It seems hard to realise, today, that pulsars were totally unknown at the time of the last General Assembly. The discovery of pulsars dates from November 28th, 1967, when the first pulses were recorded from the source now known as CP 1919. Since then we have witnessed an astonishing phase of activity, amongst observers and theoreticians alike, which may be unique in the history of astronomy. Now, with at least 50 pulsars in the catalogues, we are in possession of a wealth of information about these remarkable sources. My aim tonight is to outline the observational evidence which leads us to the physical nature of pulsars. I shall happily leave to Professor Ginzburg the far more difficult task of weaving these strands into a logical pattern and devising a model to account for what is observed.

Theorists, both physicists and astronomers, are usually greatly pleased with their choice, because to study theoretical questions is, in a sense, easier and more effective than to observe and to measure. Experimentalists and observers, on the other hand, often grumble at their fortune for their work is very labour-consuming and its success depends largely on quite non-scientific problems, such as getting money, equipment and so on. I should like to mention this because the study of pulsars can serve as an example (not, of course, the only one) when the theorists have every reason for envying the observers. At any rate as far as I am concerned, this is so. In the previous report made by A. Hewish the facts were presented and we have every reason for congratulating the observers on their success. In less than three years great work has been done. As for the theory of pulsars we have, for the time being, a shortage of exactly established facts and I would like to discuss mainly general considerations and working hypotheses. Fortunately, for the theorists the situation is not always like that. There are cases when theory goes far ahead and anticipates observations. In the case of pulsars some lag in the theory is caused by two circumstances. Firstly we deal here with exceptionally complicated tasks, for example, with the equation of state of a substance with a density ℓ ≿ 1011 g cm−3 and the electrodynamics of the magnetosphere of a rapidly rotating star with non-coinciding axes of rotation and of magnetic symmetry (say, with the direction of magnetic dipole). Secondly the observational data, in spite of their variety, give only indirect information about pulsars because their structure cannot be seen directly as, for example, in the case of the surface of the Sun or a number of nebulae.

Does our Galaxy possess spiral structure and, if so, can this structure be traced observationally with some degree of confidence? This is the major question to which I shall address myself tonight. Following my talk, Professor Lin will speak about the theoretical aspects of spiral structure. I shall be very brief in my references to spiral structure in galaxies outside our own and leave it largely to Professor Lin to speak about observations on other spiral galaxies and their relevance for possible theoretical interpretations.

A year ago, my good and distinguished friend Professor B. A. Vorontsov-Velyaminov chided me in his address at the Basel Symposium for having been too eager to accept the fact that observation suggests that our Galaxy really possesses spiral structure. I hope to show in my Discourse that for our Galaxy we have indeed evidence for the presence of an underlying structure of spiral features.

It is an honor for me to continue the discussion of galactic spirals following Professor Bok, who has contributed to the subject for many years. My experience has been relatively short. My first encounter with the study of galactic spirals occurred in 1961, when I was invited by Professor Bengt Strömgren to attend a conference on interstellar matter at Princeton, N.J. There I first learned about the winding dilemma from Professor Jan Oort. After the conference, Professor Lodewijk Woltjer, who edited the Proceedings, visited me for about a month. Thus, I began slowly to learn about spiral galaxies and to work on the subject. It turned out to be an extremely rewarding experience, for the observational data were already ripe for theoretical analysis, and the hydromagnetic theory of spiral arms was clearly encountering great difficulties. Since I was not educated as an astronomer, I owe my gratitude to Professors Strömgren, Woltjer, and Kevin Prendergast, on whom I depended for correct astronomical facts as I started my work. Without their help, I would not be standing here today. Later on, I was to receive help from many other distinguished astronomers, including Professor Bok from whom you just heard. I am greatly impressed with the community of astronomers as a dedicated group of scientists.

In successfully carrying out a manned lunar landing and return, with both operational and technological objectives, the Apollo program made possible a variety of significant scientific experiments. This important milestone in the continuing quest for knowledge took the eyes, hands, and mind of man, as well as his instruments, to a new world. The activities of highest priority carried out by the astronauts, once the landing had been successfully completed, were to collect lunar material and data, emplace sophisticated experiments, and record man’s impressions and observations.

In the missions ahead, scientific exploration of the Moon will be the principal goal. Unique features and sites on the Moon will be visited. New experiments, both on the lunar surface and in lunar orbit, will be carried out, as we probe the Moon’s past and attempt to unravel the early history of the Earth. In so doing, we will also be establishing and defining the possibilities and limitations of man as a space explorer as we extend his domain further in space.

One of the most significant advances in our understanding of the Moon has been the discovery, through the Surveyor, Orbiter and Apollo programmes, of widespread lunar igneous activity and differentiation.

Volcanic flows (Figure 1) in, and in the neighbourhood of, Tycho are up to 20 km long and up to 12 km wide. At least 5 flows, mapped by Strom and Fielder (1970) originated in, and breached, craters up to 100 km from the rim of Tycho. One flow (Figure 2) is essentially the same age as the ropy floor material of Tycho, since it is traversed by cracks that are indicative of marginal stretching of the floor unit. Other flows are found in what are probably lava lakes (Figure 3), and two flows derive from such lakes.

The objective of the passive seismic experiment is to measure vibrations of the lunar surface produced by all natural and artificial sources of seismic energy and to use these data to deduce the internal structure and constitution of the Moon, the nature of tectonic processes which may be active within the Moon, the rate of strain energy release for the lunar body, and the numbers and masses of meteroroids striking the lunar surface. The instrument used is also capable of measuring changes in gravity and tidal tilts which occur in its vicinity. To accomplish these objectives, seismic data must be combined with data from laboratory measurements of the physical and chemical properties of surface rocks, and many other geophysical and geochemical measurements. Thus far, we have had the opportunity to record data from two lunar seismic stations which were installed by the astronauts during Apollo missions 11 and 12. The combined recording time from these stations is presently over 9 months, but there was no overlap to permit recording the same event at two stations. Results from the analysis of these data have been presented by the seismic experiment team in five previous papers.

It is shown that the Moon possesses an extraordinary response to induction from the solar wind due to a combination of a high interior electrical conductivity together with a relatively resistive crustal layer into which the solar wind dynamic pressure forces back the induced field. The dark side response, devoid of solar wind pressure, is approximately that expected for the vacuum case. These data permit an assessment of the interior conductivity and an estimate of the thermal gradient in the crustal region. The discovery of a large permanent magnetic field at the Apollo 12 site corresponds approximately to the paleomagnetic residues discovered in both Apollo 11 and 12 rock samples The implications regarding an early lunar magnetic field are discussed and it is shown that among the various conjectures regarding the early field the most prominent are either an interior dynamo or an early approach to the Earth though no extant model is free of difficulties.

Prof. Fred L. Whipple kindly agreed to be the chairman for all three panels, and introduced the next speaker, Prof. F. Hoyle, who spoke on ‘The Solar Nebula’. Hoyle: I would like to begin this contribution by considering the deductions we can make by comparing the gross chemical compositions of the planets with the composition of the Sun. For this purpose I have divided the planets into the three groups shown in

The second line gives the mass fractions in the Sun of the major constituents of the planetary groups, while the fourth line gives the factors by which the present masses must be multiplied to give the amounts of solar material needed to yield the appropriate amounts of main planetary constituents. The interesting points emerge that Jupiter and Saturn require the least amount of solar material, and that Uranus and Neptune on the one hand and the terrestrial planets on the other require approximately equal amounts. The total requirement is for ≈10-2Mʘ This is less by a factor ≈10 than the amount postulated in many theories of the origin of the planetary system. However the amount we have now calculated can readily be seen to be consistent with angular momentum requirements.

Hall: On behalf of the Organizing Committee of Joint Discussion A it gives me great pleasure to introduce our next speaker, Dr B. Levin. He is Chief of the Department on the Evolution of the Earth at the Schmidt Institute of Physics of the Earth at Moscow. He will present an invited paper on the Internal Constitution and Thermal Histories of Terrestrial Planets.

Levin: The problems of the internal constitution of the terrestrial planets and that of their thermal histories are closely connected. We are interested not only in giving the distribution of density in planetary interiors and their stratification into physically distinct layers, but also in giving some insight into the chemical gross composition of planets. At the same time the thermal histories of the terrestrial planets strongly depend on the content of radioactive elements and thus on the chemical composition. The differentiation of planetary interiors into chemically distinct layers depends on their temperatures which determine their viscosity, but, on the other hand, the redistribution of radioactive elements in the course of differentiation seriously change the further course of the thermal history. The controversial problem of thermal convection in planetary interiors depends both on their constitution and on their thermal history.

De Marcus: In a discussion of the formation of the solar system’s giant planets it seems necessary, even if somewhat tedious, to define our terms. Strictly this would mean some attempt to outline the past physical steps which led these bodies to their present condition. However, it seems possible that some at least of these bodies may not yet be quite through with their formation in the more general and presumably more meaningful sense that changes may still be ahead in which such basic parameters as mean density etc. may be appreciably different than the current values. I do not mean anything here as trivial as improved measures of radii which recently appreciably altered the number we refer to as the mean density of Neptune. Rather as an example of what I mean (but without asserting anything about the probability that the example is true) it could be that Saturn has much more helium than the current models indicate, in which case at some time in future his mean density might become ≈2.0 gm/cm3 or more instead of the present value of 0.7 gm/cm3. Consequently I first like to address myself to observational evidence which might suggest that such changes could happen.

The Chairman, J. S. Hall asked I. P. Willams (Reading, England) to speak on ‘Planetary Formation’.

Williams: According to a theory by McCrea, published in 1960, after a protosun has been formed about 1000 unstable cloudlets, called floccules by McCrea, are captured in orbit around this protosun. Their orbital distance is roughly equal to the mean free path of the floccules in the original gas cloud from which both the sun and the captured floccules formed, taken numerically to the 60 AU. In order to conserve angular momentum about 600 of the captured floccules will be in prograde orbit while 400 will be in retrograde orbit. As an agglomeration of about 20 floccules is stable, when floccules adhere on collision stable condensations may be formed. We make a statistical investigation of this process. The problem is similar to that of having 400 red balls and 600 black balls in a bag which are pulled out and assembled into a pile. When 20 are in a pile a stable condensation exists and a new pile is started. If there are equal numbers of red and black balls in a pile this compounds to a condensation with very low angular momentum which falls into the Sun and so this is rejected and a new pile started. The number of stable condensations that is formed and the ratio of prograde to retrograde floccules in each of these condensations are obtained. This ratio determines the angular momentum, and hence the position, of the condensation.

I wish to thank the Organizing Committee members who planned and organized this Joint Discussion - Professor Fowler, who will preside this afternoon, Dr Hearn, Dr Reeves, Professor Tayler, Professor Underhill, Professor Wallerstein, and Professor Mathis, who has consented to edit the published proceedings.

We feel we have assembled a team of the world’s greatest experts on the helium problem, and we know that there are many other even greater experts in the audience. The general idea of the program is that the first session will be devoted to deductions about what the abundance (or abundances) of helium is (or are) in various objects, and the second session to what these abundances imply about stellar, galactic, or universal evolution. We very much want to encourage discussion and the presentation of new results following each review paper.

It is particularly appropriate to discuss helium at this IAU General Assembly in England, as this element was discovered in 1868 by Sir Norman Lockyer, who measured λ5876 in the spectrum of the chromosphere, and realized that none of the then known elements could produce it. Helium is thus a real British astronomical element. Furthermore, helium was first identified on the Earth by Sir William Ramsay, who observed the same line in the gas obtained from uranium, and thus showed it was a terrestrial element too.

In this talk I shall try to outline the subject which will be discussed in detail by the succeeding speakers. I shall concentrate on ideas and I shall not attempt to present any definite results. The two sides to this problem are:

(a) How much helium is there in the Universe?

and

(b) Where and how has it been produced?

I shall sub-divide (a) and (b) into

• (a) (i) Direct observations

  • (ii) Inferences from theories of stellar structure and evolution

• (b) (i) Helium production in stars

  • (ii) Primeval helium production

  • (iii) Production in other objects

Helium abundance in stellar atmospheres has been the subject of several recent investigations, essentially induced by progresses in the computation of non-grey model atmospheres (including line blanketing in the far U.V.), by considerable advances in the theory of Stark broadening of helium lines, by non local thermodynamic equilibrium (LTE) work for the helium spectrum and more fundamentally by the intrinsic importance of the helium over hydrogen ratio in cosmological theories.

From the titles of the other contributions to this morning’s session, one deduces that the hard facts of theory are to be sandwiched between the loose speculations of direct observation. I shall therefore feel free to discuss theoretical implications from stellar structure unencumbered by what are, after all, but peripheral considerations.

I should, however, begin with an apology. It is impossible to do even minimal justice to the vast number of investigations which have contributed to our present understanding (or lack of it). The choice of topics has been dictated in part by history, in part by current relevance, but mainly by personal whim. While much of the discussion centres, for obvious reasons, on Population II work, one or two problems associated with Population I will also be considered. Reference may be made to the exhaustive review article of Iben (1967) and also to the Warner Prize Lecture by Demarque (1968) for background and a more complete discussion of some of the points which will be touched upon here.

I shall assume today that the ‘normal’ helium content of stars is of the order of 25±5% by mass. This leaves me free to pay particular attention to systems which do not appear to adhere to this generalization, systems for which abnormally high or low helium contents have been claimed. The main point of my talk is that there is no really convincing, concrete evidence from stellar interiors alone for helium contents far removed from normal. During the course of the past decade, this has come to be largely accepted. It is in a way sad to see the radicalism of one’s youth become the orthodoxy of middle age.

During the past two or three years much work has been done on the determination of helium abundances in gaseous nebulae, using observations at both optical wavelengths and radio wavelengths. I imagine that the organisers of this Discussion must have asked whether the subject should be reviewed by an optical observer or a radio observer, and have finally resolved their dilemma by choosing a speaker who has not made observations at any wavelength whatsoever.

In order to maintain the luminosity of the Galaxy ⅓ of a solar mass of hydrogen has to be transformed into helium every year. This rate of production is too small by a factor 10 or 20 in order to give a helium content of Y = 0.3-04. within the age of the galaxy if the mass fraction Y of helium was zero at the beginning. The situation is even worse if the destruction of helium by helium burning is taken into account. In his review paper Tayler (1967) came already to this conclusion. I shall discuss the problem here using more recent model calculations, but we shall come up with the same result.