The present report covers the period 1976–1978. As with previous reports it has not been possible to write an all- inclusive account covering everything published, or even every field under investigation; the number of contributions to extragalactic research is increasing enormously fast. Summaries of the papers may be found in the Astronomy and Astrophysics Abstracts.

The report is divided into ten sections, including the four Working Groups, and they have been prepared by the Vice-President, by members of the Organizing Committee and by the Chairmen of the Working Groups.

As usual, this report contains contributions from a number of authors, as follows: § 2, P. D. Jackson and M. P. FitzGerald; §§ 3 and 4, F. J. Kerr and D. L. Crawford; §5, P. O. Lindblad; §§ 6A and C, R. Wielen; §§ 6B and 7, J. Einasto; §§ 6D and E, K. C. Freeman, § 6F, M Fujimoto. The layout follows previous practice, except that a new Section 7 on the galactic environment has been added. A longer version of the Report will be published by the University of Maryland and will be distributed to all members of Commission 33 and to astronomical institutions.

It is now recognized that diffuse matter in space plays a decisive role in the evolution of our Galaxy and of similar galaxies. Primordial gas—together with gas ejected in planetary nebulae, stellar winds, novae, supernovae, and other types of stars—has accumulated to form a complex medium containing regions with densities ranging from 10–3 to 106 particles cm-3 and with temperatures ranging from 10 K to 106 K. From time to time, part of the interstellar medium collapses to form stars. In order to understand the evolution of the Galaxy, it is essential to understand how energy, mass, trace elements, and dust grains are deposited into the interstellar medium by stars. It is also essential to understand the mechanisms that initiate star formation in certain regions, and how the ensuing collapse develops in space and time.

It is now clear that the ISM does not consist solely of the classical three components, i.e., molecular clouds, diffuse clouds, and the intercloud medium (H II regions are discussed in section 7 of this Report). High-temperature, low-density components also exist. The role of the transition regions between these components has recently been emphasized, and dynamical processes, including interchange of matter between them and the role of stellar winds and supernova remnants, have been extensively studied. Review papers or general discussions have been given by Field (18.131.115); Salpeter (17.131.143); Kahn (18.131.116, concerning hydrodynamics and MHD); and Heiles (20.156.012, on interstellar magnetic fields). McKee and Ostriker (1975, Astrophys. J. 218, 148) have given a general theory of the ISM including a hot, diffuse component; they emphasize the role of the energy and momentum input from supernovae, the evaporation of cold clouds into the hot medium, and the role of heat conduction.

During the past three years work on Clusters and Associatons has been proceeding with unprecedented vigor. Space limitations, unfortunately, make it impossible to review all of this material in detail. A listing of some of the most important recent review papers is given in section 2. Work in progress or recently published on individual clusters and associations is collected in the tables given in section 3.

At the XVIth General Assembly, Commission 40 decided to curtail its triennial reports, confining them to aspects and developments which do not naturally find a place in the report of another commission. In letters to Presidents of eighteen commissions our new policy was explained and Commission 40 members willing to serve as radio astronomy contact person with one of these commissions were introduced. All Commission 40 members received notice of our intentions, with the names of the contact persons and an outline of this report in September 1978. We trust that from now on research at radio observatories will be integrally reported in the thematically appropriate reports of relevant commissions.

It can be considered as very satisfactory and a sign of good health that, as early as 1961, and since then, the question of the role of our Commission was discussed and in particular the overlapping activities with those of COSPAR and of other IAU Commissions were put into focus. It is the opinion of some members of our Commission that an analysis of whether a partial merging of Commission 44 with COSPAR Working Group 3 should be considered. Such an organizational scheme might offer the advantage of a greater flexibility and of a closer cooperation between IAU and COSPAR in space astronomy with the guarantee that duplication is prevented. Similar schemes do exist in other ICSU organizations.

The precise value of the Hubble constant, Ho, is today a matter of considerable controversy. The well-known stuay by Sandage and Tammann is published in a series of papers in the Astrophysical Journal from 1974 to 1976. Briefly, Sandage and Tammann determine distances to nearby galaxies in which cepheid variables can be observed. In thoee galaxies, of presumably known distance, they measure the linear diameters of the largest HII regions (which they find to be correlated with galaxy luminosity class). Using these data as calibration, they find distances, and hence absolute magnitudes, of more distant spirals of various luminosity classes from the angular sizes of their largest HII regions, thereby providing a new calibration of the luminosity-class-absolute magnitude relation for Sc spirals. Finally, they use the new calibration to determine distances to still more remote Sc spirals of recognized luminosity class, and whose radial velocities are presumed to be great enough to be indicative of the cosmological Hubble flow. Prom various treatments of their data, they find values for Ho in the range 50 to 55 km s1 Mpc1.

In view of the broad range of topics covered by Commission U8 and the consequent inevitable overlap with other commissions, it/is not feasible to produce a comprehensive self-contained report. The commission therefore restricts its report to a selected list of accessible recent review articles and conference reports, wherein up-todate summaries of various topics can be found. Such a list is given below. A special report by I. S. Shklovsky on work done in the Soviet Union follows.