In the preceeding paper of this Symposium by Cayrel and Perrin (1978), HR diagrams have been discussed, in which the [Fe/H] ratio and effective temperatures are derived from model atmosphere analyses and absolute bolometric magnitude from apparent visual magnitude, parallax and bolometric correction. The use of parallaxes imposes a very severe restriction on the number of objects, as parallaxes of at least 0″.050 are needed to have an absolute magnitude good to ±0m.4.

In this paper some thoughts and problems are presented from the viewpoint that the evolution of stars may play a key role in generating magnetic fields which, in turn, may affect the mixing of nuclearly processed elements from the stellar interior to the surface. The relevant parameter is stellar rotation which, upon interaction with convective turbulence driven by thermal instabilities, leads to the generation of magnetic fields. A possible connection to Bidelman's hypothesis on the evolutionary status of Ap stars is also discussed in the context of a post-core-helium-flash hypothesis.

A basic objective of stellar-evolution theory is to provide detailed quantitative information on the evolutionary characteristics of stars with different compositions and masses. For the evolutionary phases from the zero-age main sequence (ZAMS) through core-helium burning the important physical processes are sufficiently well understood to justify a systematic investigation, and, indeed, extensive grids of evolutionary sequences have been constructed for these phases by many researchers. However, several difficulties are sometimes encountered when using the available calculations. For example, differences in numerical techniques and physical assumptions can give rise to systematic differences among the results of various investigations. Furthermore, the available calculations do not always explore the full ranges of the input parameters, and in some cases they neglect physical effects that are now believed to be important.

The purpose of this paper is to show the first results of a program designed to construct population models of stellar systems on the basis of theoretical data of stellar evolution. Synthetic HR diagrams were obtained using the isochrones and luminosity functions of Ciardullo and Demarque (1977). This last work is based on the stellar models of the Yale - IBM research group (Mengel, Sweigart, Demarque and Gross 1978; Sweigart and Gross 1976; 1978). The conversion from the theoretical HR diagram to observable parameters is made with the help of theoretical model stellar atmospheres principally due to Kurucz (1976), Relyea (1976) and Bell and Gustafsson (1975). The program provides a synthetic C-M diagram and integrated properties for the system given the following parameters: the chemical composition in terms of the (Y,Z) pair, the age, the total number of stars, the initial mass function (IMF) and the law of mass loss from stars in the system. In this brief paper, we shall concentrate on the integrated properties of theoretical star clusters and then remark on some interesting consequences for the color evolution of elliptical galaxies.

The theoretical study of the evolution of massive stars has considerable relevance to many branches of observational astronomy. As an example, one may mention, first, the direct application of theoretical models in interpreting the observed properties of supergiants, and second, the use of calculations of the nucleosynthesis taking place in these stars, together with observed supergiant mass loss rates and supernova calculations, to estimate the rate and type of nucleosynthetic enrichment of the interstellar medium by supergiants.

This review is restricted to the most recent studies of the structure of stars in the approximate range from 10 to 100 M⊙, during the core H- and He-burning phases. Other recent major reviews on this subject are by Dallaporta (1971), Massevich and Tutukov (1974) and Iben (1974). The lower limit was chosen to be just above the transition from degeneracy to non-degeneracy in the core at carbon ignition (Schwarzschild and Härm 1958). The upper limit is very uncertain. The canonical value of about 60 M⊙ for Pop I composition was set by Schwarzschild and Harm (1959), using linear pulsation theory. More recent non-linear dynamical calculations lift the limit above 100 M⊙, and also show that mass loss by vibrational instability occurs at such a rate that stars in the range from 100 to 200 M⊙ can survive for a time comparable to the total main sequence lifetime (Appenzeller 1970a, b, Ziebarth 1970, Talbot 1971a,b and Papaloizou and Taylor 1974).

Detailed spectroscopic work has begun for several stars that have been classified WN 7. All show absorption lines in the upper Balmer series and in ionized helium. The absolute magnitudes, established from cluster membership, are relatively brighter than those of most other WR stars. In this paper the outflow velocity near the stellar photosphere is shown to be similar to that in Of stars. These WN 7 stars show somewhat larger emission line widths and probably have less hydrogen in their envelopes, but otherwise have similar luminosities and temperatures as Of stars.

According to the conventional wisdom, the first stars formed in our Galaxy, including the massive ones responsible for most heavy element synthesis, must have had metal abundances much lower than solar. They may also have had somewhat lower helium abundances, corresponding to an enrichment ΔY/ΔZ = 2.7 ± 1.0 over the history of the Galaxy, a larger ratio than is readily accounted for by standard (Arnett-type) supernovae (Hacyan et al. 1977) or by mass shed from lower mass stars (Gingold 1977). But early nucleosynthesis must have occurred in stars with Pop II compositions. Several authors (Ezer and Cameron 1969; Cary 1974; Chiosi and Nasi 1974; Trimble et al. 1973) have modelled and evolved such stars, but none of the studies followed the stars far enough from the main sequence to be able to say anything very quantitative about their contribution to galactic chemical evolution. We address here the question of the effects of initial composition upon helium and heavy element production in massive stars.

In previous papers, we have discussed the calculation of model atmospheres for metal-deficient and solar-abundance giant stars and have presented a grid of such models. We have now carried out synthetic spectrum calculations at a resolution of 0.1 Å between 3000 and 7200 Å for most of the models of this grid. We have also computed synthetic spectra at wavelengths up to 12000 Å for some of the grid models. The calculations allow for the presence of thousands of atomic and molecular lines. Examples of these spectra are shown in this paper.

The structure of the HR diagram in the G-K giant region is complex, because of the funneling effect and of the intersection of evolutionary tracks for different stellar masses. Therefore it is impossible to derive the age of a cool giant from its location in the HR diagram alone. To remove this indeterminacy a third dimension is needed, and we suggest microturbulence as the appropriate observational parameter.

In the late phases of stellar evolution, evolutionary tracks of stars with different masses come together along the Hayashi line in the HR diagram. The theoretical HR diagram (log L, log Teff) is accordingly partially degenerate in the domain of late-type giants and supergiants, with respect to the third parameter, the stellar mass M. The stellar radius, R, being determined by log L and log Teff, the mass determines the surface gravity log g at the radius R. These parameters enable us to transform a point in the theoretical HR diagram to the corresponding point in the empirical HR diagram MV, (R-I) or spectral type. This transformation is conventionally carried out within the framework of the plane-parallel approximation in stellar atmospheres, and the parameters for the abscissa of the empirical HR diagram are dependant upon Teff and log g alone, irrespective of the mass itself. In this case, the parameter M indirectly affects the observable quantities through log g, but the effects of a variation by Δlog g=±0.5, corresponding to Δlog M=±0.5, are almost insignificant (cf. Tsuji 1976). The transformation between the theoretical and the empirical HR diagram is, therefore, almost one-to-one, within the framework of the plane-parallel approximation. Late-type giants and supergiants, however, have moderately extended atmospheres in general (cf. Schmid-Burgk and Scholz 1975), and their photometric colors and spectra are expected to be influenced by the sphericity of the atmospheric structure. Consequently, in comparing empirical HR diagrams with theoretical ones, it is important to know how atmospheric sphericity affects the transformation in the degenerate domains of the theoretical diagram.

The Symposium opened with a morning devoted to the accomplishments and character of Henry Norris Russell, as one of the two originators of the Hertzsprung-Russell diagram, on the hundredth anniversary of his birth. The papers presented at this session, together with some shorter remarks made at other times during the Symposium, are to appear in a separate volume as Dudley Observatory Report. No. 13. Suffice it to say here that the picture which emerged was that of a fine man of enormous energy, prodigious memory, committed to his scientific work but with many interests, always polite and pleasant to those around him but somewhat remote.

A classification scheme in the red spectral region for the S stars is described. For those stars exhibiting both ZrO and TiO bands, a modified version of the Keenan (1954) system is used for the assignment of temperature subtypes. For the pure S stars, a new system is introduced utilizing ZrO band strengths and the sodium D lines. Comparisons between the revised types and photometric colors have been made. In addition a new abundance index is proposed based on the relative strength of YO compared to ZrO and TiO. The revised system provides a connecting link between the M and C systems so that all red giants may be classified consistently from the strengths of TiO, ZrO and Na D.

Using the model of the Galaxy presented by Eggen, Lynden-Bell and Sandage (1962), plane galactic orbits have been calculated for 800 southern high-velocity stars which possess parallax, proper motion, and radial velocity data. The stars with trigonometric parallaxes were selected from Buscombe and Morris (1958), supplemented by more recent spectroscopic data. Photometric parallaxes from infrared color indices were used for bright red giants studied by Eggen (1970), and for red dwarfs for which Rodgers and Eggen (1974) determined radial velocities.

The writer is preparing for publication a complete listing of the late G.P. Kuiper's spectral classifications of stars of large proper motion north of δ=−50°. This work, resulting from his observations made at the Yerkes and McDonald Observatories some 35 years ago, has been only partially published; it is estimated that useful unpublished classifications exist for the order of 1000 stars. This paper discusses the background of his program and the procedures to be followed in making these data generally available.

In an effort to sort out BIa supergiants among B-stars for use in delineating spiral structure, a photometric discriminate has been sought in the following way. Wide slit 16Å/mm (flux) spectra were obtained of a sample of B stars over a suitable spectral and luminosity range. These spectra were digitized on the PDS Microdensitometer. Numerical filters of various widths were then slid along the spectra and numerous color-indices were formed and tested. An index ℓO = u + v − 1.9 (ℓ) plotted against cO, where ℓ is a 200Å halfwidth passband centered near 3840 Å and the other magnitudes are on the Strömgren 4-color system, succeeds in separating the Ia supergiants up to B1. For stars of spectral type B1 and earlier, confusion sets in and the separation is uncertain (although a fair percentage of success is also had for types B1 and B0). This luminosity discriminate is now in use for a large spiral structure program in which all known Be stars (and perhaps eventually all B stars) are being surveyed for Ia supergiants. Spectra for 2-dimensional classification are obtained for the Ia candidates.

About 3/4 of the sky were searched spectroscopically for stars whose ultraviolet spectra (3640–4100 Å) match that of the Sun, at 20 Å resolution. Down to the limit of the BSC, only two were found: HR 7504 and HR 2290. No G2 V star matches the Sun. Stars that do match have (B-V) = 0m.66. The search has a bearing on effective temperatures of G dwarfs and on metal-abundances relative to the Sun.

UBVRIJHKL photometry and MK spectral types have been obtained for stars illuminating nebulae in the CMa R1 association. Five stars, including the two classical Herbig-emission-stars Z CMa and HD 53367, have (K-L) excesses and/or emission line spectra indicating the presence of circumstellar matter. A number of stars also have (V-K) indices larger than expected on the basis of their spectral types and (B-V) indices, suggesting that a steeper than normal extinction law applies to these stars. The color-magnitude diagram for the association is unusual; many stars over the entire spectral range of the association (B0 to A1) lie more than 1 magnitude above the ZAMS in V. The middle and early type B stars with this property are all variable stars and most have rotationally broadened spectra and/or shell indicators. No stars later than B5 are on the main sequence, making CMa R1 probably the youngest stellar group yet identified. It suggests an age of ~3 × 105 years, compatible with the suggestion that a supernova explosion triggered star formation in this region. A comparison is made between the observations and theoretical models of pre-main sequence stars. A bolometric luminosity of Mbol < −4m.3 is derived for Z CMa implying a core mass of M > 10 M⊙, and a spectral type of ~B1, if the star is on the ZAMS.

The luminosity of individual late-type stars is estimated mainly by spectral features in the MK spectral classification system or by means of the Wilson-Bappu effect. The MK spectral type is given for spectra with a dispersion of about 100 Å mm−1 which is within the reach of some Schmidt telescopes with objective prisms. On the other hand, the Wilson-Bappu effect is measured from spectra with a dispersion of about 10 Å mm−1, obtained by coudé spectrographs. In either case the luminosities must be calibrated by knowledge of apparent magnitudes and distances for some stars.