Star clusters

A brief glance at the night sky reveals that it is not uniformly bespangled with thousands of stars. Some areas are sparse, whereas others contain a profusion of stars that tend to be grouped into clusters. There are two distinct types of clusters—open (or galactic) and globular—and each is fundamentally different in nature and appearance.

The globular cluster M3 (NGC 5272) in the constellation Canes Venatici (the Hunting Dogs) is one of the four great globular clusters in the Northern Hemisphere. It is not a very dense cluster, although it contains more than 44,000 stars. It is difficult to distinguish between individual stars without using a telescope, and so M3 is seen with the naked eye as a single star, even though it is 48,500 light-years away.

Open clusters

There are more than 1,000 open clusters in our galaxy. Most of the conspicuous ones are known only by their catalog numbers; although some of the more familiar examples have been given names—such as the Pleiades, a group of hot, young, bluish-white stars in the constellation of Taurus. The six brightest member stars can be seen with the naked eye, and modern telescopes reveal hundreds of others within the group. Another open cluster in the same region of the sky is the Hyades cluster, whose most prominent stars form the V-shape that outlines the face of Taurus, the Bull. An open cluster usually contains from ten to several hundred distinguishable stars, which are interspersed with prominent patches of gas and dust. A cluster is seldom larger than 10 parsecs across.

Globular clusters

A few globular clusters are bright enough to be seen with the naked eye, and they appear as fuzzy, luminous balls of light. But telescopic observation reveals that most are spherical stellar systems that contain several thousand stars. They are densely packed toward the central regions and become more dispersed at the edges. The stars are so tightly packed at the center of a globular cluster that if the earth were situated there, the nearest stars would be light-months away, and the brightness of the night sky would always be comparable to full moonlight. A cluster has an average size of 100 parsecs across—a small space for so many stars—and contains no interstellar gas. There are about 100 globular clusters in our galaxy.
Astronomers have recently observed that a few globular clusters are associated with intense bursts of X rays. Each burst is equivalent to about 1 million times as much energy radiated by the sun in a similar time. One current interpretation is that a massive black hole may exist in the center of many globular clusters.

Composition and location of cluster stars

Spectral analysis of the two groups shows that open and globular cluster stars have basically different chemical compositions. The open cluster stars are similar in composition to the sun—75 per cent is hydrogen, most of the rest is helium and less than 1 per cent is composed of heavier elements. In globular clusters, the abundance of heavier elements is 10 times less even than that in open cluster stars.
As well as differing in composition, the two stellar groups occupy distinctive regions within our galaxy. Open clusters occur in the spiral arms, and globular clusters tend to congregate around the central halo. Also, their motions within the galaxy—which is itself rotating—follow different patterns. The open, or galactic, cluster stars move round the nucleus of the galaxy in much the same way as the planets move around the sun, in fairly circular orbits in the same plane. In contrast, the globular clusters follow a galactic path analogous to that of comets within our solar system that have elongated orbits approaching the sun from all directions. The stars in globular clusters move in highly eccentric orbits with a high inclination to the galactic plane.
During the 1940’s, when Walter Baade was studying the stars in the Andromeda galaxy, he first noticed that there was a fundamental difference between stars in the central regions of galaxies and those in the spiral arms. He described the two kinds in terms of two different “populations.” The stars in the spiral arms, whose brightest members include young, hot, blue stars (typical of open cluster stars in our galaxy), he called Population I stars. He called the cool, red stars that he found to be typical of the central areas of Andromeda (with similar properties to the globular cluster stars in our galaxy), Population II stars.

Stellar evolution

Once the concept of two stellar populations was established, astronomers could determine the age and evolution of stars. This was done by comparing the Hertzsprung-Russell diagrams for both types of cluster stars with those for field stars, such as the sun, which move singly rather than in clusters. The Hertzsprung-Russell diagram for a typical open cluster is similar to that of stars such as the sun. The diagram for stars in globular clusters indicates that most of them fall in the red giant zone. In terms of stellar evolution, because the globular clusters consist mainly of mature red giant stars, and the open clusters are composed of young, bright, blue stars, astronomers assume that the latter must have formed at a later stage than the former. This assumption explains the lower abundance of elements heavier than helium in the Population II globular cluster stars, which must have formed when there were few heavy elements in the area.

A Hertzsprung-Russell diagram reveals the evolution of open and globular clusters. Open clusters lie on the main-sequence band—the brightest, youngest stars in the upper part and the fainter, older stars near the bottom. As they mature, the stars move toward the red giant region, as has M67, the oldest open cluster. The Hertzsprung gap contains few stars because they evolve quickly through this region, as has the Double Cluster h and chi Persei. All globular clusters, older than open clusters, occur in the red giant region—M3, for example, has reached the red supergiant zone.