The rotation of the Galaxy can best be studied with the aid of tracers for which distance and velocity can be obtained simultaneously, accurately and unambiguously out to large distances. Clearly, the best objects are the very young clusters. The local gradient of the angular velocity is contained in Oort's constant and, due e.g. to perturbations from spiral features, may not reflect the smoothly varying, underlying global component of rotation. Thus, it is necessary to reach out as far as possible in order to allow a smoothing of the rotation curve.

The present state of astrometric, spectroscopic and photometric data for stars in 63 open clusters nearer to the Sun than 750 pc is summarized. Table I reports the limits of the available data in terms of the apparent V magnitude, in the same way as the previous version (Mermilliod 1977). Information on the apparent V magnitude of the brightest main sequence star (Vbr) and on the number of stars brighter than V=10 (n∗) has been included. In addition, the bluest spectral type on the main sequence has been used as an age estimator (TS).

Well known methods for analyzing stellar distributions have been refined and new methods developed. Their application to h and χ Persei is demonstrated.

The study of star clusters occupies a central place in the interaction between astronomical observation and theory. The extensive and exponentially growing literature concerning star clusters makes the Catalogue of Star Clusters and Associations (CSCA) an indispensable research tool.

This is a review of a subject that turns out to be rather dismal. Both in theory and in observation we know and understand open clusters poorly – either in comparison with globular clusters or on an absolute basis.

From a proper motion survey by Pels and photometric measurement of selected stars it was found that the Pleiades cluster extends till at least 496 from the centre, corresponding to 10 pc at a distance of 125 pc. It turns out that the luminosity function of the Pleiades is a function of the distance to the centre, the proportion of faint stars increasing with this distance. Because of this, the luminosity function as it was determined before flattened towards fainter stars, whereas for the total field with a diameter of 20 pc one finds a luminosity function that is still increasing at the faint end. Flare star observations in the Pleiades field show that the increase amounts to at least a factor 20 in the mass range 2 to 0.4 M⊙. Accurate proper motions of stars in the projected central field show a dispersion of velocities in the cluster of 700 m/sec. This could indicate a total mass of the Pleiades cluster of the order of 2000 M⊙.

The dynamical evolution of open star clusters has been followed by numerical N-body simulations. These computations have been done considering both the interactions between stars in the cluster, and the effect of a linearized galactic tidal field, assuming circular orbits in the solar neighbourhood. Models have been studied for 250, 500 and 1000 stars, and two initial mass spectra (α=2.35 and α=2.75). We adopt a mean mass and a mass ratio of 50:1 between the heaviest and the lightest star. A simple scheme for instantaneous mass-loss from stellar evolution (neutron stars and white dwarfs) is incorporated into the models using published parameters. The models with 250 stars are found to be relatively insensitive to the adopted value of (the virial theorem scale factor) in the range of pc. The disruption half-lifes (T1/2) are about 1.7 × 108y for models with 250 stars, and 4.6 × 108 y for 1000 stars and α=2.75. Preliminary work including successive encounters of clusters with interstellar clouds appears to give shorter disruption times. Two effects play an important role in the dynamical evolution of the cluster: the formation of one or two massive and energetic binaries (≃50 per cent of the total energy) during the first few crossing times, and their disruption by mass-loss.

A forming star progresses through several evolutionary stages which are marked by different observational signs that can be identified by different techniques. Stellar birth from interstellar matter occurs in dense, cold clouds of gas due to a local increase in the density and interstellar clouds composed of molecules and grains are the preferential place where star formation can be initiated. The observational indications of this prestellar stage are the emission of millimeter radio waves by the cloud's many molecules and the optical obscuration of the background field stars by the grains mixed with the molecular gas. When fragmentation and condensation mechanisms occur in a dense cloud, the gravitationally collapsing fragments cannot be directly observed at visible wavelengths, because their light is blocked by the dust grains which heat up and radiate in the infrared. Thus the various protostellar stages are essentially studied using infrared techniques. Evidently, the spectral energy distribution of the protostars depends on their evolutionary stage and, globally speaking, one can say that the maximum of the infrared emission moves towards the short infrared wavelengths as the protostar evolves. A tentative coherent stellar evolutionary scenario may be given for the radio and infrared sources directly connected with the star and cluster formation:

The dynamical evolution of the solar neighbourhood is described by an accretion model in which the gas accretion rate decays exponentially with time. Stars form at a rate proportional to the local gas volume density and their velocity dispersion is increased after birth by star-cloud collisions. The present mass density distribution of stars and of gas perpendicular to the galactic plane (Oort 1965) and the observed increase of stellar velocity dispersion with age (Mayor 1974; Mayor and Martinet 1977) are reproduced for an e-folding time of 3 × 109 y of the gas accretion rate and a characteristic star formation time scale of 2.8 × 109 y.

Several topics relevant to the study of stellar evolution through open clusters are discussed. These include composite color-magnitude diagrams, the need for thorough studies of populous clusters, parameters affecting the core helium burning stage, and the potential importance of initial conditions and dynamical evolution on the cluster color-magnitude diagram.

The HR diagram (HRD) of clusters constitutes an especially sensitive test of physical processes in stellar interiors. Let us remember that the picture of inhomogeneous stellar evolution imposed itself by the fact that the cluster sequences turn to the right in the HRD (cf. Schwarzschild, 1958).

The radial mass distribution (obtained by counting stars in strips) of the real cluster is compared successively to the distribution functions of a simulated cluster of 100 stars, each of which corresponds to a certain dynamical age, Tdyn, The value of Tdyn, belonging to the function most similar to the observed one is taken to be the dynamical age of the cluster. The radius is given in units of R1/2 (sphere containing half of the total mass); this unit is nearly time-independent. The difference between the distribution functions is measured by the maximum Δmax of the Kolmogorov-Smirnov test which is free from assumptions on the form of the distributions. The minimum in the plot Δmax vs Tdyn, indicates the age of the cluster. It is then converted into an absolute age, Tabs (in years), by The error due to the dynamical theory (limited number of distribution functions, etc.) is estimated at 12%, the error due to the uncertainty of diameter and mass of the cluster is about 30%. Unreliable results were obtained in case of strongly inhomogeneous reddening of the cluster. As an example, the plot of the test values for NGC 457 is given in Figure 1.

The motivation for studying distant open clusters primarily arose out of a desire to gain some understanding of star formation processes in the general context of galactic structure. Of specific interest are faint open clusters (c.f. Alter, Balázs and Ruprecht 1970) near the galactic anticenter which are part of a larger survey of objects which may be located in the “periphery” of the Galaxy. Table I contains a sample of results from broad-band photometric studies for clusters near ZII=180°, bII=0°. The characteristics of these clusters are consistent with other investigations, which suggest that the disk of the galaxy extends to large galactocentric distances; for example, deep surveys of blue stars (Chromey 1979; McCarthy and Miller 1974) and investigations of the rotation curve of the galaxy (Blitz 1979; Knapp, Tremaine and Gunn 1978), to mention a few. There is also a considerable amount of evidence that the distributions of distant objects basically reflect the locations of local absorbing clouds and low absorption “windows” (Westpfah1 and Christian 1979), rather than distant galactic structure. Therefore, in this study, emphasis is being placed on the determination of ages, abundances and luminosity functions of clusters, rather than considering them as “spiral tracers”.

From the literature a list of stars with abnormal spectra belonging to open clusters was compiled. This list is included in Lyngå's catalogue of open clusters. With this data it becomes possible to derive a correlation with the age, compiled by W. Buscombe and which appears also in Lyngå's catalogue.

Spectra of 140 stars in a 1.36 square degree field centered on the open cluster Collinder , 17 outside this field, and 125 in two neighbouring fields of the same total area, have been classified on the MK system. Twenty of these were classified on slit spectra in the manner described by FitzGerald et al. (1979) and the balance on objective prism plates obtained at Cerro Tololo with a disperson of 280 A mm−1. All spectra were classified at least twice, usually by more than one author. The internal accuracy was ±0.9 spectral sub-classes and ±0.4 luminosity classes. Comparison with the Michigan 10° classes of Houk (1979) for 34 stars in common gives the difference (Houk-FHM): 0.2 ± 0.2 (s. e. mean) for spectral classes and −0.1 ±0.1 for luminosity classes. All stars brighter than V=14.35-0.65 S were classified in the above regions, where S is a numerical representation of the spectral class ranging from 2.0 to 6.7 for BO to M7 stars respectively. Members are shown in Figure 1.

The metal abundances of young stellar groups in the solar vicinity are obtained from the photometric [Fe/H] ratios observed for the red giant group members. The definition of the groups has been rediscussed with the help of a sample of B5-A0 stars whose physical properties are derived by Cramer (1979). The young G3-K5 giants have been selected according to their spatial velocities and their age derived from the Geneva colours (Grenon 1978). They show the kinematics of A stars (Eggen 1963). The star concentrations coincide in both (U,V) planes (red giants and B & A stars) in the zones listed below with the value of the mean [Fe/H] and galactocentric distance, .

The investigation of the open cluster NGC 663, based on the application of the method of UBV three colour photometry and the use of new photographic material, lead to a new distance to the cluster of 2550pc, E(B-V) & E(U-B) colour excesses of 0.74 & 0.53 mag and consequently a total visual absorption AV of 2.22 mag. An age of 3.9 × 107 y and a spectral type B2-B3 can be estimated for it; accordingly, it fits well the Perseus Arm (+1) near the outer edge and lies below the galactic plane at a distance of 47 pc.

From the distribution of 476 stars investigated, around both the ZAMS and the apparent centre of the cluster, 270 probable physical member stars could be separated from the field stars and neither late type giants nor a giant branch are in existence.

47 A mm−1 image-tube spectra show that the turnoff region and blue straggler stars in NGC 2477 are not, in general, rapid rotators, as postulated by Hartwick and Hesser (1974) to explain, via mixing induced by meridional circulation, the widely differing photometric metallicity estimates based upon uvbyβ photometry of the main-sequence stars and DDO photometry of the giant stars. However, the discrepant metallicity estimates are largely reconciled by recent revisions in the reddening corrections for the DDO system (Boyle 1979, Janes 1979); and by small color corrections of the type described by Muzzio (1978) and Schmidt and Taylor (1979) applied to the β-photometry. The mean radial velocity is found to be 5.6 kms−1 while the mean Vsini is 105 km s−1. Star “m” is spectroscopically confirmed to be an Am star.

As part of a long range program on spectral classification in open clusters we have obtained high quality spectrograms (39 A mm−1 and 125 A mm−1 both 1.2mm wide) of 35 stars in NGC 2287.

BV polarimetry of about 60 stars belonging to the clusters Tr 14, Tr 16 and Coll 228 as well as some luminous stars of the surrounding field is presented.