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The Relative Ages of Galactic Globular Clusters
Peter B. Stetson, Don A. VandenBerg and Michael Bolte
Publications of the Astronomical Society of the Pacific
Vol. 108, No. 725 (1996 July), pp. 560-574
Published by: Astronomical Society of the Pacific
Stable URL: http://www.jstor.org/stable/40680769
Page Count: 15
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We discuss the present state of knowledge and thought concerning the spread in age found among Galactic globular clusters, with some discussion of the implications for what happened during the earliest stages of the formation of the Milky Way Galaxy. Differential observational techniques to derive the relative age differences among clusters of similar metallicity are discussed in detail. We conclude that as of the current date (mid-April 1996) the state of the field is still somewhat muddled. However, we believe that there is now a substantial body of evidence—including a particularly revealing intercomparison of the color-magnitude diagrams of NGC 1851, NGC 288, and NGC 362 presented here— indicating that age is not the dominant second parameter determining the shape of globular clusters' horizontal branches. If our assertion is correct, then apart from a handful of anomalous clusters that may well have been captured from a satellite dwarf galaxy, there is no strong evidence either for a significant spread in age among clusters of a given metal abundance or for a systematic variation of mean age with Galactocentric distance. On the question of whether there is a significant age difference between metal-poor and metal-rich clusters, we feel compelled to fall back on the Scots verdict: "Not proven." Data now being collected by numerous groups in various subdisciplines may resolve the remaining controversy within a few years.
Publications of the Astronomical Society of the Pacific © 1996 The University of Chicago Press