Astronomy Bio...Chushiro Hayashi

Jay Bitterman

Chushiro Hayashi was born on July 25, 1920 in Kyoto, Japan. After obtaining his B.Sc. in 1942 at the University of Kyoto he became a research associate at the University of Tokyo. From 1946 to 1949 he was a research associate at the University of Kyoto. He then spent five years as assistant professor of Naniwa University, Osaka. In 1957 he once again returned to Kyoto University to become a Professor of Physics

In 1946 George Gamow proposed that the early dense stages of the Universe were hot enough to enable thermonuclear reactions to occur. In collaboration with Ralph Alpher and Hans Bethe, he hoped their work might account not only for the observed abundance of helium in the universe but also for the distribution of other, heavier elements. This theory assumed that matter was originally composed of neutrons which, at very high temperature and great enough density, would combine with protons. In 1948 the first detailed calculation of the formation of helium in the "hot big bang" was published.

In 1950 Hayashi exposed a fallacy in their "hot big bang" theory. He pointed out that at times in the Big Bang that preceded the first 2 seconds, the temperature would have been greater than 1010 K, which is above the threshold for the forming of electron-positron pairs. The creation of a pair of electrons requires about 1 MeV of energy, and at 1010 K many protons have this much energy. This radically changes the time scale of the a b g theory. Neutrons can react with the thermally excited positrons through the so-called weak interactions to produce protons and anti-neutrinos. The reaction time for this to happen (at temperatures above 1010 K) is less about 1 second. Hayashi proposed that at such a temperature there was complete thermal equilibrium between all forms of matter and radiation. Below that temperature, the weak interactions cannot maintain the neutrons in balance with the protons because the concentration of electron pairs is falling abruptly. This means that the ratio of neutrons to protons is "frozen in" until a few hundred seconds have gone by and neutron decay becomes appreciable. The "frozen-in" ratio at a temperature below 1010 K is about 15% and such a change in the ratio affects the helium abundance. The "frozen-in" abundance of neutrons does not depend on the material density but on the temperature and the properties of weak interactions. Thus if the density is great enough for the combining reaction between neutrons and protons to occur and faster than the expansion rate, a fixed concentration of neutrons will be incorporated into the helium nuclei no matter how great the material density, producing a "plateau" in the relationship between helium abundance and material density. Hayashi derived a percentage value for this plateau, but more recent research by physicists suggests that his value is too high.

Since that time, Hayashi has published many papers on the origin of the chemical elements in stellar evolution and on the composition of primordial matter in an expanding Universe. As yet there is not enough evidence of the primeval build-up of heavier elements. In any case, it now appears that the concentration of such elements in the galaxy has changed. With time, older stars tend to have a lower metal content than young stars. Astrophysicists are at present trying to correlate element abundance, stellar type and motion, in galactic location. It is to be hoped that such work will lead to an improved understanding of the way in which the heavier elements have been formed.

Published in the July 2000 issue of the NightTimes