An accurate relative age estimator for globular clusters

Brian Chaboyer, H. Pierre Demarque, Peter J. Kernan, Lawrence M. Krauss, Ata Sarajedini

Research output: Contribution to journalArticlepeer-review

55 Scopus citations


Globular cluster age estimates based on the absolute magnitude of the main-sequence turn-off [Mv(TO)] are generally considered to be the most reliable from a theoretical viewpoint. However, the difficulty in determining Mv(TO) in observed colour-magnitude diagrams leads to a large error in the derived age. In this paper, we advocate the use of the absolute magnitude of the point which is brighter than the turn-off and 0.05 mag redder [Mv(BTO)] as a precision age indicator. It is easy to measure this point on observed colour-magnitude diagrams, leading to small observational error bars. Furthermore, an extensive Monte Carlo calculation indicates that the theoretical uncertainty in Mv(BTO) is similar to Mv(TO). As a result, ages derived using Mv(BTO) are at least a factor of 2 more precise than those derived using Mv(TO). This technique is applied to the globular cluster M68 and an age of 12.8 ± 0.3 Gyr is derived [assuming Mv(RR) = 0.20[Fe/H] + 0.98], indicating that M68 is a 'young' globular cluster. A homogeneous set of globular cluster age estimates with this precision would provide unprecedented insight into the formation of the Galactic halo.

Original languageEnglish (US)
Pages (from-to)683-689
Number of pages7
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
StatePublished - 1996
Externally publishedYes


  • Globular clusters: general
  • Globular clusters: individual: M68
  • Methods: data analysis
  • Stars: evolution
  • Stars: interiors
  • Stars: population II

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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