Why the Sun Shines
If the sun generated heat and light by ordinary burning, it would have long since consumed itself and become dark. Nor can the sun (or any other star) keep shining simply because it is extremely hot. Scientists know that since the sun has been shining for about 5 billion (5,000,000,000) years and will continue for at least that many more years, only a very large and long-lasting source of energy could power the sun. That source is the same as the one used in a hydrogen bomb—the transformation of matter into energy during nuclear fusion.
Albert Einstein in 1905 first defined the relationship of mass to energy with the formula Emc2. In a reaction following this formula, a very large amount of energy is released from a relatively small mass.
Deep within the interior of the sun, several types of fusion reactions take place. In the principal process, groups of four hydrogen nuclei are constantly combining to form single helium nuclei. The mass of each new helium nucleus is slightly less than the total mass of the four original hydrogen nuclei, the lost mass having been converted into energy. This energy eventually reaches the sun's surface, where it is released as light, heat, and other forms of electromagnetic radiation. Although the mass loss from each individual reaction is extremely small, the total loss from the entire sun is some 4,000,000 tons (3,600,000 metric tons) every second.