How does it manage to do it? What is its source of energy? Chemical energy supplied by combustion and gravitational potential energy supplied by contraction are thousands of times too small to have kept the sun going for such a long time. The principle illustrated by Figure 4 suggests the most probable source of energy for the sun and all the other stars as well. It is known that the sun consists chiefly of hydrogen and that it has a temperature of about 40,000,000 degrees Fahrenheit near its center. Several kinds of nuclear reactions produced in atom smashers have demonstrated that hydrogen nuclei, if energized by being heated to a very high temperature, can actually combine, or fuse, to form helium nuclei. The accompanying loss of weight per particle indicated by Figure 4 must result in the appearance of sufficient energy to balance Einstein’s famous equation. In fact, calculations by the German-born American physicist Hans A. Bethe and others show that, based on reasonable estimates of the conditions within the sun, familiar nuclear reactions account for its energy. The calculations predict, furthermore, that the sun can continue to operate at its present level for many billions of years. [Illustration: _Large loop prominences on the sun, caused by a locally intense magnetic field. Project Sherwood, the U. S. program in controlled fusion, is devoted to research on fusion reactions similar to those from which the sun derives its energy._ Courtesy Sacramento Peak Observatory, AFCRL] Since fusion of light nuclei is produced by extremely high temperatures, fusion events are called _thermonuclear reactions_. The possibility of bringing about thermonuclear reactions on earth to serve as a source of energy has naturally attracted much attention. In spite of the fact that fusion of ordinary hydrogen atoms (each of which has one proton as its nucleus) supports the activity of the sun, this particular reaction seems to occur much too slowly to be usable on earth. Other isotopes of hydrogen, called deuterium and tritium, however, which contain one and two neutrons in their nuclei, respectively, fuse much more rapidly and seem to be potential earthly sources of controlled thermonuclear energy. [Illustration: _An early phase of a nuclear detonation at Eniwetok Atoll during the 1951 tests._ Courtesy Joint Task Force Three]