Star ClustersENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSStar ClustersEven a small telescope shows obvious local concentrationsof stars scattered around the sky. These star clusters arenot chance juxtapositions of unrelated stars. They are,instead, physically associated groups of stars, movingtogether through the Galaxy. The stars in a cluster areheld together either permanently or temporarily by theirmutual gravitational attraction.The classic, and best known, example of a star clusteris the Pleiades, visible in the evening sky in early winter(from the northern hemisphere) as a group of 7–10 stars(depending on one’s eyesight). More than 600 Pleiadesmembers have been identified telescopically.Clusters are generally distinguished as being eitherGALACTIC OPEN CLUSTERS or GLOBULAR CLUSTERS, correspondingto their appearance as seen through a moderate-aperturetelescope. Even in a relatively small telescope, the starsin an open cluster can be individually distinguishedright at the center, whereas a globular cluster typicallyappears to consist of a sprinkling of bright, resolved starssuperposed on a continuous background of unresolvedfainter stars. Although the original distinction of the twoclasses of clusters was based almost entirely on their visualappearance, we now know that the globulars and the openclusters are very distinct from one another in almost everyrespect.At the opposite extreme of concentration from theglobular clusters are the stellar associations, loose, un-bound groups of stars with particular spectral properties.Associations are normally observed as widespread regionswith an excess density of spectral type O and B stars (‘OBASSOCIATIONS’) or T TAURI STARS (‘T associations’). In princi-ple, there could be other types of association as well. Itis not uncommon to find one or more open clusters at thecore of a stellar association. The implication is that thestars in the central cluster or clusters and the surroundingassociation all formed more or less at the same time outof the same material. Since the association is not a gravi-tationally bound system, after a relatively short period oftime, its stars will disperse and the association will loseits identity; for that reason, associations always consist ofyoung stars.The stars in an unbound association or a clusterthat is evaporating will nevertheless all be moving withalmost identical velocities, so at some later time, onemight expect to find stars spread out in space, but stillmoving with similar velocities. Stars selected by velocityare sometimes calledMOVING GROUPS. The best known ofthe moving groups is called the Hyades moving group. Itconsists of a number of stars, located in all directions fromthe Sun, which share the space motion of the Hyades starcluster. However, it is uncertain whether these stars sharea common origin with the Hyades cluster.The category of moving groups is not to be confusedwith moving clusters. These are clusters near enough tothe Sun that all three dimensions of their motion throughthe Galaxy can be used to derive the cluster distance,independently of parallax measurements.Afinal category of clusters has recently been identifiedbased on observations with infrared detectors. These arethe embedded clusters—star clusters still in the process offormation, and still embedded in the clouds out of whichthey formed. Because it is possible to see through the dustmuch better at infrared wavelengths than in the optical,these clusters have suddenly become observable.Star clusters are of considerable astrophysical impor-tance to probe models of stellar evolution and dynamics,explore the star formation process, calibrate the extragalac-tic distance scale and most importantly to measure the ageand evolution of the Galaxy.Definition of a star cluster—cluster catalogsJust what is a cluster? How many stars does it take tomake a cluster? Trumpler in 1930 defined open clustersas ‘Star groupings which undoubtedly form physicalsystems (stars situated at the same distance and probablyof the same origin) and which at the same time aresufficiently rich in stars for statistical investigation.’ Sincevirtually all clusters have been discovered either by visualexamination of the sky with a telescope, or from inspectionof photographic or electronic images in the visual orinfrared, an operational definition of a star cluster issuggested:A star cluster is defined to be an obviousconcentration of several stars or more abovethe surrounding stellar background, apparentlylocalized in space and identifiable on visual orinfrared images covering a suitable field of view.While this definition may seem imprecise, it convenientlyincludes most of those objects (in our Galaxy at least)listed in the existing cluster catalogs. At the same time, itexcludes galaxy-sized systems or objects with insignificantnumbers of stars. Presumably most clusters defined inthis way are physical systems, although they may or maynot be gravitationally bound systems. So some of theembedded clusters or slightly older, optically revealedclusters may be unbound star-forming regions in theprocess of dispersing. The above definition avoids thedifficulty of distinguishing bound systems from unboundones—if it looks like a cluster, it is a cluster.From the theoretical point of view, a cluster mightbe better defined as a self-gravitating system of starsresiding within the Galactic gravitational potential. Usingthis definition, unbound embedded clusters or youngoptically revealed clusters would not be considered to betrue clusters. In addition, a star that has achieved escapevelocity from a cluster but is still within the tidal radiusof the cluster would be considered to be a nonmember bythis theoretical definition, but would be a member by theobservational definition.Few objects have been catalogued as extensively asstar clusters. TheMESSIER CATALOG of 107 nonstellar objectsincludes 59 objects now known to be star clusters. TheCopyright © 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, UK1Star ClustersENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSfirst major catalog that includes clusters is the NEW GENERALCATALOG(NGC), listing 487 clusters. The update to theNGC, the IC, contains another 30 clusters.There have been at least a couple of dozen othermore or less systematic attempts to search for