HANS BETHE was one of the world’s greatest scientists, unusually highly respected by the scientific community. He made considerable contributions to the development of the atomic bomb and nuclear energy during the Second World War. But he was best known for his work on developing a theory for the production of energy in stars. For this, he was awarded the 1967 Nobel Prize.

Hans Albrecht Bethe was born in 1906 in Strasbourg, Alsace (then in the German Empire), where his father was a distinguished physiologist. The family later moved to Frankfurt, where Bethe went to school at the Gymnasium from 1915 to 1924.

He read physics at Frankfurt before moving to the University of Munich in 1926. There, as a protégé of Arnold Sommerfeld, he took his doctorate in theoretical physics in 1928. His thesis was a study of the passage of electrons through crystals, a fashionable subject since quantum theory had surprisingly predicted that electrons, formerly thought to be particles, could, like light, also behave as waves.

Bethe worked for a year as a physics lecturer at the universities of Frankfurt and Stuttgart (where he worked with P. P. Ewald on X-ray diffraction in crystals), before moving in 1929 to the University of Munich. In the winter of 1932-33, he became an assistant professor at the University of Tübingen.

In January 1933, Hitler became Chancellor of Germany and many Jewish academics lost their jobs, including Bethe. Fearing that worse was to come, he emigrated to England, where he was given a temporary lectureship at Manchester University followed by a fellowship at Bristol University. In early 1935, he moved on to the US, having been appointed an assistant professor at Cornell University, at Ithaca, New York. Apart from a leave of absence during the Second World War between 1942 and 1945, and sabbatical leaves, he was to stay at Cornell for the rest of his working life.

In 1930 he and Sommerfeld wrote a review article on the electron theory of metals. The article was mainly Bethe’s work, and not only described the papers already published in the field but contributed many new ideas. Bethe also studied the problem of how particles travelling at high speeds lost energy as they passed through matter. It was an important problem at the time because the relationship between the energy and the range of (the distance travelled by) a particle was an important tool for measuring the energy of a particle.

Between 1935 and 1938, Bethe worked on the theory of nuclear reactions, predicting the cross-sections of many reactions (the probabilities that particular interactions will take place). This led him to the discovery of the nuclear reactions that provide the energy in the stars. Bethe described his research and experimental results in three articles in the Reviews of Modern Physics, which for many years were essential reading for nuclear physicists.

In his 1939 paper Energy Production in Stars, a classic, he explained where stars get their energy from. He proposed that the energy came from a series of six nuclear reactions, beginning with the reaction of a nucleus of an atom of an isotope of carbon (carbon-12) with a proton (a nucleus of a hydrogen atom), and ending with the production of another carbon-12 nucleus and a nucleus of helium (helium-4). In the process a large amount of energy (27 million electron volts) is released.

The carbon-carbon process is the way in which all stars, including our sun, produce the huge amounts of energy they need. For this work — the first explanation of stellar and solar energy, the origin of the bulk of the energy we use on Earth — Bethe was awarded the Nobel Prize in Physics.

In 1942, during the war, Bethe worked on microwave radar at the Radiation Laboratory at the Massachusetts Institute of Technology; in 1943, he went to work at the Los Alamos Scientific Laboratory in New Mexico, where he worked on the Manhattan Project, America's programme to develop nuclear weapons. Invited by Robert Oppenheimer to work on the project, he was one of the first group of scientists to join it, and he headed the Theoretical Physics Division at Los Alamos.

The success of the atomic bomb project owes a great deal to Bethe. Yet he did not at first want to get involved with it. He did not believe that an atomic bomb was a practical idea. He changed his mind when he saw the primitive nuclear reactor that Enrico Fermi had built on the tennis court at Chicago University. He was, in any case, anxious to contribute to the war effort because of his conviction that the Nazis must not be allowed to win the war.

Three nuclear weapons were fabricated at Los Alamos during the war. One was tested in the desert near Alamogordo, New Mexico, on July 16, 1945 (the Trinity test); one was exploded over Hiroshima on August 6, 1945; and the third was exploded over Nagasaki on August 9, 1945. Together the explosions over the two Japanese cities killed about 250,000 people.

Bethe watched the Trinity nuclear test. He said of it: “This was a necessary test for us. We did not know whether the device we had invented, designed, and built would explode. Like others who had worked on the atomic bomb, I was exhilarated by our success — and terrified by the event.” After the war, he became a strong advocate of nuclear disarmament.