Stars with spectral types later than or equal to F5V in the Coma Ber cluster have been measured from 1977 to 1979 with the spectrometer CORAVEL (A. Baranne et al. 1980). The aims of this investigation were to clarify the membership based on radial velocity for some faint stars, to study the binarity of the low mass stars of the cluster, and to obtain the distribution of the rotational velocities for the red dwarfs. 391 measurements have been made for 34 stars. The average dispersion over the mean VR of each star is 0.6 kms−1.

75 young open clusters have been divided into 14 age groups on the basis of their MV/U-B diagrams. Composite HR diagrams have been constructed and empirical isochronous curves estimated (Figure 1). The left envelope defines a ZAMS. The mean position of the red giants in the HR diagram has been investigated, as well as the occurrence and location of Ap, Am and Be stars and of blue stragglers. Red giants appear mainly in clumps (shaded area).

The results of a search for HI features in nine galactic clusters are reported. The clusters were selected from the catalog of Alter et al. (1970) with galactic latitudes in order to reduce the galactic background radiation. The clusters are listed together with some of their newer optical data in Table 1. Column 1 gives the cluster's designation; 2 and 3 their galactic coordinates; and 4 and 5 their distances and mean angular diameters. Only two of the clusters have a measured radial velocity.

A new method of determination of the ages of aggregates based on flare star observations is proposed. The method is specially useful in the case of young clusters and associations, for which the usual method involving their H-R diagrams encounters difficulties. The method can be used also for particular flare stars.

We present evidence that relates the phenomenon of the chemically peculiar stars of the upper main sequence to supernovae explosions in young galactic clusters and associations. In the Upper Scorpius region of the Scorpio-Centaurus association we find that the radio shell discovered by Sancisi and Van Woerden (1970) from 21 cm observations and interpreted by them as an old supernova remnant has interacted with dense clouds. This remnant is seen clearly in the deep sky, narrow-band, H-alpha pictures of Sivan (1974). We find that the peculiar stars lie close to, or along the edges of, the interaction.

An investigation has been made of the Mv/co diagrams of six young galactic clusters - three in the north and three in the south, as listed on Figure 1. The uvbyß photometry of NGC 3293 is from unpublished observations of 42 stars by the author. From the Mv/co parameters of the ß Cep stars, which have evolved to the end of core hydrogen burning (Shobbrook 1978), the ages of Lac OB I, NGC 3293 and NGC 4755 were calculated using the models of Cogan to be 18, 10 and from 8.5 to 16 million years, respectively.

Photoelectric photometry in BVRI colors has been obtained at the Kitt Peak National Observatory of many stars in the regions of the Hyades, Praesepe and Coma star clusters which had no previous photometry. The stars were limited mostly to the lower main sequence. Their range is approximately 0.6 to 1.5 (and to 1.7 in the Hyades) in B-V and 0.25 to 0.85 (and to 1.3 in the Hyades) in R-I. Their ranges correspond from mid G through early M in spectral class. The purpose of this project is to detect and examine the main sequences of the nearest open clusters. It continues the results reported earlier in this program for the Hyades cluster [Upgren (1974), Upgren and Weis (1977)] and extends them to include Praesepe and Coma as well, by means of the new sensitive gallium-arsenide photocells which permit fainter stars to be observed with moderate sized telescopes, especially in the R and I bands.

In erecting the edifice we call science, it is given to some to draw the plans by which the entire effort will be guided, to others to lay the foundation stones on which the rest is built, to another group to place the windows that let in light upon the whole, and to still others to cover the result with the paint that will make it acceptable to the populace and the sponsoring agencies. In this review, it is, I fear, my task to run around the construction site, pick up odd bricks and pieces of brick, and ask whether they do not perhaps belong somewhere in the building. After the customary bit of folk history, this will be done in the order globular clusters, open clusters, with observations and theory inextricably mingled, as indeed they really are.

It used to be that interest in the ages of the globular clusters was primarily motivated by cosmological considerations. In recent years, remarkable progress in the quality and quantity of observations and advances in the understanding of stellar atmospheres and in the construction of theoretical isochrones make possible the use of globular clusters as probes of galactic evolution. A new picture, still tentative at this point, is emerging of the formation and chemical enrichment of the halo, in its nucleus and in its outer reaches through observations of globular clusters, dwarf spheroidal galaxies and the Magellanic Clouds.

We begin with Fig. 1, which somewhat aged though it may be still illustrates important aspects of the subject (Gascoigne 1971)

1. (i) There is a clear division of cloud clusters into a blue and red group. The division corresponds to the mass around 2.51M⊙, at which core degeneracy first develops in stars approaching the giant branch. Such stars spend about three times as long on the giant branch and travel higher up it than the slightly heavier non-degenerate stars, and so dominate the colours of the clusters in which they occur.

2. (ii) The red clusters are somewhat less luminous than the globular clusters in the galaxy. Freeman and Chun (1972) have shown from dynamic arguments that the cloud clusters are also less massive, by enough to keep the M/L ratios roughly the same as those in the galaxy.

3. (iii) For a long time it has been taken that the blue clusters are young and the red clusters old. Thus the clouds present us with a truly two-parameter family of globular-like clusters, the parameters being of course age and abundance.

The young globular clusters of the LMC have ages of 107–108 y. Their masses and structure are similar to those of the smaller galactic globular clusters. Their stellar mass functions (in the mass range 6 m⊙ to 1.2 m⊙) vary greatly from cluster to cluster, although the clusters are similar in total mass, age, structure and chemical composition. It would be very interesting to know why these clusters are forming now in the LMC and not in the Galaxy.

I will talk about the “young globular” or “blue populous” clusters of the LMC. They were first identified as a family by Hodge (1961). The ages of these objects are 107 to 108 y, and their masses are 104 to 105 m⊙, so they are populous enough to be really useful for studying the evolution of massive stars. I will not discuss this aspect (see the extensive work by Flower and Hodge and Robertson since 1974), but will concentrate on the structure and stellar content of these young clusters.

N-body calculations have shown that hard binaries play a dominant role in the evolution of small stellar systems. However, only one or two such binaries form by dynamical interactions and since the final components tend to be massive, their effect and observability in open clusters are limited to relatively short time-scales. In the present investigation we consider an initial population of binaries with physical evolution times exceeding the cluster age. These calculations may be applicable to a later phase when the heavier stars have suffered mass loss.

The number and distribution of visual binaries in the Pleiades agree with the frequencies in the solar neighbourhood, except for an apparent deficiency of systems with faint secondaries. A full account of the work is scheduled to appear in Astrophysics and Space Science.

Deblanketing vectors derived from subdwarf photometry and spectrophotometry are used to transform published UBV photometry of globular cluster main sequences into an effective temperature scale defined by model atmosphere surface fluxes. Conversion of apparent visual magnitudes into apparent bolometric magnitudes is accomplished using subdwarf bolometric corrections, which are functions of effective temperature and metallicity. The results do not depend on stellar evolution models.

Because globular clusters are the oldest stellar systems, they should be sensitive probes of the early dynamical and chemical evolution of galaxies. Since some success has been achieved in understanding these problems in our own galaxy, it seems appropriate to see what can be learned from the clusters in the Large Magellanic Cloud (LMC).

Radial velocities based on 81 image tube spectra at 30 Å mm−1 are given for 25 stars (17 early type supergiants or upper main sequence stars and 8 late type stars) in the young SMC cluster NGC 330. An upper (one standard deviation) limit to the velocity dispersion is found to be 2 km s−1. The mass-to-light ratio is likely to be less than 0.1 in solar units. Results for other young Magellanic Cloud clusters derived on the assumption that they are tidally limited are consistent with this result. Spectroscopic binaries with semi-amplitudes, K, greater than about 10 km s−1 appear to be rare or absent amongst the stars studied.

The star density distribution in NGC 1818, 2004, 1806 and 1846, which are representative for the “blue” and “red” globular clusters of the LMC, have been determined from star counts on B- and V- ESO Schmidt telescope plates. These have limiting magnitudes of about 21, thus stars with absolute magnitudes of about Mv ~ +2 are reached. It was found that the two cluster types can be treated as “polytropic stargas spheres” according to the Schuster law with an index n ~ 2.75.

Some C-M diagrams and photometric abundances are presented of clusters and their adjoining fields in areas of the Clouds, in an attempt to derive properties which would allow a comparison with regions in the Galaxy.

The discovery of three RR Lyrae stars beyond the limiting radius of NGC 1841, a globular star cluster in the extreme halo of the Magellanic Cloud system, led Kinman, Stryker and Hesser (1976) to suggest that they might represent a stellar component of a tenuous halo that could have resulted from tidal interactions with the Galaxy. The field of NGC 2257, a “red” globular about 7° east of the LMC, is ideal for evaluation of the gradient in RR Lyrae surface density at intermediate distance in the Magellanic Cloud halo system. Sixty plates taken by Graham, Hesser and Kinman are available (6 from the 1.5-m telescope taken in 1971, and 54 from the 4-m telescope taken in 1974–1977); the 4-m plates are usually IIaO+GG385 exposed for 5 to 10 minutes with a 7 to 21 day Moon (avoiding full Moon by ± 3 days). Extreme hour-angle coverage during two 4-m telescope runs consisting of three consecutive nights each and separated by one month aided in detection and period determination for the short-period variables. Walker's (1972) electronographically measured stars were used as standards, after being made brighter by B=0.10 mag to bring them nearer to Gascoigne's (1978) photoelectric scale. Care was taken to account for increasing background near the cluster center in the iris-to-magnitude conversion. Twenty-eight variables were known in the cluster from the early work of Alexander (1960) who was able to determine periods of six of them. He has kindly made his original step estimates available to us, and they have yielded corresponding increases in the precision of our period estimates. Among our results are: (1) We have found 16 new RR Lyrae variables in the cluster and ~ 12 in the surrounding field; 3 of Alexander's variables appear constant on the CTIO plates. (2) The period-frequency diagram for stars within 8 arcmin of the cluster shows a high ratio, N(c)/N(ab), in contradistinction to the variables in the surrounding field. (3) If the effects of the variable background have been properly removed, then the mean B magnitudes of the cluster variables implies that they, like the cluster, lie about 9 kpc closer to the Galaxy than do the field variables. (4) Walker star 95 is a c-type variable with P=0.360d, 〈B〉 =19.44 mag, and AB ≃ 0.60 mag, and should not be used as a local standard star.

The cluster NGC330 is compact, populous, and one of the brightest objects in the Small Magellanic Cloud. Because of its size and its large number of blue supergiants, it has been referred to as a “blue globular”, as have several other clusters in the Clouds. To learn whether this appellation is more than superficial, we have undertaken a general study of the cluster. We have obtained DDO photometry, photoelectric and photographic UBV photometry, and low-dispersion spectroscopy of stars in and near the cluster. We report here our preliminary analyses of the data.