Even the philosophers of antiquity noted that some stars are grouped together form clusters like the Pleiades. Photographs of the sky show that many stars are found arranged into a number of different groups. In addition to isolated stars in space, known as FIELD STARS , and to the binary stars and multiple stars, there are star clusters and associations.

OPEN CLUSTERS (or GALACTIC CLUSTERS, the terms are synonymous) are loose and irregular aggregations of stars containing a few hundred to a few thousand members. Individual stars in the cluster are easily resolved, and indeed their brighter members are visible to the naked eye in the eases of the Hyades. Pleiades, and K crucis .About two dozen open clusters are visible to the unaided eye; over a thousand more are now catalogued as a result of systematic searches with telescopes. The open clusters are mainly located close to the galactic plane, or Milky Way, and most of the known ones are within 3 kpc of the Sun Beyond this distance the clusters undoubtedly exist but they merge with the starry back ground.

GLOBULAR CLUSTERS are tightly-packed collections of hundreds of thousands, or even millions, of stars. They have a spherical shape and are distributed throughout a sphere surrounding the whole Galaxy. This sphere is often called the HALO. Figure 2.22 shows schematically the Galaxy and halo and the distribution of open and globular clusters. Many of the globular clusters are rather distant objects (3-60 kpc) of which just over a hundred are known in the Galaxy. The brightest globulars are co Centauri and 47 Tucanae in the southern hemispheres, and M5 and Ml 3 (Hercules) in the north. It is estimated that some globular clusters contain millions of stars. Their linear diameters are not large by astronomical standards : the range is 7-120pc, and co Cen at 20 pc is a typical example (figure 2.23). Globular clusters are known to contain large numbers of faint, evolved stars (e.g. white dwarfs), and the presence of these senile objects is an indication that the clusters are ancient.

There are also rather looser star families called ASSOCIATIONS, which lack any obvious structure. These are composed mainly of 0 and B stars, share a common motion through the Galaxy, and are expanding in total size. The signs are that associations are star groups of relatively recent origin.

Star clusters are of immense importance to observational and theoretical astrophysics because each cluster is a collection of objects that condensed from the matter between the stars at more or less the same time and with similar composition. Therefore the careful study of a cluster will show how the only remaining variable, he masses of the individual stars, affects its life history In relation to other stars. We return again to this important point in chapter 3 In our discussion of stellar evolution

In addition to the families we must consider the stellar populations into which stars can be categorized the STELLAR POPULATIONS into which stars can be categorized .In the 1940s German American observer walter baade resolved for the first time individual stars in the central regions of the Andromeda nebula ,or M31, a giant spiral galaxy 700kpc from the milky way .This Technical achievement led to an astronomical advance because Baade found that the stars in the central region of the M31 galaxy were quite unlike those in the spiral arms of M31. From this observation he developed the concept of stellar populations: that the properties of the broad range of observed stars differs from place to place in a galaxy. The young material in a galaxy tends to be found in the spiral arms and comprises hot, young stars, gas nebulae and dust; this is called POPULATION i, and it is distinguished chemically by a relatively high proportion of the heavier elements. Older material is found in globular clusters, galactic nuclei, and elliptical galaxies, and it is made up of old evolved stars (red giants and white dwarfs, for example); this is referred to as POPULATION n, and chemically it is matter that contains only a small amount of the heavier elements. Population I objects are made from interstellar gas that has been enriched by the debris from exploding stars, whereas Population II objects formed long ago from relatively unprocessed raw materials. The concept of stellar populations is particularly valuable to discussions of galactic structure.

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