Besides, if burning produced its energy, it would have run out of fuel many millions of years ago. Various theories have been advanced to explain the sun's tremendous energy output. One said that all the bits of matter in the sun were exerting gravitational attraction on each other and causing the sun to shrink and become more tightly packed. This process, called gravitational contraction, does occur in some stars and can release a great deal of energy. Atomic theory finally provided an explanation.
Scientists now agree that thermonuclear reactions are the source of solar energy.
Albert Einstein's theoretical calculations showed that a small amount of mass could be converted to a great amount of energy. The vast amount of matter in the sun could provide fuel for billions of years of atomic reactions. The sun's core is believed to be a superhot, extremely dense mass of atomic nuclei and electrons. Under these conditions, nuclei can collide and fuse into new and heavier nuclei. This is a type of thermonuclear reaction called a fusion reaction.