Star Clusters

A star cluster is defined to be a visible concentration of several stars or more above the surrounding stellar background, localized in space and identifiable on visual or infrared images covering a suitable field of view. From the theoretical point of view, a cluster might be better defined as a self-gravitating system of stars residing within the Galactic gravitational potential. Using this definition, unbound embedded clusters or young optically revealed clusters would not be considered to be true clusters. In addition, a star that has achieved escape velocity from a cluster but is still within the tidal radius of the cluster would be considered to be a nonmember by this theoretical definition, but would be a member by the observational definition.

Even a small telescope shows local concentrations of stars scattered around the sky. These star clusters are not chance juxtapositions of a number of unrelated stars. They are physically associated groups of stars, moving together in the Galaxy. The stars in a cluster are held together permanently or temporarily by their mutual gravitational attraction.

The classic, and best known, example of a star cluster is the Pleiades, visible in the evening sky in early winter (from the northern hemisphere) as a group of 7–10 stars

More than 600 Pleiades members have been identified telescopically.

Clusters are generally distinguished as being either Galactic Open-clusters or Globular Clusters, corresponding to their appearance as seen through a moderate-aperture telescope. Even in a relatively small telescope, the stars in an open cluster can be individually distinguished right at the center, whereas a globular cluster typically appears to consist of a sprinkling of bright, resolved stars superposed on a continuous background of unresolved fainter stars. Although the original distinction of the classes of clusters was based almost entirely on their visual appearance, we now know that the globulars and the open clusters are very distinct from one another in almost every respect.

The known globular clusters in our Galaxy number about 147 and, taking into account obscured regions of the Galaxy, the total Galactic population of globulars is probably about 200. The globular clusters cover a substantial range in size. The average core radius is close to one parsec, but a few clusters in the outer halo of the Galaxy have core radii in excess of 10 parsecs. Some 1200 open clusters have been catalogued, but as many as 250 of these appear to be simply slight enhancements in the stellar density along a line of sight or small asterisms of unrelated stars. Most of the open clusters in the Galaxy have not yet been found, since open clusters are found near the Galactic plane where the interstellar dust is the most opaque. There may be as many as 105 open clusters in the Galaxy. The smallest of the open clusters contain fewer than a dozen stars, so the minimum mass of an open cluster is about 10 times the Solar Mass. At the other extreme, some of the most massive open clusters are old ones such as NGC 6791, with more than 10 000members, again with typical masses near one solar mass.

Questions to Ponder

• What are "blue stragglers" in globular clusters?
• How can we discern different galaxies/clusters along the same line of sight?