An age difference of two billion years between a metal-rich and a metal-poor globular cluster


Globular clusters trace the formation history of the spheroidal components of our Galaxy and other galaxies1, which represent the bulk of star formation over the history of the Universe2. The clusters exhibit a range of metallicities (abundances of elements heavier than helium), with metal-poor clusters dominating the stellar halo of the Galaxy, and higher-metallicity clusters found within the inner Galaxy, associated with the stellar bulge, or the thick disk3,4. Age differences between these clusters can indicate the sequence in which the components of the Galaxy formed, and in particular which clusters were formed outside the Galaxy and were later engulfed along with their original host galaxies, and which were formed within it. Here we report an absolute age of 9.9 ± 0.7 billion years (at 95 per cent confidence) for the metal-rich globular cluster 47 Tucanae, determined by modelling the properties of the cluster’s white-dwarf cooling sequence. This is about two billion years younger than has been inferred for the metal-poor cluster NGC 6397 from the same models, and provides quantitative evidence that metal-rich clusters like 47 Tucanae formed later than metal-poor halo clusters like NGC 6397.

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Figure 1: The 47 Tucanae white-dwarf cooling sequence.
Figure 2: Luminosity function comparison between 47 Tucanae and NGC 6397.
Figure 3: Model comparison to white-dwarf colour-magnitude distribution.
Figure 4: Age–metallicity relation based on white dwarfs.


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Support for the programme GO-11677 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. This work is supported in part by the Natural Science and Engineering Research Council of Canada.

Author information




B.M.S.H. and A.D were primarily responsible for the modelling efforts. J.S.K. and J.A. were primarily responsible for the analysis of the data. H.B.R., R.M.R. and D.R. were responsible for the scheduling of the observations. All authors, including M.M.S., G.G.F., J.R.H., I.R.K. and P.B.S. were involved in the conception and planning of the project and in the writing of the paper.

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Correspondence to B. M. S. Hansen.

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The authors declare no competing financial interests.

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Hansen, B., Kalirai, J., Anderson, J. et al. An age difference of two billion years between a metal-rich and a metal-poor globular cluster. Nature 500, 51–53 (2013).

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