The attempt to make three-dimensional sense of the Milky Way goes back to a most unlikely origin: the English antiquary of the early eighteenth century, William Stukeley, remembered today for associating the Druids with Stonehenge. Stukeley came from Lincolnshire and so was a fellow-countryman of Isaac Newton, and as a result he was privileged to talk with the great man from time to time. In his Memoirs of Newton Stukeley records one conversation they had in about 1720, in which Stukeley proposed that the Sun and the brightest stars of the night sky make up what we today would term a globular cluster, and this cluster is surrounded by a gap, outside of which lie the small stars of the Milky Way in the form of a flattened ring.

I am greatly pleased to have a chance to speak at this Symposium about Harlow Shapley, the man who was probably responsible, more than any other, for the shape my life has taken.

In my talk this morning I am repeating a few of the remarks I made at the Centennial Celebration for the birthday of Harlow Shapley last October 17 at the Harvard College Observatory. Some of the people here today were present then, and I trust they will excuse the repetition. But my talk today has a different slant.

Harlow Shapley's legacy can be divided into three aspects: his scientific contributions, the institutions he built, and his multi-faceted efforts to publicize astronomy. Today's public funding of science undoubtedly owes much to Shapley's enthusiasm for astronomy.

Harlow Shapley (1918) used the positions of globular clusters in space to determine the dimensions of our Galaxy. His conclusion that the Sun does not lie near the center of the Galaxy is widely recognized as one of the most important astronomical discoveries of this century. Nearly as important, but much less publicized, was his realization that, unlike stars, open clusters, HII regions and planetary nebulae, globular clusters are not concentrated near the plane of the Milky Way. His data showed that the globular clusters are distributed over very large distances from the galactic plane and the galactic center. Ever since this discovery that the Galaxy has a vast halo containing globular clusters, it has been clear that these clusters are key objects for probing the evolution of the Galaxy. Later work, which showed that globular clusters are very old and, on average, very metal poor, underscored their importance. In the spirit of this research, which started with Shapley's, this review discusses the characteristics of the globular cluster system that have the most bearing on the evolution of the Galaxy.

Constraints on cluster kinematics proper motions, radial velocities and tidal radii are reviewed. Analysis of the cluster radial velocity distribution suggests a rotation law for the system in which the specific angular momentum is nearby independent of galactocentric distance, and the residual velocity dispersion is isotropic. However, the absence of severely tidally truncated clusters indicates that nearly radial orbits are absent from this distribution. The kinematic properties of the remote halo clusters remain largely indeterminate. Absolute proper motions measured directly with respect to background galaxies and quasars are needed to determine the kinematics of these objects, and also to elucidate the process of tidal stripping.

This review concentrates on new generalities emerging from this, the second “golden era” of B,V color-magnitude diagrams (CMDs), and remarks about future work in other photometric systems.

At a meeting on globular clusters held five years ago, Zinn (1981) made the following comments:

“In summary, there are clearly large differences in metal line strength between 47 Tuc giants and the giants in the other clusters. These differences suggest that there are substantial differences in metal abundance, which conflicts with the measurements obtained from echelle spectrograms, and there appears to be no simple way of reconciling these results. Until this is done, the metallicity scale for globular clusters hangs in limbo.”

Twenty years ago it was believed by most astronomers that globular clusters were chemically homogeneous - where by homogeneous one means that the outer layers of all stars within a given cluster are the same to within a few tens of percent. Today it is possible to defend the case that no Galactic globular cluster has this characteristic. The reason that this phenomenon has exercised the minds of so many groups in the past 15 years is exciting and obvious: if one can ascertain which are the relevant physical processes in operation, one stands to gain significant insight into both the way in which globular clusters formed and/or the way in which individual low mass stars evolve and mix the products of their nucleosynthesis into their outer layers. A second important driver at the back of the minds of workers in this field is the possible ramifications of an understanding of the phenomenon; for example, if one concludes that the abundance anomalies are being driven today by some particular effect (angular momentum, magnetic fields, interactions within binary systems, stellar collisions - or whatever) this may lead to insight into other important globular cluster phenomena (eg bimodal horizontal branches, gaps at the base of the giant branch, horizontal branch rotation, etc.)

Using stellar evolutionary models which have been computed for the preferred set of element abundances, ages of 13-14 Gyr are derived for the Galactic globular clusters. No significant (> 1.5 Gyr) spread in age is found as a function of either metal content or galactocentric distance. The lack of an age-metallicity relation is shown to be contingent upon the assumption that [O/Fe] is anticorrelated with [Fe/H]: the use of scaled solar abundance calculations would predict that the most metal-poor systems are about 3 × 109 yr older than the most metal-rich clusters. Some evidence in support of high oxygen-to-iron ratios is discussed.

The use of luminosity functions in the following areas is reviewed: (a) the determination of the helium content and ages of the globular clusters; (b) the testing of stellar structure theory; and (c) the determination of the initial mass function of globular clusters.

Recent work on the chemistry and kinematics of the field halo population stars is reviewed, including the metallicity distribution function, elemental abundance patterns, primordial abundances, and their relations with stellar kinematics. The important role played by these stars in determining the ages of the globular clusters is discussed. A comparison is made between the kinematic and chemical properties of the field and cluster stars to ascertain if they share a common history.

The old globular clusters in the Magellanic Clouds are important links between our understanding of globular clusters in our own galaxy and similar unresolved objects in more distant galaxies. The Cloud clusters spread over a large range in age. Several contain RR Lyrae variable stars. High weight abundance data are needed for individual cluster members as well as deeper color-magnitude diagrams.

In this paper, I discuss some of the new facts that have been learned about Magellanic Cloud clusters, mostly thanks to new detectors and associated reduction code. I first show the extent of the LMC cluster system, in order to note that studies of age, abundance, and kinematics of the cluster system have been missing clusters to the north and south of the Hodge and Wright atlas, and to point out that star formation has gone on in places far from present day neutral hydrogen. I will concentrate on the intermediate age clusters (108-1010 y) in the discussion concerning new stellar evolution results, neglecting the 107 y clusters and 30 Doradus. I further restrict my choice of topics to 1) the luminosity of clump giants, 2) the youngest possible RR Lyrae stars, and 3) the patterns and history of cluster formation. The discussion of abundances of Cloud clusters leads readers to the excellent poster papers presented at this meeting.

The present status of the search for globular clusters in M31 is reviewed and some outstanding properties of the cluster system as a whole are briefly discussed.

We review what has been learned about the globular clusters in NGC 5128 (Cen A). Two independent programs (our work at CTIO and Sharples” work at the AAT) have confirmed 87 objects as clusters in NGC 5128 from their velocities. We discuss the colors of the clusters, which are indicative of a wide range of metallicities, their velocities, and their origin as genuine globular clusters in this elliptical galaxy.