The Second Conference on Co-ordination of Galactic Research was held from 17 to 22 June 1957 at Saltsjöbaden near Stockholm, at the invitation of Dr B. Lindblad, director of the Stockholm Observatory. It was prepared by the sub-commission on Co-ordination of Galactic Research of Commission 33 of the International Astronomical Union. The local organization was in the hands of the Stockholm Observatory, particularly of Professor B. Lindblad and Dr J. Ramberg. By a grant from the Swedish Government the participants received free board and lodging at Vår Gård, a boarding-school belonging to the Swedish Co-operative Union. A considerable part of the travel expenses was defrayed from funds made available by UNESCO and the International Astronomical Union.

Renewed interest in the study of star clusters and associations has been aroused by at least three developments.

(a) Interpretation of the color-magnitude (C-M) diagrams of clusters and associations with a theory of stellar evolution promises to give data on galactic structure problems. Information of kinematic relations, origins, and past history of stars over the face of the H-R diagram will be available when proper evolutionary interpretation is achieved. Only by assembling C-M diagrams of many clusters of different ages and richness can the empirical approach to evolution be fully exploited.

In connexion with the importance of present views on stellar evolution for discussions of the evolution of the Galaxy, the sessions dealing with galactic problems were preceded by a review of items relating the evolution of stars with that of the Galactic System. The following represents with only few modifications the introductory paper on this subject as presented by J. H. Oort.

This section deals with a few remarks on stellar systems outside the Galaxy which were considered to be of importance in the interpretation of galactic observations.

The objects from which the structure of the galactic halo may be derived by optical means are the extreme population II objects such as RR Lyrae variables with periods longer than 0-4 days, the sub-dwarfs, globular clusters, stars similar to those found in globular clusters such as the bright red giants, and the blue stars found in the galactic polar regions. The longperiod variables with periods shorter than 200 days and the ordinary highvelocity stars are now regarded as belonging to an intermediate population II, somewhat younger than the halo population II proper.

The nuclear region may be explored by means of both radio and optical data. Optical methods include surveys of variable stars and various other objects, but of special interest are measurements in the far infra-red, which may provide a connexion between optical and radio observations.

According to the general evolutionary ideas outlined in section (A), p. 13 above, we should find in the disk both old objects, formed immediately after the contraction of the primeval cloud into the disk, and recently formed stars. The study of the interarm population of our Galaxy is a difficult problem due to the interstellar absorption. Information from nearby galaxies, especially from M 31, is in some ways more readily obtainable. In the disk of M 31 we find stars of absolute magnitude about – 3 photovisual, which are similar to the brightest stars observed in globular clusters. Observations with the 200-inch telescope have shown that the number of these stars of population II decreases rapidly from the nucleus to the adjacent regions, and then slower towards the limit to which the system can be traced by optical means.

Investigations of the structural form of the Galaxy are being actively carried on at a number of observatories. The portion of the Galaxy surveyed has been markedly increased since the time of the first Symposium for Coordination of Galactic Research. Many new results have been obtained, particularly in the southern hemisphere; additional observing programs are in the planning stage. New observational techniques for some problems have been found and are under investigation.

The region within about a kiloparsec of the sun contains no globular clusters, and few associations, planetary nebulae, novae, or Cepheids; the interpretation of 21-cm observations is complicated by peculiar motions which are comparable with galactic rotation effects. Nevertheless, this is an extremely important region for the study of galactic structure. Not only are all distance calibrations ultimately based on objects near the sun, but only in this region do we have the resolution necessary to examine the detail of galactic structure and only here can we study the numerous fainter members of the various stellar populations. As elsewhere, the problems are two-fold, the efficient selection of those objects whose study is the most informative and the choice of the methods of obtaining the greatest amount of information about those objects.

The Magellanic Clouds offer a unique opportunity to study fully resolved stellar systems at, optical and radio wave-lengths. Optically, with the largest southern telescopes available it is possible to reach absolute magnitudes + 1 to + 2 by direct photography and absolute magnitudes - 6 to - 4 spectroscopically. The greatest value of the Clouds to galactic research is the opportunity to calibrate the absolute magnitudes of luminous objects within the Galaxy. Uncertainty regarding location in depth within the Clouds adds a random error of about ± 0·13 m, i.e. considerably less than the uncertainty of the distance modulus of the Clouds themselves.

The meeting was concluded by a summary presented by J. H. Oort and subsequent discussion of the principal desiderata for future work. The following paragraphs include the principal topics of this summary and the desiderata.