they will fuse and energy will be released because the mass of the combination will be less than the sum of the masses of the individual nuclei. If the combined nuclear mass is less than that of iron at the peak of the binding energy curve, then the particles will be more tightly bound than they were in the lighter nuclei, and that decrease in mass comes off in the form of energy according to the Einstein relationship,
Even though a lot of energy is required to overcome the Coulomb repulsion and initiate hydrogen fusion, the energy yields are enough to encourage continued research which may one day lead to fusion reactors on earth to produce energy.
In the universe the most important fusion reactions, because they power the stars, are those that produce helium from hydrogen.
are neutrinos and gamma rays respectively, which serve to carry away energy and momentum.
This reaction can be followed by either
Either way the effect is that hydrogen is converted into helium and energy is released. This is just the start in many start of a long chain of reactions that converts light elements into heavier elements. This chain can continue right up to iron. Beyond this point, the energy balance changes so that energy is no longer released by fusion, but by fission in which heavier elements are broken down into lighter elements.