In nuclear fusion, when two atomic nuclei combine to form a heavier nucleus, a small amount of mass is converted into energy. This is according to Einstein's famous equation, E=mc^2, which states that energy (E) is equivalent to mass (m) multiplied by the speed of light squared (c^2).

When the mass of the resulting nucleus is less than the sum of the masses of the original nuclei, the difference in mass is released as energy. This energy is carried away by particles such as photons (gamma rays) and neutrinos.

The energy released in nuclear fusion is enormous. For example, the fusion of two hydrogen isotopes, deuterium and tritium, releases about 17.6 MeV (megaelectronvolts) of energy. This is equivalent to the energy released by burning about 10 tons of coal.

The process of nuclear fusion is what powers the sun and other stars. In these celestial bodies, the immense pressure and temperature in their cores cause hydrogen atoms to fuse together, releasing vast amounts of energy that sustain the stars' luminosity and heat.

On Earth, scientists are working on developing nuclear fusion as a clean and safe energy source. The challenge lies in creating and controlling the extreme conditions necessary for fusion reactions to occur. Achieving this could revolutionize energy production by providing an almost limitless and sustainable source of power.