Hans Bethe; Nobel physicist, helped develop atom bomb

Hans Bethe, a Nobel Prize-winning physicist who elucidated the origins of the sun's energy and played a crucial role in the development of the atomic bomb, died Sunday in his Ithaca, N.Y., home. Cornell University, where Dr. Bethe had served on the faculty since 1935, announced his death yesterday. He was 98.

Dr. Bethe (pronounced BAY-tuh) was one of the last surviving members of a remarkable group: the European physicists who fled Adolf Hitler to come to the New World and then unlocked the secrets of the atom. Working on the Manhattan Project, the top-secret effort to build an atomic bomb, he served as head of the Theoretical Physics Division at Los Alamos.

Before that, though, Dr. Bethe had helped unlock the secrets of the sun. He formulated the carbon-cycle theory of stellar energy in 1938. Astrophysicists generally assumed that stars generated their energy by fusion, with atomic nuclei of a given substance fusing together to produce a different element and, with it, tremendous amounts of energy. What was unclear was what did the fusing and what the original material then became.

Dr. Bethe hypothesized a six-step cycle in which four hydrogen nuclei, with carbon and nitrogen acting as catalysts, are transformed into one helium nucleus. The atomic weight lost becomes energy, which fuels the sun. His calculations exactly coincided with the actual amount of energy the sun produces.

It was for this work that Dr. Bethe was awarded the Nobel Prize in Physics, in 1967.

''He was so amazingly versatile," said Edwin Salpeter, 81, an astrophysicist and professor emeritus of physical sciences at Cornell who first met Dr. Bethe in 1947. Salpeter added that Dr. Bethe ''sort of taught other scientists how to do science simply," because he ''worked like other people, only much better."

In a 60th-birthday tribute to Dr. Bethe, Victor Weisskopf of the Massachusetts Institute of Technology and R.F. Balcher of the California Institute of Technology wrote, ''The great craftsman of our profession, the master of the trade, is Hans Bethe."

Hans Albrecht Bethe was born in 1906 in Strasbourg, which was then part of Germany. He came from a long line of academics. His father, Albrecht T.J. Bethe, was a professor of physiology. His mother, Anna (Kuhn) Bethe, was the daughter of a professor.

Dr. Bethe, who demonstrated an aptitude for mathematics at an early age, realized as an undergraduate at the University of Frankfurt that he should pursue theoretical physics.

''It had become plain to me by this time that I could not do experimental physics," he once told his biographer, Jeremy Bernstein. ''My hands are just no good. I do everything wrong. . . . Besides, I had realized by then that because of my interest in mathematics I wanted to do theoretical physics. I loved mathematics, but I wanted to apply it to physics."

It was a time of unparalleled ferment in physics, and Dr. Bethe quickly rose in the profession. He received his doctorate from the University of Munich, and while on a Rockefeller Fellowship he did work overseas with Enrico Fermi, Ernest Rutherford, and Niels Bohr. He held a series of posts at the universities of Frankfurt, Stuttgart, Munich, and Tubingen, and then Manchester and Bristol in England.

The reason Dr. Bethe left Germany was political: His mother was Jewish, and with the Nazis' rise to power, restrictions were being placed on Jewish professors.

From England, Dr. Bethe came to the United States to teach at Cornell. He attained emeritus status in 1975. After his retirement, he continued to do research. He returned to astrophysics, doing groundbreaking work on supernovas.

During 1936 and 1937, Dr. Bethe published three articles in the American Physical Society's Journal, Reviews of Modern Physics, that taken together amounted to the first comprehensive overview of developments in nuclear physics. They became known as ''Bethe's Bible." They are only the best known of some 300 research papers Dr. Bethe published.

''After the fall of France," Dr. Bethe later recalled, ''I was desperate to do something to make some contribution to the war effort." He became a US citizen in 1941 and began doing military research on armor penetration and on radar. J. Robert Oppenheimer, director of the Manhattan Project, then sought him out. ''The essential point is to enlist Bethe's interest," Oppenheimer explained to a colleague.

At Los Alamos, Dr. Bethe became known as ''the Battleship" because of his blocky build and imperturbability. As chief theoretician, he supervised research upon which all other divisions depended.

Notable honors that came Dr. Bethe's way include the Presidential Medal of Merit in 1946, the Max Planck Medal in 1953, the Atomic Energy Commission's Enrico Fermi Award in 1961, the National Medal of Science in 1976, the National Science Foundation's Vannevar Bush Award in 1985, and the Einstein Peace Prize in 1993.

Long regarded as a senior statesman of science, Dr. Bethe became a leading advocate of arms control. He helped found the Federation of Atomic Scientists and was a founding director of the Bulletin of the Atomic Scientists. He was one of the first to propose what later became the Nuclear Test Ban Treaty and vigorously backed the Anti-Ballistic Missile Treaty.

''One of the things that was very special about Hans was his strong moral motivation," said astrophysicist John Bahcall of the Institute for Advanced Study in Princeton, N.J. ''He did things because he believed they were right and not because they were convenient or helpful to him or promoted his career. His work on the bomb was motivated by a desire to preserve freedom and open society in the face of a spreading Nazi tyranny, which he knew about firsthand."

Dr. Bethe told the Associated Press in 1996 that he and his Manhattan Project colleagues had been startled by the power of the atom bomb. It ''was worse than we expected," he said. ''After Hiroshima, many of us said, 'Let's see that it doesn't happen again.' "

Dr. Bethe leaves his wife, Rose (Ewald); a son, Henry, of Ithaca; and a daughter, Monica, of Kyoto, Japan.

Material from the Associated Press was used in this report.