BASIC OBSERVING SERIES

by Greg Burnett (gburnett)

Star Clusters

[This article was first published in Star Stuff, the newsletter of the Ford Amateur Astronomy Club, in November, 1993.]

There are two types of star clusters, "galactic' or "open" clusters, and "globular" clusters. The Pleiades and the Hyades are well known examples of nearby open clusters. M13, the Hercules Cluster, is a prime example of a globular cluster. The two types differ radically, being almost opposites of each other in terms of their defining characteristics. They also call for different observing techniques.

Open clusters generally contain some tens to some hundreds of stars. They occur most often within the spiral arms of our galaxy, hardly ever being found at higher galactic latitudes (i.e. they are all near the plane of the galaxy). As a result, astronomers estimate that there may be as many as 100,000 open clusters that remain undiscovered because they are obscured by dust in the galactic plane. Only about 1200 open clusters have been catalogued. Most open clusters are young, as evidenced by their populations of hot, blue stars of "early" spectral types. Many still contain nebulosity left over from their formative period (e.g. the Pleiades). The stars in an open cluster are not gravitationally bound to each other. They will eventually disperse into the galaxy due to galactic gravitational tidal forces and other perturbations. But while they remain a cluster, they are useful for computing distances beyond the range of parallax measurements. Because the members of a cluster are travelling in more or less the same direction through space, observations of the convergence (or divergence) of their proper motions, combined with measurements of their radial velocities, allow us to triangulate the distance to the cluster.

Open clusters are comparatively large in angular size, so observing them calls for low powers and wide fields. Many open clusters are admirable, even spectacular, binocular objects, and a few can be easily seen with the naked eye. Generally speaking, open clusters are composed of reasonably bright stars distributed over a generous area, so large telescope apertures and high resolution are not mandatory for satisfying views. The challenge in observing open clusters is often to separate them from the background stars. In this respect great light gathering power can be a hinderance rather than an asset. In a large-aperture scope the cluster can be lost among the many fainter background stars revealed by the large scope. The lowest magnification that provides adequate contrast usually yields the best views of open clusters. Move to higher powers to examine double stars and asterisms within the cluster.

Globular clusters generally contain thousands to hundreds of thousands of stars. They occur throughout a spherical "halo" that encompasses the galaxy, and they are found at all galactic latitudes. Within our galaxy there are 138 known globulars; probably fewer than 100 remain undiscovered. Globulars are among the oldest objects in the galaxy. Some are estimated to be up to 10 billion years old. Most of their stars are poor in "metals" (in astrophysics, any element other than hydrogen or helium is called a "metal"), having been formed at a time before much of the heavier elements had been formed. The stars of a globular cluster are gravitationally bound to each other. They form a stabile system in which each star orbits the gravitational center of the cluster as a whole. Globulars at high galactic latitudes are sometimes less dense than those closer to the galactic plane. Gravitational tidal forces generated by the galaxy impose a lower limit on the density of globulars close to the plane. Many globular clusters have been identified in other galaxies, and are important "standard candles" used in estimating intergalactic distances. Some 300 have been observed in M31, the Andromeda Galaxy, and a hoard of over 6000 globulars surrounds M87.

Observing globular clusters calls for moderate- to large-aperture telescopes with excellent resolution. Many globulars are somewhat faint, and the best views are those that resolve as many individual stars as possible. Moderate to high magnifications can be used for improved contrast, as globulars are comparatively small in angular diameter.

Star clusters are among the most beautiful objects in the heavens. Most of the best clusters (accessible from the northern hemisphere, at least) are contained in the famous catalogue of Charles Messier. Many additional objects worthy of observation can be found in the NGC and IC listings (New General Catalogue and Index Catalogue). One of the best books about stars clusters for the amateur is the Webb Society Deep-Sky Observer's Handbook, Volume 3: Open and Globular Clusters, which contains a wealth of information about clusters and a very useful catalogue of the best objects of both types.