31 redshifts have been obtained for A1367, 34 for A262 and 61 for the Centaurus cluster (HMS 1247-4102). Full details of this work have been and will be presented in M.N.R.A.S. Both A1367 and A262 are spiral-rich in Oemler’s classification, while Centaurus is intermediate in type between spiral-rich and poor. In all 3 cases, the distributions of E and L galaxies are centrally concentrated, whilst the spirals are distributed in a more extensive and ragged fashion. The mean corrected redshifts and velocity dispersions for 2 morphological subsets are given in Table I. There is no significant difference in the mean velocities of the 2 subsets for each cluster. However, the velocity dispersions for the spirals are significantly greater than those for the E,L galaxies in both A1367 and A262. The differences in velocity dispersions of the 2 subsets for Centaurus are not statistically significant. Gott and Gunn have suggested that irregular clusters of the sort presently described have not undergone collapse. However, all 3 clusters show morphological separation and are x-ray sources. This is consistent with a collapsed core surrounded by a shell of infailing spirals.

The motion of G8-K5 III, IV stars near galactic poles (|b|>60°) was investigated. The main attention was paid to motion in the z-direction. The spectral types, visual magnitudes, colors and radial velocities (vr) were taken mainly from Schild (1973), Heard (1956), Blanco et al (1968) and Wilson (1953). To reduce the values vr by Heard to those by Wilson, the correction of -1 km/sec was made. The stars with variable vr and spectroscopic binaries were excluded from the investigation.

A very faint intergalactic background light component (to be called IBL) was photographically discovered by Zwicky (1957) in the centre of the Coma Cluster. More recently Welch & Sastry (1971, 1972) have mapped the area using isodensity tracings of photographic plates. In order to provide an accurate calibration of the intensity and to determine the colour of the IBL in the Coma Cluster, photoelectrical observations have been made of a region located ˜2′ east of IC 3949 and ˜12′ south-west of the cluster centre. The observations were made using a 60-cm Ritchey-Chretién telescope at the Metsähovi Observatory near Helsinki. The diaphragm size was 115″. In Table I the results of these observations are given together with three other available photoelectrical values for the IBL intensity in clusters. For comparison, the surface brightness due to visible galaxies has been estimated. The resulting values are: IV = 11.6 S10 (≙ 25.1 mag sec-2), IB = 4.6 S10 (≙ 26.1 mag sec−2), and thus the IBL intensity amounts to 25 percent and 39 percent in V and B, respectively, of the light of visible galaxies. For the colour index of the IBL one obtains using the values in Table I: B-V = 0.54 ± 0.18. Two mechanisms have been hitherto proposed in the literature to explain the IBL in the Coma Cluster: (1) light from dwarf galaxies, intergalactic globular clusters or individual intergalactic stars, and (2) thermal bremsstrahlung from a hot intergalactic gas with Te = 0.5 − 10 × 105 K. A third possible mechanism can still be mentioned, namely the scattering of the light of galaxies by intergalactic dust grains.

Typical orbital periods for galaxies in rich clusters are observed to be several billion years. Consequently these large systems have had little time to come to equilibrium, and indeed if they formed from small initial density perturbations they must have taken a substantial fraction of the age of the Universe to separate from the Hubble flow and recollapse (Gunn and Gott 1972). Despite the relatively short time available, dynamical effects can cause appreciable evolution both in the distribution of galaxies within clusters and in the observable features of individual galaxies. The overall aspect of a cluster changes as violent relaxation irons out in homogeneities and establishes a relaxed core-halo structure. At the same time collisional relaxation causes massive galaxies to lose energy to lighter objects in an attempt to reach equipartition of kinetic energy, and to spiral towards the cluster centre. These processes are relatively easy to analyse in any particular scheme for cluster formation, and below we concentrate on comparing their predicted effects with observation.

The papers presented this morning form a good illustration of the old adage that you should never publish a paper that is completely correct, for then you can write only one paper. But if you publish a paper with a lot of errors, you are sure to be criticized, and you can write a second paper to answer your critics, and if the heat proves to be too much you can write a third paper withdrawing everything.

First a bit of history. It is difficult to pin-point when exactly this question of the star density in high latitudes first came up, but I feel sure that it is Implied, to say the least, in the early work on faint blue stars by Malmquist (1927, 1938) and by Humason and Zwicky (1947). This morning Ivan King showed us a table of the changes in the frequency of occurrence among the stars in high galactic latitude as one goes to fainter and fainter magnitudes. This is exactly what I did in 1960 when I published a color-magnitude diagram for 4000 stars down to 19th photographic magnitude near the South Galactic Pole (Luyten 1960). These data had been obtained with the Palomar 48-inch telescope using Haro’s three-image method. I also made up a similar diagram calculated from what I thought were then the best available data on the luminosity and density functions. The conclusion of my analysis was that there seemed to be rather fewer M stars than had been expected, and a great many more stars with the color of F and G stars than expected. This idea was not popular at the time; hence this paper has been carefully ignored until these ideas were used by others twelve years later without reference to the 1960 paper.

In research in the galactic polar caps we may roughly distinguish two categories:

I. Research on problems concerning the galactic population at large distances from the plane, including the density and velocity distribution and the field of force at large z.

II. Research on problems concerning the population in the immediate neighbourhood of the sun; at high galactic latitudes it is easier to discriminate the nearest stars than at low latitudes - see, for in stance the investigations of the M dwarf population.

Most of the papers presented in part II of this Joint Discussion deal with the first category, and of these, the distances reached are mostly within a few Kpc. Classical investigations, some of them dating from more than thirty years ago, have provided our current knowledge of the force K(z) perpendicular to the galactic plane up to about 1500 pc and the large scale features of the density distribution. What emerges from recent and current work, is attempts to improve the knowledge of K(z) and to extend it to greater distances; and refinement of the density and velocity investigations by more and more detailed discrimination of population types. The dominating problem is that of the separation of the stars according to two basic parameters: age and chemical composition. Kinematic and space distributional properties for subgroups according to these two parameters will be the necessary elements on which a theory of the (local) evolution of the Galaxy is to be built. The vague notion, that metal abundance may be taken to be an equivalent of age is abandoned and replaced by the recognition that at any epoch, star formation may have occurred with a considerable spread in the resulting stellar metal abundances.

I would like to report on some work that Ed Turner and I have done on groups of galaxies and how groups may be used to obtain useful cosmological information. Of course the classic study of groups of galaxies was done by de Vaucouleurs (1975) who made a catalogue of groups and loose associations of galaxies in the local supercluster. Partly to obtain a larger sample of groups with uniform and well defined statistical properties and partly to obtain a sample of denser groups suitable for virial analysis, Ed Turner and I decided to make a new catalogue of groups of galaxies based on picking the groups as surface density enhancements in the Zwicky catalogue (Turner and Gott 1976) (TGI).

The problem of the existence of high-order clustering of galaxies is a basic one in extragalactic astronomy and cosmology. In a conventional formulation this problem consists of indicating the largest configurations in the spatial distribution of galaxies.

All treatments are carried out generally on the material presented in the catalogues of Abell (1958) and Zwicky et al. (1961-1968). However catalogues of the Jagellonian type (Rudnicki et al. 1973) may turn out to be extremely valuable. The nature of the observational material demonstrates that only statistical methods may be applied effectively. It is already clear that the expected effects are not large even if high-order clustering is a basic property in the Universe.

In the days following our Joint Discussion the Co-editors were joined by two of the participants, Dr. Ivan King and Dr. Uli Steinlin, in an effort to evaluate results reported as well as the problems posed for future work. As a result of these discussions at Grenoble the present listing was made. This list was presented to members of the Organizing Committee but since it was impossible in the last days at Grenoble to convene this group or to speak collegially with all participants it must remain the responsibility of the above-named authors. It cannot be a complete resumé nor can it presume to represent adequately the varied opinions expressed at Grenoble on 25 August. We hope here only to attempt a synthesis of certain evident results and to outline certain prospects for exploring further the exciting problems of structure and evolution in the galactic polar caps.

A new updated list of clusters of galaxies found within x-ray error boxes is presented. Since the last published list (Vidal 1975) four new clusters A478, A2142, A2255 and an anonymous cluster were found within 3U0405+10, 3U1555+27, 3U1639+40 and MX1329−31, respectively. This extensive list is used to look for phenomenological correlations between x-ray luminosities and other physical cluster parameters. We analysed 3 such correlations: (a) The x-ray luminosity-velocity dispersion diagram is presented using newly determined velocity dispersions in the clusters A1060 (Vidal & Peterson 1975), A1367 (Tifft & Tarenghi 1975) and the Centaurus cluster (Vidal & Wickramasinghe 1976). First, it is shown (Yahil & Vidal 1976) that the observed velocity distribution of galaxies in clusters in general is consistent with a gaussian and, therefore, the standard deviation of the distribution can be used as a good statistical measure for the velocity dispersion. But, inspecting the Lx-∆V diagram, it is shown that a linear relationship between these parameters is not compelling yet. Furthermore the slope of such an assumed linear relationship is relying too much on the isolated Perseus point. Consequently the velocity distribution in the Perseus cluster is critically analysed.

The discovery of the extended sources of X-ray emission associated with clusters of galaxies was undoubtedly one of the most significant observations carried out by the Uhuru satellite (Gursky et al (1971), Giacconi et al (1974)). At the present time, there are more than 30 identifications and suggested associations of X-ray sources with clusters of galaxies although extended emission has been directly observed in less than 10 cases.

The mechanism for X-ray emission from such large objects is of considerable interest. X-rays could be generated by the inverse Compton interactions of microwave background photons with populations of relativistic electrons distributed throughout the clusters. Alternatively the radiation may be due to Bremsstrahlung from hot (T ≃ 108 °K) gas which constitutes an intracluster medium. Progress in understanding the extended X-ray sources and in determining the emission mechanism has come from observations of the X-ray structure and spectra of the cluster sources and it is the purpose of this review to present the current status of these observations.

Welcome to this Joint Discussion on stellar atmospheres and interiors. For stellar structure, the atmospheres are of importance for two reasons, first as a source of information on effective temperature, gravity (or mass: luminosity ratio), chemical composition and dynamical effects, and secondly as an outer boundary condition for stellar models.

In the last few years there have been substantial advances in stellar atmospheres as a result of both improved theories and new measurements. In the particular field of abundance determination, non-LTE theory has at last come of age and provided a certain number of corrections that are both realistic and significant, for example in reconciling the neon abundance in B stars with that in nebulae and solar flare particles. Solar abundances have been improved by the provision of better oscillator strengths and better treatment of line broadening, generally with the effect of leading to still closer agreement with carbonaceous meteorites. Nevertheless, he would be a bold man who claimed to know the initial solar abundance parameters Y and Z to better than, say 25 per cent, even if we grant that the photosphere is a true sample of the initial interior abundances - an assumption that is now being questioned (again) in view of the solar neutrino problem.

Sixty-five radio sources in the 4C catalogue lie within 0.3 of the radius, Rc, defined by Abell (1958), of the centre of an Abell cluster of galaxies. Statistically few of these are expected to be chance coincidences and hence they provide a well defined statistical sample of sources in rich clusters of galaxies. Over the last 6 years sources from this sample having declinations greater than 10° have been observed with high resolution using the Cambridge One-Mile telescope by Slingo (1974(a) and (b)), Riley (1975) and by myself (1977, in preparation). The number of sources observed and the number expected by chance at different distances from the cluster centre are shown in Table I; the radio positions used are from the 4C catalogue.

As a star burns its nuclear fuel, its radius R and its luminosity L are modified. Its mass may as well be affected if the mass loss rate has a time scale comparable to the nuclear time scale; this is likely to occur for stars of very high luminosity. Currently, the change in radius R and luminosity L of an evolving star is described in the socalled theoretical Herzsprung-Russel diagramme with in abscissa the logarithm of the effective temperature defined by:

It is reasonable to suppose that the state of the convective zone underlying the atmosphere of cool stars correlates with the atmospheric kinematics. Following Iben (1967), as a star evolves from the red giant tip to the helium burning core phase, the convection rapidly decreases. Then the microturbulence can be expected to decrease too.

“The properties of radio sources in clusters” presupposes that we know something about radio sources out of clusters, or that we even know whether a radio source is “in” or “out” of a cluster. Thus we are faced with the problem of defining what we mean by a cluster. Most of us use Abell’s catalogue of RICH clusters and assume that we are really “in” a cluster. However, most radio sources are identified with faint, distant objects and it is often difficult to know whether the remark “galaxy in a group” or “galaxy in a cluster” indicates a cluster such as the Coma Cluster, a cluster similar to an “open” Zwicky cluster, or a group of galaxies which may be gravitationally bound.

This uncertainty must not be forgotten, and in the following discussion, we will try to limit the effects of this by concentrating on catalogued clusters; ignoring most distant radio galaxies, many of which may be in rich clusters; and also by neglecting quasars, some or all of which may be in clusters.

Abundance peculiarities in successive stages of stellar evolution are reviewed. Main-sequence stars show anomalies in lithium and, on the upper main sequence, the Am, Ap and Bp effects, which may be largely due to separation processes, and helium and CNO anomalies to which nuclear evolution and mixing could have contributed. Red giants of both stellar Populations commonly show more or less extreme variations among the C, N, 0 isotopes, sometimes accompanied by s-process enhancement, due to mixing out in various evolutionary stages. Detailed anomalies expected from galactic evolution are also briefly considered. Novae show strong effects in C, N, 0 and synthesis of heavier elements is displayed by the supernova remnant Cassiopeia A.

The idea that gas may be present in clusters of galaxies originated with the missing mass problem—the observation that galaxies in clusters were moving so fast that the inferred galaxy masses (using conventional mass to light ratios for the galaxies) were not enough to hold them together caused astronomers to speculate that the “missing mass” was in the form of gas. Subsequent observations in the 21 cm and La lines as well as radio, optical and x-ray continuum observations have effectively ruled out this possibility in a large number of clusters. However, there is evidence for the presence of gas in smaller quantities in many clusters.