By forcing the light nuclei together, they will be fused by yielding energy, as the mass of combination will always be less than the mass of masses of single nuclei. It is the process of nuclear reactions, where light elements are combined for making the heavier elements. In some cases where the interacting nuclei are belonging to the elements that are having the low atomic numbers such as hydrogen or its isotopes, then it leads to the release of a substantial amount of energy. For the first time, the vast energy potential of the reaction of nuclear fusion was exploited in the thermonuclear weapons, or in the hydrogen bombs, which were immediately developed after world war II.
For the stars including the sun, the fusion reactions are constituting the fundamental sources of energy. It is possible to view the evolution of stars as passing through the various stages like the thermonuclear reactions, and nucleosynthesis, that causes the changes in composition over the longer periods. The burning of hydrogen causes the initiation of fusion energy, of the stars and causes the formation of helium. By the generation of fusion energy for particular purposes, scientists have to rely on the reactions between the lighter elements that are burning to make the helium. The heavy isotopes of hydrogen can react more efficiently when they are undergoing fusion and they are yielding more energy per 1 reaction than the nuclei of two hydrogens.
The reactions of fusion between the lighter elements are like fission reactions that cause the splitting of heavy elements and also release the energy due to the main feature of the nuclear matter known as the binding energy.
By looking at the fusion reaction, shown in the above figure, it can be seen that deuterium and tritium are fusing. Their components are recombined into the fast neutron and helium atom. These heavier isotopes are resembling the helium atom, so there is the extra mass that is converted to the kinetic energy of the neutron. For the start of the nuclear fusion reaction, the two nuclei should be brought so close that there is the activation of the nuclear forces, and they cause the joining of nuclei together. These nuclear forces are acting against the electrostatic forces where there is the repulsion of positively charged nuclei. Due to this reason, most of the nuclear fusion reactions are occurring at high temperatures and high-density environments.