Globular Star Clusters

Globular star clusters usually contain about 105 stars. The distribution of the stars is spherically symmetric, and the central densities are about ten times larger than in open clusters. Stars in globular clusters are among the oldest in the Milky Way, and therefore they are of great importance for studies of stellar evolution. There are about 150-200 globular clusters in the Milky Way.

The colour-magnitude diagram of a typical globular cluster is shown in Fig. 16.7. The main sequence only contains faint red stars; there is a prominent giant branch, and the horizontal and asymptotic branches are clearly seen. The main sequence is lower than that of the open clusters, because the metal abundance is much lower in the globular clusters.

The horizontal branch stars have a known absolute magnitude, which has been calibrated using principally RRLyrae type variables. Because the horizontal branch stars are bright, they can be observed even in distant clusters, and thus using them the distances of globular clusters can be well determined.

Using the known distances, the linear sizes of globular clusters can be calculated. It is found that most of the mass is concentrated to a central core with a radius of about 0.3-10 pc. Outside this there is an extended en-

Fig. 16.7. Colour-magnitude diagram of the globular cluster M5. In addition to the main sequence one can see the giant branch bending to the right and to its left the horizontal brunch. (Arp, H. (1962): Astrophys. J. 135, 311)

velope with a radius that may be 10-100 times larger. At even larger radii stars will escape from the cluster because of the tidal force of the Galaxy.

The masses of globular clusters can be roughly estimated from the virial theorem, if the stellar velocities in the cluster have been measured. More precise values are calculated by fitting theoretical models to the observed density and velocity distributions. In this way masses in the range 104-106 Me have been obtained.

The globular clusters in the Milky Way fall into two classes. In the classification given in Table 17.1 these correspond to intermediate and halo population II. The disc globular clusters are concentrated towards the centre and the plane of the Milky Way and they form a system that is rotating with the general rotation of the Milky Way. In contrast, the halo clusters are almost spherically distributed in an extensive distribution reaching out to at least 35kpc. The system of halo clusters does not rotate, but instead the velocities of individual clusters are uniformly distributed in all directions. The abundance of heavy elements is also different in the two classes of clusters. For disc clusters it is typically about 30% of the solar value, for halo clusters it is only about 1%. The smallest known heavy element abundances, about 10—3 times the solar value, have been detected in some halo globular clusters. They therefore give important information about the production of elements in the early Universe and during the formation of the Milky Way.

All globular clusters are old, and the halo clusters are among the oldest known astronomical objects. Determining a precise age is difficult, and requires both accurate observations of the turn-off point of the main sequence in the HR diagram, as well detailed theoretical stellar evolution models. The ages obtained have been about (13-16) x 109 years. This age is close to the age of the Universe calculated from its rate of expansion (see Chap. 19).

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