What type of reaction occurs when light nuclei combine to form a heavier nucleus?

The reaction in which light nuclei combine to form a heavier nucleus is known as nuclear fusion. This process is fundamental to the energy production in stars, including our Sun, where hydrogen nuclei fuse under extreme temperature and pressure conditions to create helium, releasing a vast amount of energy in the form of light and heat.

Nuclear fusion is characterized by its ability to produce energy on a much larger scale compared to other reactions like fission, where heavy nuclei split into lighter nuclei. The fusion process is essential in understanding not only stellar phenomena but also the potential for creating cleaner, sustainable energy sources on Earth.

In contrast, neutron capture involves a heavy nucleus capturing a neutron, which can lead to fission in some cases, but it does not involve the combining of light nuclei. Isomeric transitions refer to the changes in energy states of a nucleus without a change in the number of protons or neutrons, and they do not involve the fusion of nuclei either. Thus, the interaction described in the question is distinctly that of nuclear fusion.