Fusion is the process by which lighter elements are fused into heavier elements while under tremendous pressures and temperatures. Lighter elements, such as those with lower atomic masses than iron, have exothermic fusion reactions, meaning they release more energy through the reaction than was needed to initiate the reaction. The most simple fusion reaction is that of hydrogen. When two hydrogen nuclei collide, they fuse together to form a helium nucleus, releasing roughly 18 MeV (million electron volts), or about 2.8e^-12 joules. Heavier elements fuse through more complex processes, such as the Triple Alpha process and Carbon Burning, which have a lower net energy release than hydrogen fusion.