Les clichés du Grand et du Petit Nuage décrits dans cet article ont été pris à travers un filtre interférentiel d'une qualité analogue aux précédents (voir dans le même Symposium: “Distribution des vitesses radiales des régions HII dans la direction du Centre Galactique”), mais centrés évidemment sur la longueur d'onde correspondante au déplacement Doppler-Fizeau dû à la vitesse radiale des Nuages qui, comme on le sait, est de l'ordre de +200 km/sec. La largeur de la bande passante était de 10 å.

A considerable proportion of the radio emission at high frequencies from the Galaxy is of thermal origin. This thermal radiation appears to originate both in very extended regions and in discrete sources. It has always been difficult to compare the radio and optical data for these HII regions as galactic optical observations are hampered by heavy obscuration. However, the HII regions in the Magellanic Clouds are relatively free from obscuration, and spectrophotometry by Dickel, Aller, and Faulkner (this volume, paper 63) and Henize (1956) have provided very good measurements of Hβ and Hα flux densities and the relative intensities of the OII, OIII lines. Also the distance to the Clouds is fairly well known (about 55 kpc) so that the linear dimensions of the HII regions may be estimated with some degree of accuracy in contrast to the galactic emission nebulae.

The neutral hydrogen gas in the Large Magellanic Cloud has been observed with the 14′.5 beam of the Parkes 210-foot telescope and the 48-channel H-line receiver. We wish to present a progress report based on computer reductions of integrated brightness and median radial velocities for some 4200 profiles. The survey is incomplete for some of the southern regions below dec. −73°. Assisting in the observations were our colleagues, M. W. Sinclair, C. J. Ohlston, and G. H. Trent and the staff at ANRAO, Parkes.

Photometric measurements of the emission nebulosities in the LMC are not numerous. Doherty, Henize, and Aller (1956) microphotometered both widened and unwidened objective-prism photographs covering a narrow wavelength range near Hα to obtain cross-sectional “intensity profiles” for certain nebulosities in the Large Cloud. These intensities were converted to surface brightnesses by tracing the widened spectra of stars of known magnitude and colour and using the method of Ambartsumian (1933). Peak intensities for each scan across a nebulosity are given in erg cm–2 sec–1 steradian–1.

The program of observations of the 30 Doradus and η Carinae nebulae described here had its origin in the general study of Magellanic Cloud nebulae considered in the previous paper. The 30 Doradus nebula is considerably larger and brighter than the other HII regions in the Clouds, and may be treated in greater detail. The η Carinae nebula was chosen as a comparison object in our own Galaxy.

Objects in the Small Magellanic Cloud classified by Lindsay (1961) as planetary nebulae or probable planetary nebulae have been studied in detail on large-scale photographs (Henize and Westerlund 1963). Of Lindsay's 50 objects, 11 are clearly resolved, 2 are probably resolved, 12 show stellar images, 13 are below the limit of the plates, 11 (fairly faint) are outside the photographed regions, and 1 is of uncertain identification. The masses of the resolved nebulae lie between 2 and 33 solar masses. They are therefore classified as small diffuse nebulae; it appears unlikely that the mass of a planetary nebula can exceed a few tenths of a solar mass.

With the five-colour photometer of the Leiden Southern Station (system V, B, L, U, W) stars can be classified according to luminosity (Th. Walraven and J. H. Walraven 1960).

This paper gives the results of a spectroscopic study of individual stars in the SMC cluster NGC 330. Before this, however, it may be useful to review briefly the spectroscopic evidence bearing on the questions of reddening and chemical composition in the Magellanic Clouds—matters about which there has not been entire agreement in the past.

During the past three years, Priscilla F. Bok, Jane Basinski, and I have concentrated in our work on the Large Magellanic Cloud upon the study of young associations. One colour-magnitude array, that of NGC 1955–68–74, has been published (Bok, Bok, and Basinski 1962), and work on three additional groupings, NGC 1929–34–35–36–37, NGC 1770, and NGC 1814–16–20 is well under way. We shall report here on NGC 1929–37, which is often referred to as “Shapley's Constellation I”. Its position is α = 5h22m0; δ = −68°00′ (1960).

It is important to study some of the less bright and less rich clusters in the Magellanic Clouds in order to complete the picture of their populations as derived from studies of their giant clusters and associations. The region around NGC 419 in the Small Cloud is fairly rich in small- and medium-sized blue clusters and has, in addition, a photoelectric sequence (Arp 1958). Several clusters and groups of stars in this region have now been investigated (Westerlund, unpublished data). The resulting colour-magnitude arrays for four clusters in this region are presented in Figure 1 together with the arrays for three clusters in the wing. The clusters are identified by their numbers in Kron's (K) or Lindsay's (L) catalogues (Kron 1956; Lindsay 1958).

Some years ago (1960) I discussed the HR diagram of the variable star field investigated at Herstmonceux as the result of Drs. Sandage and Eggen's visit to Pretoria, and found colour-luminosity arrays showing mainly very blue giant stars; and later (1961) I suggested that a second and older population could be seen in the same area of the LMC, showing a rather different x,y distribution. Recently Miss Epps has assisted me to make further investigations into the matter. We are now discussing stars fainter than V = 14 · 5 and therefore too faint for us to determine their proper motions. In order to get rid of foreground stars which obscure the HR diagram we have had to make use of the x, y distribution to discriminate between LMC members and foreground stars. Firstly, we examined an open cluster which appears as a faint object on the 74-inch Pretoria plates. It is away from the main lane occupied by the bright blue objects, and its appearance on the B and V plates showed it not to be excessively blue. On a Radcliffe plate (74 in. stopped to 44 in.) with an exposure of 20 min, 29 stars can be seen within a circle of diameter of 1′ of arc. We have been able to determine V accurately for only 15 of these stars, one of which proved to be a variable of period 4–75 days. The HR diagram shows a gap between B – V = 0 · 2 and B – V = 1 · 0, the variable being in this gap. It is comparable with those of NGC 129, NGC 2287, and NGC 6067, and, by analogy with these objects, the open cluster now described probably has an age of about 108 years. The object is of course very different from the young or blue (but tight) cluster NGC 1818 not far away from it.

It is becoming increasingly evident that the older stellar components of the Magellanic Clouds are significantly younger, on the average, than the old star component of the Galaxy. Magellanic Cloud red globular-like clusters of the NGC 419 variety, and the common field stars of similar type, are almost certainly to be classified as intermediate-age populations by galactic standards. The oldest galactic population, seen in the galactic halo, globular clusters, and oldest galactic clusters, is apparently very sparse in the Magellanic Clouds, although in the SMC at least something very similar is seen in and near NGC 121.

There are about 50 clusters in the Magellanic Clouds which from their spherical symmetry, integrated colours, and luminosities appear similar to the globular clusters in the Galaxy. The colour-magnitude diagrams of these clusters should give moduli for the Clouds, indications of the age and chemical composition of the clusters themselves, and perhaps some information about the evolutionary tracks of old stars generally. The first investigation of this kind was carried out by Arp on the SMC clusters NGC 361 and 419. This was followed by papers by Eggen and Sandage and by the writer on NGC 1783 in the LMC, and by Tifft (1962) on NGC 121 in the SMC. Of these four clusters only NGC 121 appeared really similar to a galactic globular cluster, the others displaying features not reproduced by any known cluster in the Galaxy. Further work was clearly needed to clarify the problems raised by these results, and a program for the systematic observation of the colour-magnitude diagrams of red clusters in the Clouds was accordingly begun here in September 1961. This contribution is a progress report on this program.

The present investigation is concerned with the energy distributions in globular clusters. In a sense, energy-distribution measurements are comparable with multi-colour photometry. The chief advantage is that narrower band-passes may be used and the entire spectrum traced, whereas in multi-colour photometry one is limited to effective wavelengths determined by the filter and a rather broad band-pass. On the other hand, broad band-pass photometry often permits one to cover a broader spectral range, and to work much faster. Since the time required for a single observation is much shorter, the observer is less at the mercy of the sky transparency. Hence a greater accuracy can be obtained.

Our knowledge of short-period variables in the Magellanic Clouds remains rather meagre despite the considerable effort that has been put into discovering them. Original failures to detect RR Lyrae variables at the expected magnitude of 17 · 5 led to a temporary belief that perhaps the Clouds contained virtually no Population II. However, the Radcliffe discoveries of RR Lyrae variables in the globular clusters NGC 121 (3), 1466 (44), 1978 (2), and 2257 (17 + 8), put the existence of a Population II component in both Clouds beyond doubt. Moreover, the published light curves and periods of variables in NGC 121 (Thackeray 1958) and NGC 2257 (Alexander 1960) showed the typical characteristics of Bailey types a, b, and c. Thus the comparison, so far as it goes, between the Population II of the Magellanic Clouds and the Galaxy shows no outstanding difference.

This is a brief account of some photoelectric observations of Magellanic Cloud cepheids, begun in 1951 with Dr. G. E. Kron on the 30-inch reflector, and since continued there and on the 50- and 74-inch reflectors. The initial emphasis was on colour, the Cloud cepheids, as measured, being considerably bluer than those in the Galaxy. It is now known that this discrepancy is due mostly to reddening of the galactic cepheids, and interest has shifted rather to the differences in the period-luminosity relations in the two Clouds, as revealed by measurements made in the SMC by Arp (1960), and in the LMC by Woolley et al. (1962).

This work was performed in the same way as that corresponding to the Small Cloud (Landi Dessy 1959a, 1959b). Two hundred and seventy-seven new variables have been discovered, whose positions are shown by arrows in Figure 1. The pairs of plates used for blinking have from 80m to more than 1000d intervals.

It must be obvious to all of us that no such Symposium on the Magellanic Clouds as we have been holding over the past few days would have been possible 10 years ago. At that time, radio work on the Clouds had scarcely begun. de Vaucouleurs, Buscombe, and Gascoigne were preparing their summary on Cloud problems as a preliminary to work with the Canberra 74-inch reflector, while the first results of work with the Radcliffe reflector were slowly beginning to emerge. Even five years ago, such a Symposium could scarcely have been fruitful, but the interest of many northern astronomers in the Clouds had certainly been excited. Now there are far more instruments and astronomers concentrated on Magellanic Cloud problems than ever before and the string of papers presented to this Symposium with lively discussions gives striking evidence of their activity.