Nuclear fusion is the process by which several separate nuclei 'fuse' together to form one heavier nucleus - the byproduct of this reaction being either the release or absorption of energy. The factor that dictates whether energy is absorbed or released is the binding energy of the molecule's nucleus (i.e., the energy required to split the molecule apart). Hence, the higher the binding energy, the more stable the molecule and the more difficult it is to disrupt it through fusion or fission. Nickel and iron have the highest binding energy of the elements, and are the most stable; this is also the reason iron and nickel are quite common in planetary cores, since they are produced profusely in supernovas. Fusion produces energy by combining very light elements into more tightly-bound elements, such as hydrogen into helium, while fission produces energy by splitting very heavy elements, such as uranium and plutonium, into tightly-bound elements.

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