The process by which stars produce energy has always been of great interest to scientists. Not only would the answer to that puzzle be of value to astronomers, but it might also suggest a method by which energy could be generated for human use on earth.

In 1938, German-American physicist Hans Bethe suggested a method by which solar energy might be produced. According to Bethe's hypothesis, four hydrogen atoms come together and fuse—join together—to produce a helium atom. In the process, very large amounts of energy are released.

This process is not a simple one, but one that requires a series of changes. In the first step, two hydrogen atoms fuse to form an atom of deuterium, or "heavy" hydrogen. In later steps, hydrogen atoms are regenerated, providing the materials needed to start the process over again. Like nuclear fission, then, nuclear fusion is a...