When two smaller nuclei fuse to form one larger nucleus, an enormous amount of energy is released. The masses of two combining nuclei should be equal to the mass of the product (larger one) but it is not. The difference in masses of two nuclei and final nuclei appears as energy i.e. the leftover mass takes the form of energy and leaves the system.
As the laws of conservation of mass and energy suggest,
Mass and energy can neither be created nor be destroyed but can be changed to interchangeable forms.
Einstein’s mass-energy equation gives a relationship between mass and energy. It says that mass and energy are interconvertible.
The fusion reaction occurs in stars and makes them glow or heat up by the energy produced as a result of the reaction. Two hydrogen atoms fuse together to form a helium isotope and a large amount of energy that runs the life processes on nearby planets and as far as we know till today, just the earth.
A number of different types of fusion reactions are occurring on the sun every second. In fact, every second, about 620 million metric tons of hydrogen atoms fuse together to form 616 million metric tons of helium along with the production of such a large amount of energy that the temperature on the core of sun reaches 27 million degrees Fahrenheit (~15 million degrees Celcius).
Although fusion reactions are not feasible on earthly conditions, researchers are figuring out ways to induce fusion reactions, especially the Deuterium-Tritium (D+T) fusion plant plans are being sorted, and soon, there will be a perfect fuel alternative, the D+T fusion system.
Comparison to fission energy
Fission reactions are also driven on the same principle except on reverse. Even then, a large amount of energy is produced by the fissure of a large nucleus into two small nuclei. This energy is released due to the lower mass total of smaller nuclei produced as compared to the main larger one. This mass defect is covered by releasing a large amount of energy but not large enough to equalize fusion reaction energy.