Hans Albrecht Bethe | Biography

Prior to 1938, scientists were puzzled about how stars produce their light and energy. One popular theory about the production of stellar energy relied on chemical combustion; others proposed gravitational forces or electrical fields. In 1938, German-American physicist Hans Bethe finally provided the answer: nuclear fusion.

Bethe was born in Strassburg, Germany on July 2, 1906. He studied physics at universities in Frankfurt and Munich, and received his Ph.D. in 1928. He went on to work with the eminent physicists Ernest Rutherford in England and Enrico Fermi in Italy. When Adolph Hitler (1889-1945) rose to power in 1933, Bethe left Germany and went to England, where he taught for two years. He then moved to Cornell University in Ithaca, New York to teach.

Bethe, who was one of the scientists who worked on the development of the atomic bomb, combined Henri Becquerel 's information about subatomic physics with Sir Arthur Eddington's theory about stellar temperatures. He came to the conclusion that the presence of nuclear fusion in stellar bodies was consistent with their physical characteristics and tremendous energy output. Since the nuclear process converts a little mass into a great amount of energy, fusion could also account for the longevity of stars.

Bethe suggested that at the temperatures found in the core of many stars (millions of degrees), atomic nuclei of hydrogen can fuse with carbon nuclei. Each time a pair of these nuclei fuses, a complex chain reaction, called the CNO cycle (Carbon-Nitrogen-Oxygen Cycle) is initiated, ending with the fusion of four hydrogen nuclei into a helium nucleus, and the regeneration of one carbon nucleus. In the process, one percent of the mass of the hydrogen is converted into energy. Hydrogen is the "fuel" of the star, carbon is the catalyst and helium is the by-product. For stars whose interior temperatures were not hot enough to fuse hydrogen and carbon, Bethe suggested a different reaction, called the proton-proton chain, in which four hydrogen nuclei (i.e., protons) fuse together to form helium. This reaction also releases a large amount of energy.

Analysis of the amount of helium in the sun shows the fusion of hydrogen to helium has been going on for about five billion years. It is estimated the sun converts about 600,000,000 tons of hydrogen to helium every second, yet a star the size of the sun has enough hydrogen to produce energy at this prodigous rate for about ten billion years. Therefore, no one need worry about the Sun burning out just yet -- it has another five billion years of life left.

Bethe received the Fermi award in 1961 for his part in the development of atomic energy and the Nobel Prize in physics in 1967. In 1996, the Hans Bethe prize was established, annually honoring a scientist who has made distinguished contributions to astrophysics or nuclear physics.