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Chushiro Hayashi (1920–2010)

Published onDec 01, 2011
Chushiro Hayashi (1920–2010)

Chushiro Hayashi, the greatest Japanese theoretical astrophysicist, died of old age at a hospital in Kyoto on 28 February, 2010; he was 89 years old. C. Hayashi was born in Kyoto on July 25, 1920 as the fourth son of his parents Mume and Seijiro Hayashi. His father Seijiro managed a small finance company and the family “Hayashi” can trace its history back to honorable master carpenters who engaged in construction of the historic Kamigamo-shrine and Daitokuji-temple in Kyoto.

In his high-school days in Kyoto, Hayashi enjoyed judo, and he was interested in philosophy and read a lot of philosophy books. Some of his schoolmates thought that Hayashi would become a philosopher. After graduating high school, he moved to Tokyo and entered the University of Tokyo, Department of physics in 1940, where he encountered astrophysics through a paper by G. Gamow and M. Schönberg on the URCA process (1941), A.S. Eddington’s book “Internal Constitution of the Stars” (1926), etc. It was a difficult time of World War II. After a short time at university of two and half years, he graduated and was conscripted into the Navy. In 1945 the war was over he returned to his hometown Kyoto, where he joined a group of Professor Hideki Yukawa at Kyoto University, and studied elementary particle physics as well as astrophysics.

In his early outstanding paper (1950), Hayashi pointed out an important effect of neutrinos in the expanding early hot universe, resulting in chemical equilibrium between neutrons and protons, while Gamow et al. (1948) did not notice the effect in their abg-theory, where they assumed a pure neutron state as an initial state. Also Hayashi investigated the structures of red giant stars; he showed how red giant stars kept such large radius structures, in terms of stellar models with energy source of nuclear shell-burning (1949, 1957). He received a DSc in 1954; the title of his thesis was “Hamiltonian Formalism in Non-local Field Theories”.

After that, Hayashi concentrated on astrophysics. In 1957 he was appointed as Professor at Kyoto University. In the study of pre-main-sequence stellar evolution, he discovered the famous “Hayashi phase”, which was described in a three-page paper published (1961). He also compiled his studies of stellar evolution into a thick paper of 183 pages published in Supplement of Progress of Theoretical Physics with co-authors R. Hoshi and D. Sugimoto (1962). The paper was quite comprehensive, involving the whole stellar evolution from birth as protostars through death as supernovae, and frequently referred to as HHS. It was a bible in the field of stellar evolution for a long time, and may be so still.

The study of pre-main-sequence stellar evolution made Hayashi himself become interested in star formation and then planetary formation. Hayashi and his co-worker T. Nakano found that dynamical collapse of an interstellar cloud (which we should call a molecular cloud core, today) proceeded isothermally, by comparing the cooling time with the free-fall time (1965). Also, Hayashi and his co-workers made computer simulation of spherical collapse of a cloud to form a star (1970), resulting in rather high flare-up luminosity than Larson’s simulation (1969). These studies were really pioneer works in the field of star formation.

From 1970s through 1980s, Hayashi investigated the origin of the solar system extensively together with his co-workers (mostly his graduate students or former students). Once a year at Kyoto University there was held a small workshop on the origin of solar system by Hayashi; in addition to astrophysicists and astronomers, geochemists, cosmochemists and mineralogists came to the workshop from everywhere in Japan. Discussion was always active and tough. Hayashi and his co-workers presented many theoretical studies in the workshop every year, and they compiled those studies into a chapter in the Protostars and Planets II Book (1985). Like HHS above, the chapter gives a quite comprehensive planetary cosmogony, which includes formation of solar nebula, solid particle settling, planetesimal formation due to gravitational instability, coalescence of planetesimals, formation of terrestrial and Jovian planets, and, finally, nebula dissipation. It is called the “Kyoto model” and is now considered as a standard model of solar system formation.

In his tenure at Kyoto University was 30 years long, Hayashi had many graduate students and thoroughly drummed physics into them. Every Saturday afternoon, Hayashi held a colloquium in his office, but presenting in front of him was the most fearful training for his students. His disciplined methods of education and training, however, resulted in many of his students becoming university professors.

Hayashi was honored with many prizes; Eddington Medal from RAS (1970), Imperial Prize of the Japan Academy (1971), Order of Culture (1986), Order of the Sacred Treasure, the first class (1994), the Kyoto Prize of Inamori Foundation (1995), the Bruce Medal for outstanding lifetime contributions from ASP (2004), etc.

In 1984 Hayashi retired from Kyoto University. Even after that, Hayashi kept a small private seminar with his former students S. Narita and M. Kiguchi at a guest room of the university once a week and later at his home less frequently, and enjoyed discussion on astrophysics. The seminar lasted for 25 years until he was hospitalized for old age, i.e., a few months before his death.


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