4 - The cluster population

Summary

Stellar clusters occupy a central position in research aimed at the structure and the evolution of our Galaxy and of those of our neighbours in which clusters can be identified. Often the integrated cluster properties, magnitudes, colours, spectra, are the only ones within reach. In the Magellanic Clouds most of the clusters can be sufficiently resolved for the investigation of individual members by photometry and spectroscopy even if the stars in the cores in some cases are too crowded for ground-based observations. As the clusters have a range of age that covers the whole lifetime of the Clouds, this should permit the study of the complete evolution of the Clouds. En route, a number of steps have to be taken. It is necessary to determine their distances, ages, and metallicities, and, before these, their reddening. The latter is difficult to determine for an individual cluster without knowledge of its physical properties, and is, therefore, frequently assumed known. As the reddening is small over most of the Clouds (see Chap. 2), the astronomer may feel entitled to use any low value recommended in one survey or another. However, even a small error in the colour excess, E_B-V, may have noticeable effects on the other quantities. Also the distance to the cluster, i.e. to a particular part of the SMC or the LMC, is frequently assumed known or determined by isochrone fittings: isochrones for different compositions and ages are fitted to the main sequences (MSs) and/or the red-giant branches (RGBs) in the colour–magnitude diagrams (CMDs) and the best fitting one is accepted as defining the cluster properties.