Final answer:
Fusion reactions occur when lighter-mass nuclei join together to form a single nucleus. They are responsible for the power source of stars.
Step-by-step explanation:
Nuclear reactions, in which two or more lighter-mass nuclei join together to form a single nucleus, are called fusion reactions or nuclear fusions. Fusion reactions occur at extreme temperatures such as those in the cores of stars, and they are the power source for stars. Fusion reactions involving carbon and nitrogen can also occur in more massive stars.