Frank Shu (1943-2023)

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Portrait of Frank Shu.

Credits: AIP Emilio Segr Visual Archive, Physical Collection Today

Chinese American astrophysicist Frank Hsia-San Shu has made fundamental contributions to the understanding of galaxies and star formation, leaving a legacy of scientific work characterized by originality and profound insight. He wrote two widely consulted textbooks and, later in his life, embarked on a second career investigating energy technologies to help fight the climate crisis. For his lifelong contributions to theoretical astrophysics, in 2009 he was awarded the Shaw Prize in Astronomy, often described as the Nobel of the East, and the Catherine Wolfe Bruce Gold Medal from the Astronomical Society of the Pacific. He died at his California home on April 22, at the age of 79.

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Shu was born in 1943 in the southern Chinese city of Kunming, a refuge for intellectuals during World War II, partly due to the evacuation of many universities in the city. He immigrated to the United States at the age of six when his father, mathematician and engineer Shien-Siu Shu, joined the mathematics faculty at Purdue University in West Lafayette, Indiana. Raised in the Midwest, he fervently pursued interests in math and physics. He was admitted to the Massachusetts Institute of Technology in Cambridge, as one of its junior undergraduate physics students, and graduated in 1963.

At MIT, Shu began working with his mentor Chia-Chiao Lin on the dynamics of disk galaxies. This collaboration continued at Harvard University, also in Cambridge, where Shu received his PhD in 1968. Using wave methods, Shu and Lin postulated that the spiral arms observed in disk galaxies were a wave phenomenon consisting of spiral patterns propagating through the fluid medium of stars and gas. How these models were amplified to their enormous size remains a matter of debate, but density wave theory has allowed predictions of spiral galaxy properties to be derived from first principles. It has also been used to study the gravitational stability and structure of many astrophysical objects, including galactic disks, binary stars, planet-forming disks orbiting young stars, and the rings of Saturn.

Shu’s work was motivated by fundamental theoretical questions. He paid particular attention to stars not just as points of mass, but as points of light. He was always interested in how they form, observing the massive OB stars, which illuminate the spiral structure of disk galaxies seen at optical and ultraviolet wavelengths. At Stony Brook University in New York City, where Shu moved after his PhD, he led a team that showed how density-wave-induced shocks could compress a diffuse stellar medium into a higher-density phase than giant molecular clouds forming stars. In collaboration with radio astronomers from the University of Groningen in the Netherlands, Shu provided further evidence for the density wave theory.

After joining the faculty of the University of California, Berkeley in 1973, Shu turned his attention to the formation of individual stars, the subject of his most influential contribution. Although molecular clouds were identified as the birth sites of new stars and the big picture of star formation through the collapse of material under its own gravity was considered, few specific details were known. Shu and his collaborators had to address how nature overcomes barriers to gravitational collapse, such as angular momentum and ambient magnetic fields.

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The paradigm that emerged was the now-standard four-step framework: the formation of higher-density nuclei in a molecular cloud; their subsequent dynamic collapse; an unexpected (but observable) phase in which strong winds of gas blow out of the still-forming protostar; and the emergence of stars accompanied by planet-forming disks. This framework tightly linked theory and observations, and was further strengthened by independent developments, such as the idea that once-radioactive relics found in the most primitive meteorites may have originally been the products of ancient solar flares.

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While some of Shu’s theoretical hypotheses remain controversial, he has never hesitated to tackle unresolved fundamental concepts and explore innovative explanations of the most startling observations. He was also a prolific and lucid writer. In addition to many highly cited articles, Shu wrote the universally adopted introductory textbook The physical universe (1982) and the two-volume graduate textbook The physics of astrophysics (199192).

Shu served as president of the American Astronomical Society from 1994 to 1996. He engaged with a burgeoning astronomical research community in Taiwan, spearheading efforts toward launching the Institute of Astronomy and Astrophysics, part of the Academia Sinica research institution with based in Taipei. In 2002, he became president of National Tsing Hua University in Hsinchu, Taiwan, following in the footsteps of his father, who held that position from 1970 to 1975.

Shu returned to the United States in 2006 at the University of California, San Diego, where he became a professor emeritus in 2009. Over the past twelve years, his concern about the climate crisis has led him to study the use of molten salt reactors to generate energy from nuclear waste and to convert waste biomass into inert products that can be sequestered, removing carbon from the atmosphere. He was also writing a college textbook called The history of science, which was to tell how science has underpinned human progress and the lessons that come with it for tackling climate change. That book remains unfinished, but Shu’s place in the history of science is assured.

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Conflicting interests

The authors declare no competing interests.

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