Hans Bethe, who died Sunday at his home in Ithaca, N.Y., at age 98, was the last of the titans of 20th-century physics.

In a career that stretched from the 1920s into the new millennium, Bethe made seminal contributions to the understanding of atomic nuclei, elementary particles, and astrophysics. His model of the process that fires the sun, still accepted six decades after he developed it, won him the 1967 Nobel Prize in physics.

During World War II, Bethe ran the theoretical physics division of the Manhattan Project, where he oversaw the design and eventual detonation of the first atomic bomb. He soon became an outspoken advocate of arms control and helped convince President Kennedy to sign the Test Ban Treaty of 1963. Yet Bethe also supported work on the hydrogen bomb, whose fusion mechanism owed much to his theoretical work. “Until we have international control,” Bethe once said, “we cannot afford not to have the hydrogen bomb.” His public stances included advocacy of nuclear power and opposition to the Strategic Defense Initiative.

Bethe’s was a fertile mind, much given to systematization. His massive three-issue series in Reviews of Modern Physics in 1936-37 on the structure and dynamics of atomic nuclei formalized a field of study and were often reprinted as a textbook on the subject, nicknamed the “Bethe Bible.” His work also served as the basis for the emerging study of quantum electrodynamics, for which his student Richard Feynman won the Nobel in 1965.

After his retirement, Bethe focused more on astrophysics, and in recent years he published papers on the life cycles of supernovae and the “missing mass” problem of neutrinos, work that referred directly back to the carbon cycle of solar radiation he first developed in 1938.

Bethe was raised in Alsace-Lorraine. His physiologist father – properly styled “doktor doctor,” as he was both an M.D. and Ph.D. – imbued him with a life-long love of abstraction. Young Hans was a mathematical standout, and by the time he was in gymnasium it was obvious he would have a career in science. Although he later took delight in hiking in the Rockies and Alps, Bethe credited his clumsiness for his choosing theoretical instead of experimental physics.

He received his doctorate in 1928 and immediately began to collaborate with an all-star list of physicists including Rutherford, Fermi, and Bohr.

Bethe’s mother was a singer, whom a bout of influenza left with progressive deafness. She became prone to bouts of depression that led to occasional institutionalization and, when Bethe was 21, his parent’s divorced. Though a Lutheran convert, she had been born a Jew, and in 1933, after Hitler came to power, Bethe was dismissed from his university posts at Tubingen and Munich for having two Jewish grandparents.

He quickly found appointments in England, and then in 1936 took a position at Cornell, where he remained on the faculty for nearly 70 years. He managed to extract his reluctant mother from Germany in 1938. Bethe used half of a $500 prize he won for his paper on solar radiation to liberate her furniture from Nazi authorities.

With the outbreak of World War II, Bethe first worked on microwave radar at the Massachusetts Institute of Technology and then joined Robert Oppenheimer at Los Alamos for the project that led to the atomic bomb. His indefatigable bearing earned him the nickname “The Battleship.”

“Looking at the photographs from Hiroshima, I, at least, was terribly shocked how big the destruction was,” Bethe later recalled in 1995. “We had calculated what would happen. But, it is something different when you actually see it.” He was nevertheless proud of having a role in shortening the war. While Bethe’s calls for arms control – delivered in person to Presidents Eisenhower, Kennedy, and Ford – were heartfelt, he was hardly naively optimistic. A stamp collector from his teenage years, Bethe said he liked the hobby because it “was the one place in the world where all countries sat together peacefully.”

After the war, Bethe returned to Cornell, where his students included Feynman and Freeman Dyson, both of whom soon became colleagues in the physics department. His contributions to quantum electrodynamics and meson theory helped set the agenda for postwar physics.

As his role in the development of the bomb became known, Bethe’s public visibility increased. He helped to found the Federation of Atomic Scientists, and was on the original board of the Bulletin of the Atomic Scientists. He was outspoken in his support of breeder reactors, and later joined Carl Sagan and others in the Union of Concerned Scientists to oppose the Strategic Defense Initiative.

Bethe’s public pronouncements were hardly limited to doomsaying. Quoted in Time magazine soon after his solar radiation findings became known, Bethe made the vivid financial comment: “At the rate of one cent per kilowatt hour, we should have to pay a billion billion dollars to keep the sun going for a single second.”

Bethe (his name was pronounced like the Greek letter) once collaborated in a waggish physics joke in which he added his name to a scientific paper by George Gamow and Ralph Alpher. The result was that the April 1, 1948, issue of the journal Physical Review included an article titled “The Origin of Chemical Elements” by Alpher, Bethe, and Gamow.

Bethe never went in for computers or even electronic calculators, preferring to rely upon his trusty 70-year-old slide rule for the complex formulas that helped make him a legend. In later years, he took solace from numbers and said he spent several hours each day working with them. He also took long baths. “You sleep and things get somewhat unscrambled in your mind,” he said in 1996.”Then in the bath, I can become conscious of that.”

Hans Albrecht Bethe

Born July 2, 1906, in Strassburg, Alsace-Lorraine, Germany; died March 6 at home in Ithaca; survived by his wife, Rose, a son and a daughter, and three grandchildren.