The main source of energy in Sun is :

The primary source of energy in the Sun is nuclear fusion, a process occurring in its core where lighter atomic nuclei combine to form heavier ones [2]. Specifically, the Sun undergoes thermonuclear fusion, primarily through the proton-proton (PP) chain reaction. In this process, hydrogen nuclei (protons) collide under extreme temperature and pressure to fuse into helium atoms. This reaction releases a tremendous amount of energy because the resulting nucleus has slightly less mass than the original nuclei; this 'lost' mass is converted into energy according to Einstein's equation, E=mc² [2]. While other processes like the CNO cycle exist, they contribute less than two percent of the Sun's total energy. Chemical reactions and mechanical energy are insufficient to power the Sun's massive and long-lasting energy output [2].

Sources

1. [2] https://education.nationalgeographic.org/resource/solar-energy/