What occurs during nuclear fusion?

During nuclear fusion, two small nuclei combine to form a single, larger nucleus. This process occurs under extreme conditions, such as in the cores of stars where high temperatures and pressures exist. The fusion of these small nuclei releases a significant amount of energy, which is why fusion is the process that powers stars, including our Sun. In general, fusion involves light elements, such as hydrogen isotopes, merging to create heavier elements, often resulting in helium as a product. This phenomenon not only contributes to the energy output of stars but also is the reason behind the formation of heavier elements in the universe.

The other options do not accurately describe fusion: energy is not absorbed but released; large nuclei are not involved in this process since fusion typically involves lighter elements; and while photons can be emitted as a result of the energy changes during fusion, the primary defining characteristic of fusion is the merging of smaller nuclei.