1938: Fourth Washington Conference on Theoretical Physics: Stellar energy and nuclear processes

At this conference, Hans Bethe was inspired to investigate those processes that produce energy in stars. On his way back to Cornell University, Bethe determined all the important nuclear reactions involved, and within a week calculated the observed luminosity of the sun. He received the 1967 Nobel Prize for this work.

Attendees included: H. Bethe, G. Gunn, J. von Neumann, E. Teller, G. Beck, L. Hafstad, R. Roberts, M. Tuve, G. Breit, K. Herzfeld, H.N. Russell, W. Wildhack, S. Chandrasekhar, N. Heydenburg, R. Seeger, ?. Wulf, C. Critchfield, J. Mayer, H. Stromgren, J. Fleming, S. Menzel, A. Sterne, G. Gamow, F. Mohler, ?. Talbot


The Fourth Washington Conference on Theoretical Physics

The fourth Washington Conference on Theoretical Physics was held March 21 to 23, 1938, under the joint auspices of the George Washington University and the Carnegie Institution of Washington. This year the Conference was devoted to problems connected with the sources of stellar energy, particularly as these relate to nuclear physics. For this purpose several investigators whose work has been specially concerned with the internal constitution of stars (Drs. Chandrasekhar, Menzel, Sterne, and Strömgren) were brought together with physicists working on the problems of nuclear transformations (Drs. Bethe, Breit, Gamow, Hafstad, Neumann, Teller, and Tuve). The Conference was attended by a number of other physicists and astronomers from Washington and Baltimore, with a total attendance of nearly forty different investigators at the several sessions in addition to the meeting of the Washington Colloquium.

The first meeting was opened by Dr. Strömgren, who outlined in some detail the mathematical treatment and present status of the problem of temperature and density distribution in the interior of stars, with special reference to the critical features of the various particular stellar models used for these calculations. In his talk Dr. Strömgren presented as yet unpublished numerical tables, which give solutions of the fundamental equations of internal equilibrium for different conditions in the center of the star. These tables, a copy of which was left at George Washington University, will be of great help for calculations relating to the evolution of stars having a shell-source of energy, calculations which have been in progress for several months by Mr. Critchfield under the direction of Dr. Gamow.

For the theory of internal structure of stars, two physical properties of stellar matter are of extreme importance: The opacity and the rate of energy-production. The calculation of opacity, in its dependence on temperature and chemical constitution of stellar matter, was one of the primary topics under discussion. A report on calculations of this kind as based on our present knowledge about atomic structure was given by Dr. Strömgren at the second meeting; in his own studies he has made many important contributions to this question. Dr. Menzel reported unpublished calculations on stellar opacities carried out by Professor P. M. Morse of the Massachusetts Institute of Technology. Questions regarding the validity of present derivations of opacity-formulae led to an animated discussion between all participants of the Conference. In particular, Dr. Bethe now proposes to take up a more detailed study of this question.

The bearing of our present knowledge of nuclear reactions on the evaluation of the behavior of stars with nuclear sources of energy was reported by Dr. Gamow. On the basis of his recent investigations he proposed that the reactions important for the life of a star probably must be characterized by a resonance phenomenon. Such an hypothesis leads to an entirely new model of the star (the shell-source model) which it now appears may approximate more nearly the actual behavior of stars than do the models hitherto used for calculations of stellar interiors. Dr. Gamow also reported formulae for the absolute rates of energy-production of different types of nuclear reactions (ordinary and resonance-reactions), as obtained by him in collaboration with Dr. Teller. Such formulae are of great help to astronomers for their estimates of the actual energy liberation in stars.

The more detailed study of particular nuclear reactions which would lead to liberation of energy and to the building up of heavy elements in stars was reported by Dr. Bethe. He indicated that the so-called Weizsäcker’s chain reaction, which hitherto has been tentatively accepted as the fundamental course of nuclear reactions in a star's interior, is in contradiction with the recent results of nuclear research (in particular, the instability of helium-five). According to Bethe, the only possible reaction-chain which appears possible, and which would lead to the desired results, is obtained if we accept the stability of the nucleus beryllium-six. Evidence for the stability of this nucleus, however, has not yet been obtained experimentally. For the laboratory production of such a nucleus a voltage of about four million volts is required, and a study of this question will be feasible using the new high-voltage equipment under construction at the Department of Terrestrial Magnetism of the Carnegie Institution of Washington.

It was also indicated by Critchfield and Bethe that the reaction involving proton-capture by another proton with the emission of a positive electron and the formation of a deuteron has, under intra-stellar conditions, a rate which would be just enough to account for the energy-liberation of the Sun (but not of other much brighter stars). The introduction of a resonance level for two protons at a level-height of several kilovolts would considerably raise this probability, but the existence of such a level is highly improbable, as can be concluded from the investigations of proton-proton scattering by Drs. Tuve, Hafstad, Heydenburg, and Breit.

From the standpoint of experimental nuclear physics and the possible relation of these crucial astrophysical problems to the laboratory program of Drs. Tuve and Hafstad and their colleagues, two questions had previously appeared to constitute key problems, namely, the degree of stability of beryllium-eight and that of helium-five (or lithium-five). It became clear during the Conference that these two problems could be considered as eliminated (the answers being of a negative character) and that two other key problems take their place, namely, the degree of stability of beryllium-six and the production of deuterons by a primary process involving two protons (or possibly a proton and a light-quantum).

Another interesting question which brought about much discussion concerned the degree of central condensation of stars, together with the possible existence of a super-dense stellar nucleus, at least in some stars, as recently proposed by Landau. Dr. Chadrasekhar reported his investigations concerning the possibility of high central condensation in various known stars. His results lead to the conclusion that, whereas for giant stars the degree of central condensation is necessarily slight, there are stars for which as much as 90 per cent of the total mass is concentrated within less than half the radius from the center. Another aspect of the problem of central condensation was given by Dr. Sterne, who indicated the possibility of direct estimates of the density-distribution of double-star components from the observed characteristics of their orbits. The study of the stellar model having a highly condensed neutron-core (Landau’s star-nucleus) was reported by Teller. By direct integration of the equations of stellar equilibrium, one arrives for such models at extremely high temperatures (~109 C°) and densities (~109 gm/cm3) near the surface of the core. Since under such conditions the already-known nuclear reactions would proceed with extremely high velocities, it was concluded that such a star model is inherently unstable. Valuable contributions to the discussion of such superdense state of matter in a stellar interior from the point of view of general theory of gravitation was given by Neumann.

At the conclusion of the Conference a joint meeting was held with the Washington Physics Colloquium at which Dr. Strömgren reported his extremely interesting calculations and deductions regarding the so-called “transparent star” Epsilon Aurigae, as recently obtained at Yerkes Observatory.

After the Conference, from the comments of those who participated, it became clear that several specific contributions had been made toward formulating the next line of attack on the problems of stellar energy from the points of view of both physics and astronomy. Specific problems needing solution were crystallized in these discussions, but an equally important result was the stimulation of an active mutual interest on the part of the two groups represented, with a new appreciation of the extent to which definite answers can now be given to questions of long standing. "Nec frustra signorum orbitus speculamur et ortus".

G. Gamow
M. A. Tuve
March 28, 1938