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14.16 Globular Star Clusters

One of the most spectacular sights through a small telescope is a globular star cluster. Globular clusters are much more massive, more tightly packed, and more symmetrical than open star clusters or associations. They are also very old: often 11 billion or more years in age. They typically contain 20,000 to several million stars, although many of the stars in the central regions are too close together and blurred to be resolved by Earth-based telescopes. A particularly high density of luminous stars has been observed in the core of 47 Tucanae, as imaged with the Hubble Space Telescope. 


Astropedia Image
47 Tucanae, as imaged with the Hubble Space Telescope. Click here for original source URL

To imagine conditions inside a globular cluster, picture 10,000 stars placed around the Sun at distances no farther than Alpha Centauri, our nearest star. Typical diameters of the central concentrations range from only 5 to 25 pc. If we lived in the core of a globular cluster, our night sky would blaze with starlight ten times brighter than the light of the full Moon! Even the nearest globular clusters are thousands of parsecs away from us. It is only because they have so many and such very bright stars that we see them at all. Yet a modest backyard telescope can reveal many prominent examples. Using larger telescopes, astronomers have found over 200 globular clusters around our galaxy as well as globular clusters circling many nearby systems. For example, the Andromeda galaxy (M31) has over 500 globular clusters.

A globular cluster is not a static collection of stars in space. In general, the stars are on elliptical orbits around the cluster center. However, the orbits of individual stars deep inside a globular cluster may be very complex. The cluster's overall gravity, the spatial distribution of its stars, their relative speeds, and the effects of near encounters among stars are all important in determining how an individual star orbits in complex loops around the cluster's central regions. Remember that even in these crowded conditions, actual collisions between stars are rare. Furthermore, as the entire cluster orbits the galaxy, it passes through the galactic disk every 100 million years or so. We can imagine that such passages might allow spectacular close-up views of globular clusters from planets in the galactic disk.

In the 1970s, globular clusters were found to be sites of strong X-ray radiation. Globular cluster X-rays do not have the smooth periodic variations that are caused by orbital motions in binary pairs. Instead they have irregular variations, sometimes over weeks or months, but sometimes doubling in intensity within a few minutes. These surprising findings led to new thinking about conditions inside globular clusters. Generally, the X-ray sources are not exactly at the centers of clusters. They seem to come from binary systems. Studies in the 1980s showed that inner core regions of globular clusters are so crowded with stars that binary pairs may form more often than usual under these conditions. One idea is that globular clusters may contain many neutron stars and other stellar remnants and, because of the overcrowding, these may often capture passing stars into binary orbits. As the captured star evolves and blows off mass, gas is transferred onto the dense stars, and the resulting high-energy impact of gas onto the neutron star or its accretion disk may produce X-rays. The dense packing of stars in the central few cubic parsecs of a globular cluster makes these regions extraordinary stellar environments.

The shape of these aged-systems has been hard to explain. In astronomy, we encounter some cosmic systems that are flattened disks and others that are spherical. The systems that are disk-like have a large amount of rotation — for example, the rings of Saturn and the planets of the Solar System. The systems that are spherical have little or no rotation — for example, the Oort cloud of comets and the distribution of globular clusters around our own galaxy. The physical quantity that describes rotation is angular momentum. Astronomers have found that globular clusters are not exactly spherical, however, but are slightly flattened. They therefore rotate slowly and have a small amount of angular momentum. Since angular momentum is a conserved quantity, the gas clouds from which globular clusters formed must have had small amounts of rotation. Further more, the complex interactions of stars, and especially three-body encounters in the cores of globular clusters, lead to the overall structure of the system slowly beating like a heart.