Final answer:
Nuclear fusion is a reaction in which two nuclei combine to form a larger nucleus, releasing energy. It is the source of energy in stars and requires extreme temperatures to occur. Fusion reactions involving carbon and nitrogen produce more energy than hydrogen fusion reactions.
Step-by-step explanation:
Nuclear fusion is a reaction in which two nuclei are combined to form a larger nucleus. This process releases energy. Fusion is the source of energy in stars, where the necessary temperature to ignite the fusion reaction is provided by massive gravitational pressure. In stars more massive than our sun, fusion reactions involving carbon and nitrogen are possible, producing more energy than hydrogen fusion reactions.