Fusion reactions are nuclear reactions in which two or more atomic nuclei join together to form a heavier nucleus, releasing large amounts of energy in the process. Fusion reactions are the most powerful energy source known, producing several million times more energy than chemical reactions, such as burning gasoline. Fusion reactions occur naturally in stars as a result of gravitational pressures, producing the heat and light that we see at night. At the same time, scientists have been attempting to replicate this process here on Earth through controlled fusion reactions, which would provide an unlimited source of clean energy.