Globular ClustersENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSGlobular ClustersGlobular clusters (GCs) are aggregates of approximately104–106gravitationally bound stars, highly concentratedto the center, spread over a volume ranging from a fewdozen up to more than 300 light-years (ly) in diameter.They resemble shining, old islands orbiting the MilkyWay. As the name indicates, GCs show a largely sphericalsymmetry about their centers. A picture of the classic GCω Centauri is shown in figure 1.The stellar density in the cluster’s center is sohigh (up to a few 103stars ly−3) that it is generallyimpossible to separate the individual stars from ground-based observations. Only recently has the refurbishedHubble Space Telescope (HST) allowed astronomers to diginto the very central regions of many Galactic globulars,where members (sometimes peculiar or even exotic) moverandomly like molecules of gas, interacting according tothe basic laws of gravity.Early studies of GCs date back to the birth of modernastronomy. Since then, GCs have continued to offerexcitement to both professional astronomers and sky-lovers with surprising results, and they constitute a basicbenchmark for our astrophysical understanding.The Milky Way hosts about 200 GCs. They form a haloof roughly spherical shape which is highly concentratedaround the Galactic center, in the Sagittarius–Scorpius–Ophiuchus region. The most distant Galactic globulars(such as NGC 2419) are located far beyond the edge of theGalactic disk, at distances out to 300 000 ly.RADIAL VELOCITY measurements have shown that mostof the GCs are orbiting the Galaxy in highly eccentricelliptical orbits (see figure 2), with orbital periods of about108yr or even longer.While following their orbits around the Galacticcenter, GCs are subject to a variety of perturbations (tidalforces from the parent galaxy, passage through the Galacticplane, star escape, internal dynamical evolution, etc)which make the existing GCs perhaps just the survivors ofa much wider population, partially disrupted and spreadout throughout the Galactic halo and far beyond. Inthis respect, it has been estimated that, within the nextten billion years or so, most of the present Galactic GCscould disappear. On the other hand, we know todaythat four clusters in Sagittarius (M54 in particular) arelikely members of the Sagittarius Dwarf Elliptical Galaxy(discovered in 1994), currently merging into the centralregions of the Milky Way.A large majority of the galactic GCs have high relativevelocities (100–300 km s−1) with respect to the Sun, asthey do not participate in the Galactic disk rotation. Thereis, however, a subsample, commonly referred to as ‘diskglobulars’, which show properties closely connected to thedisk.Spectroscopic observations of stars in Galactic GCshave revealed that their chemical composition differs fromthat of the Sun in heavy elements content. GC stars are infact typically metal poor and old. This is a signature thatFigure 1. The GC ω Centauri.Figure 2. Orbits of GCs in the Galaxy.they were presumably born during the early stages of theGalaxy’s formation and thus represent a sort of archeo-astronomical site where the universe in its youth can bestudied.Globular clusters seem to be ubiquitous. EdwinHubble pionereed the search for globulars in the galaxiesof theLOCAL GROUP with the detection of about 100 GCsin M31, the Andromeda Nebula (more than 350 GCs areknown nowadays). However, it was only in the 1970sthat the identification of any significant number of GCcandidates became feasible around galaxies beyond theLocal Group. The main reason for this difficulty in thesearch is that, with increasing distance, a typical clusterbecomes progressively indistinguishable in shape fromforeground stars or distant background galaxies.It is now fairly well established that almost allgalaxies have GC systems, in some cases (e.g. M87)containing several thousands of globulars (seeGLOBULARCopyright © Nature Publishing Group 2001Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2001Dirac House, Temple Back, Bristol, BS1 6BE, UK1Globular ClustersENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSCLUSTER SYSTEMS IN NORMAL GALAXIES). There are, however,important differences. While all the globulars in ourGalaxy and in M31 are old (ages of about 10 Gyr, at least),there are galaxies, such as the two Magellanic Clouds andM33 (the Triangulum Galaxy), hosting much younger GCs(ages of a few Gyr, or less).The latest GC searches also reveal that dense, massivestar clusters seem to be currently forming in the halosof some interacting galaxies (seeGLOBULAR CLUSTER SYSTEMSIN INTERACTING GALAXIES). These objects are commonlyinterpreted as young and metal-rich GCs. This idea isnot universally accepted, however. In fact, observationalevidence, still quite meager, needs to be confirmed.Furthermore, astronomers seem somehow reluctant tochange their traditional view of the GCs and to admit thatmassive, young and metal-rich GCs could possibly be evenmore frequent in the universe than the ‘classic’, ancient andvery metal-poor ones.Historical backgroundPerhaps the first historical detection of a globular clustergoes back to the mists of time, when human eyes first sawω Centauri, the biggest Galactic GC, barely visible in thesouthern hemisphere.The first ‘astronomical’ detection dates back to the18th century. John Herschel, in the 1830s, realized thata large number of these clusters are concentrated in arelatively small portion of the sky in the direction ofSagittarius. Later on,HARLOW SHAPLEY detected variablestars in several GCs and, on the assumption they wereCepheids of known (calibrated) absolute magnitude,derived distances to them and to the Galactic center. Doingthis, in 1917, Shapley understood that the Galactic centeris located very far away from the Sun, in the direction ofSagittarius, and was also able to estimate the size of theMilky Way.Today we know that Shapley significantly overesti-mated (by a factor of 2 or so) the size of the GCs system andof the Milky Way as a whole, mainly because the clustervariables he identified as Cepheids are actually RR Lyraes,whose absolute magnitude is about 2–4 mag fainter thanCepheids.A key role in astronomyIndividual clusters as well as GC systems are of greatworth as specific targets but also represent a powerful toolto obtain a deep insight into a large variety of