It has become more evident during the last three years that the study of interstellar matter is paramount to understand the evolution of the universe and its constituents. From observations of the present state of the interstellar medium, in our galaxy, in other galaxies, and between galaxies, it is possible to test theories of: evolution of the universe, formation and evolution of galaxies, formation and evolution of stars and of the evolution of the interstellar medium itself. The amount of information on the interstellar medium that has been gathered during the 1982-1984 period has been very large and the theoretical models that have been ellaborated to explain these observations have been very numerous, these facts show that the subject of our Commission constitutes a very active field of astronomical research.

This report of Commission 35, as in past reports, consists of some details of only a few selected topics. This is necessary because a survey of the entire field of stellar formation, structure, stability, evolution, pulsation, and explosions for the three year period from mid-1981 to mid-1984 would be excessively long. Our topics here, in order from the most massive stellar classes to the least are: Massive Stars (R.M. Humphreys), Rotation in Late Type Stars (W. Benz), Helioseismology (J. Christensen-Dalsgaard), Planetary Nebula Central Stars (E.M. Sion), Pulsations in Hot Degenerate Dwarf Stars (A.N. Cox and S.D. Kawaler), and White Dwarfs (V. Weidemann). There is some overlap in the reviewing of these last three reports because the topics are very closely related. Concentration in this dying stage of stellar evolution seems appropriate because of the great current interest in these matters.

Commission 36 acts as a sponsor or co-sponsor at the following symposi and colloquia: IAU Colloquium No. 90 “Upper Main Sequence Stars with Anomalous Abundances”, Crima, USSR (May 1985), IAU Colloquium No. 89 “Radiation Hydrodynamics in Stars and Compact Objects”, Copenhagen, Denmark (June 1985), IAU Symposium No. 120 “Astrochemistry”, Goa, India (December 1985), IAU Colloquium No. 87 “Hydrogen Deficient Stars and Related Objects”, Bangalore, India (December 1985).

According to dynamo theories, the interaction between convection and rotation is responsible for the generation of magnetic fields in solar-type stars. Stellar observations can explore the ways the global parameters, like the surface rotation and internal structure, affect the production of magnetic fields. In this very brief review I wish to concentrate on recent progress in understanding the evolution of stellar rotation, and the dependence of stellar cycles and magnetic activity on rotation, internal structure, and age.

Wolf-Rayet (WR) stars are the helium-burning remnants of massive stars (initial mass ≲30 M©), which have lost their outer hydrogen-rich layers through the processes of Roche lobe overflow to a companion or mass loss by a strong stellar wind. The characteristic emission-line spectrum which defines the WR spectral type is produced by a stellar wind that is so dense and opaque, that the radiation of all lines and continua arise from material in the wind. Because the wind completely screens any radiation emitted by the hydrostatic core of the star, the spectra of WR stars are nearly impossible to interpret quantitatively, and the basic parameters — such as mass, luminosity, temperature, and chemical composition — are poorly determined.

The main characteristic of white dwarf atmospheres is a pressure that is at least two orders of magnitude higher than in main sequence star atmospheres of the same effective temperature. This is due to the high gravity (log g ≈ 8), extreme metal under-abundances (Δlog εM ≳ 2.5), and (in many cases) the replacement of hydrogen by helium as the main constituent. As consequences the atmospheres are very thin (ΔR ≲ a few km) and the level populations of all species are given by the Boltzmann distribution (perhaps with the exception of the extreme outer layers, Greenstein 1973, Pilachowski 1984). Thus, models can be calculated under the assumption of plane parallel radiative transfer and local thermodynamic equilibrium, which facilitates the numerics very much;but special care has to be taken of pressure effects (e.g., broadening of spectral lines, quenching of levels, changes in the dissociation-ionization equilibria). In addition, the proper consideration of convection, which is very effective and may reach into the optically thin layers, makes the construction of model atmospheres for white dwarfs rather tedious and costly.

Accretion discs are a popular ingredient among theorists for modelling a number of high energy astronomical objects like quasars, active galactic nuclei (Rees 1984) and galactic X-ray sources (Levin and van der Heuvel 1983). However, the observational evidence (as opposed to the strong theoretical presumption) that accretion discs exist in these objects is weak, and in only one case has some attempt been made to argue the case for a disc on the basis of its spectral properties (Malkan 1983). Indeed the structure of accretion discs is sufficiently ill-understood that any progress in this area must rest on a strong interaction between theoretical modelling and the actual observation of accretion discs in action.

The last three years have been very productive for cluster research. This report, on activities in the field, has been compiled by the members of Commission 37. It begins with sections on recent meetings, and on data catalogs (G. Lynga). Detailed tables of work on associations (P.E. Nissen), open clusters (G.L.H. Harris) and globular clusters (R.E. White) are then given. A section on cluster dynamics (D.C. Heggie) follows, and the final section concerns present trends in cluster research (C. Pilachowski).

While the chief function and methods of operation of the Commission have remained much the same as in the past, there has been a gradual evolution in the nature of the proposals submitted. A greater fraction now come from countries in which the study of astronomy on a professional basis is as yet very recent and more proposals are being received from relatively young astronomers, although admittedly the distinction between a “young” and an “established” astronomer is not always easy to make. The commission may wish to consider whether or not it is advisable to reconsider its guidelines. Grants awarded during the interval 30 November 1981 and 31 January 1985 were the following.

The last triennium marked the 50th anniversary of the paper describing the first observations of cosmic radio emission by Karl Jansky in 1933. Sullivan (82 Classics in Radio Astronomy, Reidel) has published a collection of the major historical papers in radio astronomy, and collections of papers discussing the historical development have been published by Sullivan (84 Early Years of Radio Astronomy, Cambridge Univ. Press) and by Kellermann and Sheets (84 Serendipitous Discoveries in Radio Astronomy, NRAO).

Commission 41 is one of the largest specialist components of the IAU. At the General Assembly in 1982 at Patras the number of ordinary members was increased from 79 to 86 as a testimony to the growing interest among astronomers in the history of their subject. At the same time the number of consulting members rose from 32 to 40, which shows that an increasing number of highly competent historians of science is engaged in research in the history of astronomy without being professional astronomers. However, many valuable contributions to the subject come from historians who are not members. In consequence, the principal purpose of Commission 41 is to serve as a link between all historians of astronomy whether they be members or not, by initiating research into particular areas, by disseminating new results, or simply by creating possibilities of personal contacts between scholars from different parts of the world. This is mainly achieved by meetings of various kinds.

Research on close binary systems has continued at a high level during the past triennium, although the rate of growth is noticeably slower – probably reflecting the cutbacks in funds to which many of us are subject. There have also been changes of emphasis within the field, which are commented on in the pages that follow. These reflect both changing opportunities for observation and the natural development of the subject. In many areas, the time is ripe for a more critical look at ideas that previously seemed adequate.

A rapid and dramatic change in our views of the Universe which we have witnessed during the past two decades or so is often compared with what happened at the time of Galileo. Revolutionary role of the optical telescope then may be analogized with that of space-astronomy today which has drastically opened the new observational window to the Universe. The revolution is ongoing with a rapid pace or even being accelerated.

The period leading up to the last IAU General Assembly was dominated in X-ray Astronomy by the results from the Einstein Observatory. This first application of a large, satellite-borne, high resolution X-ray telescope to the study of cosmic sources had led, by the end of orbital operation in April 1981, to the detection of X-ray fluxes from a wide variety of astronomical objects and the full maturing of X-ray Astronomy. During the past three years a strong scientific output has continued to flow from the analysis of the more than 5600 separate Einstein observations, many of which are now widely available via the Einstein Data Bank. In sharp contrast, the peak of activity and scientific output from Einstein will apparently be followed, in the United States, by almost a decade with no new X-ray satellite. Fortunately, successful launches have been achieved in both the Japanese and European programmes, with Tenma launched in February 1983 (to join Hakucho in orbit) followed by EXOSAT three months later. Both these new missions have attracted world-wide interest and observing time on both is at a premium. Of a range of instruments on board the Tenma satellite the most productive is proving to be the large (~800 cm2) array of gas scintillation proportional counters. These detectors, flown for the first time on Tenma (and EXOSAT), provide a factor-of-two increase in spectral resolution over the more conventional proportional counters, an advantage which is particularly valuable in resolving the key emission line and absorption features associated with the K-shell of iron, near ~7 keV. One particularly exciting result reported from this Tenma instrument was an apparent absorption feature at ~4.1 keV in X-ray burst spectra from the source 2S 1636-536 (later supported by EXOSAT observations), the feature being attributed to red-shifted iron absorption in the strong gravitational field close to the neutron star burst source! At the time of writing (December 1984) a power storage problem is limiting the operational efficiency of Tenma, but the scientific payload continues to function well.