Answer:
Nuclear fusion
Explanation:
The energy of the star comes from combining of the light elements into the heavier elements. The process is known as nuclear fusion, or nuclear burning.
Thus, Nuclear fusion is the reaction which involves combination of the two or more nuclei and form one or more different nuclei and also subatomic particles.
It is believed that most of elements in universe which are heavier than the helium are created/synthesized in the stars when the lighter nuclei in the space fuse to make the heavier nuclei. This process is known as nucleosynthesis.