Nuclear fusion is a reaction that can be inside a star where two or more atomic nuclei are combined to form one or even more different atomic nuclei and subatomic particles. It is important because stars are powered by nuclear fusion in the course mostly by converting hydrogen into helium. This production of new elements during nuclear reactions is called nucleosynthesis. A star's mass determines what other types of nucleosynthesis occur in its core. or during the explosive changes in its life.