The age of the Milky Way inner halo

  • A Corrigendum to this article was published on 11 July 2012


The Milky Way galaxy has several components, such as the bulge, disk and halo. Unravelling the assembly history of these stellar populations is often restricted because of difficulties in measuring accurate ages for low-mass, hydrogen-burning stars1,2. Unlike these progenitors, white dwarf stars3, the ‘cinders’ of stellar evolution, are remarkably simple objects and their fundamental properties can be measured with little ambiguity4,5. Here I report observations of newly formed white dwarf stars in the halo of the Milky Way, and a separate analysis of archival data in the well studied 12.5-billion-year-old globular cluster Messier 4. I measure the mass distribution of the remnant stars and invert the stellar evolution process to develop a mathematical relation that links this final stellar mass to the mass of their immediate progenitors, and therefore to the age of the parent population. By applying this technique to a small sample of four nearby and kinematically confirmed halo white dwarf stars, I calculate the age of local field halo stars to be 11.4 ± 0.7 billion years. The oldest globular clusters formed 13.5 billion years ago. Future observations of newly formed white dwarf stars in the halo could be used to reduce the uncertainty, and to probe relative differences between the formation times of the youngest globular clusters and the inner halo.

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Figure 1: Spectroscopic examination of white dwarfs in Messier 4 and the Milky Way halo.
Figure 2: The remnant mass and population age of the Milky Way halo.


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The data presented in this paper were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. I acknowledge the significant cultural role and reverence that the summit of Mauna Kea has within the indigenous Hawaiian community. I am fortunate to have had the opportunity to conduct observations from this mountain. This work was also based on observations obtained at the Paranal Observatory of the European Southern Observatory. I thank A. Dotter, P. Bergeron, and P.-E. Tremblay for discussions related to stellar evolution and ages. I also thank U. Heber for providing us reduced spectra from the SPY Survey.

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Correspondence to Jason S. Kalirai.

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Kalirai, J. The age of the Milky Way inner halo. Nature 486, 90–92 (2012).

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