When two lighter nuclei moving at very high speeds fuse together to from a single heavier nucleus, then this phenomenon is called nuclear fusion. The source of huge solar energy is the fusion of lighter nuclei. About 90% of the solar mass is composed of hydrogen and helium and the rest 10% contains other elements, most of which are light elements.
The estimated temperature of the interior of the sun is about 2×107K. At such a high temperature the atoms of all elements present in the sun are completely ionised to form a hot plasma. Fusion of hydrogen nuclei into helium nuclei is continuously taking place in this plasma, with the continuous liberation of energy.