|| © CAMBRIDGE UNIVERSITY PRESS 1998
``Perhaps the most wonderful of all the star clusters are those in which hundreds upon hundreds of faint stars are all gathered together in the shape of a globe.'' Although this description of globular clusters by Reverend James Baikie (1911) is numerically faulty, it summarizes the visually striking features of these objects accurately. Globular clusters are characterized by high central stellar densities. and tend to be extremely round. Figure 1.1 shows an image of the Milky Way globular cluster M92 illustrating these features. Globular clusters like M92 formed early in the history of the universe and contain some of the oldest stars known. Systems of globular clusters appear to surround all bright galaxies. as well as many dwarf galaxies.
|Fig. 1.1. The globular cluster M92 (Lick Observatory).|
Individual globular clusters and globular cluster systems are of interest in their own right, but they also provide unique insights into a wide range of astrophysical processes and systems. As individual objects, they constitute isolated laboratories in which many aspects of stellar evolution and dynamics can be studied. When considered as a system, the globular clusters surrounding a galaxy provide a fossil record of the dynamical and chemical conditions when the galaxy was in the process of formation. Thus observations of globular cluster systems can be used to constrain models of the formation and evolution of galaxies. Perhaps the most renowned impact of globular cluster research on other fields of astronomy is provided by age estimates of Milky Way globulars, which provide a minimum age for the universe.
The interplay between globular cluster research and other areas in astronomy is not a recent development. One historical example is provided by studies of the Milky Way globular cluster system and the role they played in uncovering the size and shape of the Galaxy. Early in the twentieth century, Kapteyn investigated the size and structure of the Milky Way (then regarded as the entire universe) by measuring the positions and magnitudes of stars on photographic plates. He concluded that stars in the Milky Way had a somewhat flattened distribution with the Solar System close to the center.
A key clue that eventually produced a major revision in this model was contained in the observations of globular clusters carried out by John Herschel in the 1830s. He noticed that a large number of these clusters occurred in a relatively small portion of the sky in the direction of Sagittarius. In 1909, Karl Bohlin remarked on the related fact that the majority of globular clusters are found in one half of the sky. He suggested that the Milky Way globular cluster system was distributed symmetrically about the Galactic center, so that the observed distribution implied that the Solar System was displaced from the center of the Milky Way. There were several oddities in Bohlin's model which obscured this important insight of moving the Solar System away from the Galactic center.
The crucial advance in this area was provided in 1918 by Harlow Shapley, who observed variable stars in globular clusters which he assumed were Cepheids. His calibration of the absolute magnitude of these variables allowed Shapley to derive distances to globular clusters. (It was later realized that the RR Lyrae variables in globular clusters are fainter than Cepheids, and that Shapley had somewhat overestimated globular cluster distances as a result.) Like Bohlin, Shapley assumed that the spatial distribution of globular clusters was symmetric about the Galactic center. With this assumption, Shapley's distances enabled him to estimate the overall size of the system, as well as the displacement of the Solar System from the Galactic center. Not only did Shapley's results overturn the accepted orthodoxy of the Solar System located towards the inner regions of the Milky Way, they also gave a size of the Milky Way an order of magnitude greater than the Kapteyn model.
We now know that, to within a factor of two or so in scale, Shapley's model was basically correct. Kapteyn and Herschel had not fully accounted for interstellar extinction produced by dust in the Galactic plane, which gave the impression that the number of stars in the plane of the Galaxy fell off fairly uniformly in all directions. The large number of globular clusters in Sagittarius observed by Herschel marks the direction of the Galactic center.
Globular clusters have also played an important role in our understanding of the differences between stellar populations and the resulting implications for galactic structure and evolution. In the 1940s, Baade pioneered work on stellar populations, noting the distinction between Population I stars, such as those in the solar neighborhood, and Population II stars that he identified in the bulge of M31 and its satellite galaxies M32 and NGC 205. Importantly, Baade noted that stars in Galactic globular clusters belonged to Population II. Subsequent observations by Arp, Baum and Sandage led to the conclusion that the stars in Milky Way globular clusters, and thus Population II stars in general, were extremely old. The great age of Milky Way globular clusters is one of the attributes that makes them valuable as probes of the formation of the Galaxy, since they probably formed when the Milky Way itself was forming. Because all their constituent stars are at the same distance, globular clusters are also invaluable as testbeds for understanding stellar structure and evolution, and for serving as the basis for models of stellar populations.
The observational work on stellar populations and the study of globular cluster color-magnitude diagrams in particular led to spectacular advances in stellar evolution theory. In the 1940s and 1950s, laboratory work in the field of nuclear physics allowed calculations to be made of energy generation and transport in stars. Much of the influential work in this area was carried out by Schwarzschild, Hoyle, Henyey and their collaborators. The great age of Milky Way globular clusters allowed a quantitative comparison between the observed colors and luminosities of evolved stars with theoretical predictions. One of the key developments was the realization that nuclear reactions in stars brought about chemical changes. It was found that evolution off the main sequence was a consequence of hydrogen being exhausted in the central stellar regions. Massive stars evolve more rapidly because they use up their hydrogen on a shorter timescale. These and other ideas, which were tried and tested through comparison with observations of globular cluster stars, provide the basis for modern stellar evolution theory.
Another valuable property of globular clusters is their ubiquity. Shortly after the realization that spiral ``nebulae'' were galaxies similar to the Milky Way, Hubble detected globular clusters in M31. It was not until the 1970s, however. that globular clusters were observed in any significant number around galaxies beyond the Local Group. Since that time, it has become apparent that all bright galaxies probably have globular cluster systems, as do many dwarf galaxies. The presence of relatively accessible tracers of the early conditions in galaxies provides a key tool in studying the galaxy formation process.
One important development in recent years is the evidence that dense, massive star clusters are currently forming in certain environments. It has been suggested that these objects are young globular clusters. This idea is not universally accepted, both because current observations are not definitive and possibly because the notion of young globular clusters flies in the face of the traditional view of globular clusters as ancient objects. However, if globular clusters are forming at the present epoch, we will have the opportunity to study the formation process directly. It seems inevitable that this will greatly enhance our understanding of how and why globular clusters form, as well as deepening our knowledge of the galaxy formation process to which globular cluster formation is intimately related.
In this book, our primary focus is a discussion of globular cluster systems. We describe the observational properties of such systems and the theoretical inferences and constraints that can be obtained from these observations. However, to put matters in context, we first describe the internal properties of globular clusters. This material is covered in Chapter 2, where we concentrate on the characteristic properties of globular clusters in the Milky Way. In Chapter 3 we look at the Milky Way globular clusters as a system, and discuss how the properties of this system have influenced (and continue to influence) ideas on the formation of our Galaxy. The globular cluster systems of galaxies in the Local Group and slightly beyond are discussed in Chapter 4, whereas more distant extragalactic globular cluster systems are addressed in Chapter 5. Correlations between the properties of globular cluster systems and properties of their host galaxies are also discussed in Chapter 5. Chapter 6 provides an overview of theories of galaxy formation and discusses how such theories are constrained by the observed properties of globular cluster systems. Formation models of globular clusters and globular cluster systems are covered in Chapter 7. A summary and some speculations concerning the future directions of globular cluster research are presented in Chapter 8.
Throughout this book, we attempt to highlight areas where globular cluster research has had an important impact on other astronomical fields, and, perhaps more importantly, where the interplay continues to advance astronomical understanding.