Thermonuclear reactions (nuclear fusion) deep in the sun; same as in hydrogen bombs...light elements (hydrogen) are fused into heavier ones (helium). The mass of the heavier element is slightly less than the sum of the masses of the lighter elements from which it was formed. The missing mass is converted into energy, per the equation E=mc^2.

The sun shines because some of the nuclear energy is transformed into radiant energy. Enormous compression due to gravity and extremely high temperatures in the deep interior of the Sun fuse the nuclei of hydrogen atoms together to form helium nuclei. This is called thermonuclear fusion (a process that releases radiant energy, a small part of which reaches Earth).

Defiantly Sun

the sun

heat