Final answer:
Nuclear fusion in medium-sized stars like the sun involves the fusion of hydrogen nuclei to form helium nuclei. In larger stars, fusion reactions can produce elements as heavy as iron.
Step-by-step explanation:
Nuclear fusion is the process in which nuclei combine to form a new element. In medium-sized stars like the sun, nuclear fusion primarily involves the fusion of hydrogen nuclei to form helium nuclei. However, in larger stars, fusion reactions can produce elements as heavy as iron. This process occurs in the cores and shells of massive stars, where elements like carbon, oxygen, neon, magnesium, and silicon are formed through fusion.