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(Ed. note: The custom in these Symposia has been to have a summary-introductory presentation which lasts about 1 to 1.5 hours, during which discussion from the floor is minor and usually directed at technical clarification. The remainder of the session is then devoted to discussion of the whole subject, oriented around the summary-introduction. The preceding session, I-A, at Nice, followed this pattern. Christy suggested that we might experiment in his presentation with a much more informal approach, allowing considerable discussion of the points raised in the summary-introduction during its presentation, with perhaps the entire morning spent in this way, reserving the afternoon session for discussion only. At Varenna, in the Fourth Symposium, several of the summaryintroductory papers presented from the astronomical viewpoint had been so full of concepts unfamiliar to a number of the aerodynamicists-physicists present, that a major part of the following discussion session had been devoted to simply clarifying concepts and then repeating a considerable amount of what had been summarized. So, always looking for alternatives which help to increase the understanding between the different disciplines by introducing clarification of concept as expeditiously as possible, we tried Christy's suggestion. Thus you will find the pattern of the following different from that in session I-A. I am much indebted to Christy for extensive collaboration in editing the resulting combined presentation and discussion. As always, however, I have taken upon myself the responsibility for the final editing, and so all shortcomings are on my head.)

In discussing the absolute magnitudes of RR Lyrae stars, I concur in the importance of the question but I would, at the outset, like to insert a word of caution: It seems to me most likely that the Mb of RR Lyrae stars is not an immutable constant but depends on the original composition of the star. I will come back to this point later.The principal point of view which I shall emphasize is rather different from that of most observers. The observer is usually interested in using the RR Lyrae stars as a means to calibrate distances and thereby looks out from the star to the Galaxy and then to the Cosmos. In contrast, the interests of the theorist look inward and he views the RR Lyrae stars as convenient fixed points for the comparison of theory with observation.At present, the theory of stellar structure is tied to fitting the Sun. We all know the difficulties in the solar neutrino experiment which casts some doubt on whether we even understand the Sun. But we have few if any cases of evolved stars where we know M, L, R, to check our stellar evolution calculations.We are now in desperate need of new fixed points where we can compare observation and the theory of stellar structure and evolution. The RR Lyrae or Cepheid type variables provide excellent examples for this comparison of observation and calculation. They are in late stages of evolution where tests are needed, they are readily identifiable, and well observed, and already the knowledge of the period provides a very precise mass-radius relationship so that a complete determination of the model will be provided by only a few additional measures.

Corrections to the measured energies of nuclear reactions associated with the binding and excitation of atomic electrons are calculated.

In discussing the absolute magnitudes of RR Lyrae stars, I concur in the importance of the question but I would, at the outset, like to insert a word of caution: It seems to me most likely that the Mb of RR Lyrae stars is not an immutable constant but depends on the original composition of the star. I will come back to this point later.

In this paper we report methods of computation which have been developed to provide a theoretical understanding of the RR Lyrae and Cepheid type pulsating stars. The results reported are intended to illuminate the methods of calculation and to provide insight into the physical processes in these stars. A survey of pulsation in RR Lyrae models using these methods has also been carried out and will be reported soon in another journal. A survey of pulsation in Cepheid models has been initiated and will be continued.

biological effects of ionizing radiation by any means, we probably have a better understanding of these than we do of the effects of all sorts of other pollutants in our atmosphere. This area is fraught with uncertainty, but less uncertainty than many other pollution problems that beset us. The second area I am going to talk about is routine exposures from the nuclear power industry. The industry has many stages and each of these various stages produces exposures to nuclear radiations. These can be designed to be quite small, and indeed they are so designed. A third area is probably more on the public mind-the large accidental releases of radioactivity from various stages, primarily reactor stages, at the nuclear fuel site. Fourth, we come to the strangest area, the one dealing with the effects of long-lived radioactivities on

Non-linear calculations of intermediate amplitude to investigate stability have been carried out for one hundred RR Lyrae models of varying composition, mass, luminosity, and effective temperature. Of these, the calculations have been extended to the maximum amplitude for fifty models. The dependence of the location of the instability strip on composition, mass, and luminosity has been explored. In particular, the high-temperature boundary of the strip increases by 500° K for each 15 per cent increase in the mass fraction of helium. The dependence on effective temperature of the phase shift, the ratio of radius and luminosity amplitudes, and the shape of the light-curve has been explored, and the correlations of these three measures have been found. These measures may then be applied to observed variables to determine their location in the instability strip and the helium content of their envelopes. The examples studied give best agreement at about 30 per cent helium by mass. The calculations reveal many details of the shapes of the light- and velocity-curves which may be compared to observation. A number of features of these curves of known variables are discussed, and the underlying explanation is investigated. The sources of the driving energy of these models have been explored. In the models of the most favored composition, 30 per cent helium, the hydrogen ionization region is almost as important to the driving as is the He II ionization. The effects that cause the amplitude to cease growing and level off at the observed maximum amplitude are explored: they appear to involve several non-linear effects, particularly a steep velocity front, in the He II ionization zone. A new kind of period-luminosity relation has been discovered. The shortest period at which the fundamental mode persists, before it changes to the first harmonic, has been found to be independent of mass and composition and to depend only on the luminosity. This relation was found to be Ptr = 0 057 (L/L☉)0.6 days. This has been used to interpret the varying period distributions of RR Lyrae stars in clusters and in the field with the result that the luminosities are found to vary from Mbol = 0.57 for very metal-weak clusters to Mbol = 0.96 for clusters only slightly weak in metals. Metal-rich variables are slightly less luminous still. These results lead to the conclusion that the masses of RR Lyrae stars are near 0.5 M☉, which must imply a significant mass loss. These conclusions appear to be consistent with the possible interior models.

Nonlinear calculation methods for model representing W virginis, noting rapid amplitude growth and luminosity-to-mass ratio

Computation method used in pulsation of Lyrae models extended to models of Cepheid variables, noting instability of appropriate period, mass size, luminosity and velocity curves

The relation of the hydrogen ionization zone to stellar pulsation is discussed. A detailed analysis of the behavior of this zone in SU Draconis, shows that it is an important energy source for driving pulsation. An examination of conditions in other RR Lyrae stars, cepheids, and W Virginis stars suggests that the hydrogen ionization zone is important in determining the phase lag and in driving their oscillation. It is proposed that non-spherical pulsationtype motions, driven by the hydrogen ionization instability, are present in all the late-type giants and supergiants and provide the principnl means of energy transport in the hydrogen ionization zone of such stars. These motions could account for indications of variability and supersonic velocities in their photospheres. Numerical calculations have been initiated to develop these ideas further. It is also shown that the dissociation transition of molecular hydrogen may provide the heat sink that is the key to the existence of the class of cool variables that include the Mira and semiregular red variables. (auth)