Globular clusters

The largest and most massive star clusters are the globular clusters, so called because of their roughly spherical appearance. The Galaxy contains approximately 130 globular clusters (the exact number is uncertain because of obscuration by dust in the Milky Way band, which probably prevents some 10 or so globulars from being seen). They are arranged in a nearly spherical halo around the Milky Way, with relatively few toward the galactic plane but a heavy concentration toward the centre. The radial distribution, when plotted as a function of distance from the galactic centre, fits a mathematical expression of a form identical to the one describing the star distribution in elliptical galaxies, though there is an anomalous peak in the distribution at distances of about 40,000 light-years from the centre.

Globular clusters are extremely luminous objects. Their mean luminosity is the equivalent of approximately 25,000 suns. The most luminous are 50 times brighter. The masses of globular clusters, measured by determining the dispersion in the velocities of individual stars, range from a few thousand to more than 1,000,000 solar masses. The clusters are very large, with diameters measuring from 10 to as much as 300 light-years. Most globular clusters are highly concentrated at their centres, having stellar distributions that resemble isothermal gas spheres with a cutoff that corresponds to the tidal effects of the Galaxy. A precise model of star distribution within a cluster can be derived from stellar dynamics, which takes into account the kinds of orbits that stars have in the cluster, encounters between these member stars, and the effects of exterior influences. The American astronomer Ivan R. King, for instance, has derived dynamical models that fit observed stellar distributions very closely. He finds that a cluster's structure can be described in terms of two numbers: (1) the core radius, which measures the degree of concentration at the centre, and (2) the tidal radius, which measures the cutoff of star densities at the edge of the cluster.

A key distinguishing feature of globular clusters in the Galaxy is their uniformly old age. Determined by comparing the stellar population of globulars with stellar evolutionary models, the ages of all those so far measured range from 12 billion to 18 billion years. They are the oldest objects in the Galaxy and so must have been among the first formed when the system condensed out of the pregalactic gas. That this was the case is also indicated by the fact that the globulars tend to have much smaller amounts of heavy elements than do the stars in the plane of the Galaxy—e.g., the Sun. Composed of stars belonging to the extreme Population II, as well as the high-latitude halo stars, these nearly spherical assemblages apparently formed before the material of the Galaxy flattened into the present thin disk. As their component stars evolved, they gave up some of their gas to interstellar space. This gas was enriched in the heavy elements produced in stars during the later stages of their evolution, so that the interstellar gas in the Galaxy is continually being changed. Hydrogen and helium have always been the major constituents, but heavy elements have gradually grown in importance. The present interstellar gas contains elements heavier than helium at a level of about 2 percent by mass, while the globulars contain as little as 0.02 percent of the same elements.

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