Globular Clusters

Globular clusters are among the most beautiful objects in the sky. They are highly spherical star clusters held together by mutual gravitational attraction, with about ten thousand to a million stars in a typical cluster. Although the number of stars in a globular cluster is large, the size of a cluster is surprisingly small. A typical diameter is only 20-30 pc. As a consequence, the density of stars near the center of a large globular cluster like M13 may be 500-1000 times greater than the density of stars near the Sun (see the discussion in the right panel).

Distribution of Globular Clusters

Globular clusters are satellites of galaxies and are distributed around their parent galaxies in a roughly uniform manner. Our own galaxy has a halo of approximately 250 globular clusters, which is typical for a large spiral galaxy. The number of globulars around a galaxy varies over quite a wide range; the giant elliptical galaxy M87 in the Virgo cluster has over 10,000 globular clusters, whereas the irregular dwarf galaxy WLM has just one. Here is a link to clusters (both globular and open) that are Messier objects and here is a set of links to images of globular clusters.

Omega Centauri

From Earth, our galaxy's globular clusters appear to be faint "fuzz balls" in binoculars and a very few are visible to the naked eye as slightly fuzzy stars. However, even small telescopes can resolve the outer regions of the brightest globulars into individual stars. An example of a bright globular is shown above: Omega Centauri (visible in the Southern Hemisphere), which is the largest and brightest globular cluster in our galaxy. Omega Centauri is about 40 pc across at a distance of approximately 5 kpc, and appears to contain about 5 million solar masses and as many as 10 million stars. The brightest globular cluster visible in the Northern Hemisphere is the apparent visual magnitude 5.8 cluster M13, in the constellation Hercules (often called The Great Globular Cluster in Hercules). It contains several hundred thousand stars and lies at a distance of about 7 kpc.

Globular Clusters and the Size of our Galaxy

Globular clusters are interesting both in themselves and because they can tell us much about the galaxies in which they reside, including our own. One of the earliest "spinoffs" of studying globular clusters was their use by Harlow Shapley in 1918 to locate the center of the galaxy and determine its diameter.
It was clear that globular clusters were large, distant clusters of stars and Shapley assumed correctly that they were approximately symmetrically distributed around the galaxy. As seen from Earth, however, most globulars lie in one hemisphere (more than half lie in a small region of the sky near Sagittarius). This suggests that we are far from the galaxy's center. From estimates of their distances (see the later discussion of period-luminosity relations for variable stars in Chapter 21), Shapley concluded that the center of our galaxy's family of globular clusters--and hence the center of the galaxy--lay in the direction of the constellation Sagittarius. He further concluded that the diameter of the galaxy was about 100 kpc (more than 300,000 light years) and that the Earth was about 15 kpc from the center. This was ten times larger than previous estimates of the size of the galaxy!
The diameter of the galaxy was too high in Shapley's original estimate because effects like the dimming caused by galactic dust were not yet appreciated when Shapley made his historic estimate. The accepted modern value for the diameter of the galaxy is about 25 kpc and the Earth is about 8 kpc from the center. Nevertheless, Shapley made the first realistic estimate of the size of our galaxy and demonstrated conclusively that the Earth was not the center of the galaxy (as had often been assumed prior to Shapley), and that the Universe was a much bigger place than had been thought.