Letter | Published:

A spin-down clock for cool stars from observations of a 2.5-billion-year-old cluster

Nature volume 517, pages 589591 (29 January 2015) | Download Citation



The ages of the most common stars—low-mass (cool) stars like the Sun, and smaller—are difficult to derive1,2 because traditional dating methods use stellar properties that either change little as the stars age3,4 or are hard to measure5,6,7,8. The rotation rates of all cool stars decrease substantially with time as the stars steadily lose their angular momenta. If properly calibrated, rotation therefore can act as a reliable determinant of their ages based on the method of gyrochronology2,9,10,11. To calibrate gyrochronology, the relationship between rotation period and age must be determined for cool stars of different masses, which is best accomplished with rotation period measurements for stars in clusters with well-known ages. Hitherto, such measurements have been possible only in clusters with ages of less than about one billion years12,13,14,15,16, and gyrochronology ages for older stars have been inferred from model predictions2,7,11,17. Here we report rotation period measurements for 30 cool stars in the 2.5-billion-year-old cluster NGC 6819. The periods reveal a well-defined relationship between rotation period and stellar mass at the cluster age, suggesting that ages with a precision of order 10 per cent can be derived for large numbers of cool Galactic field stars.

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S.M. acknowledges support through NASA grant NNX09AH18A (The Kepler Cluster Study), NSF grant 1312882 (The Kepler Cluster Study: Planets and Gyrochronology) and the Smithsonian Institution’s Competitive Grants Program for Science in 2012 and 2013. S.A.B. acknowledges support from the German Science Foundation (DFG) during a crucial phase of this work via a Mercator Guest Professorship at the University of Potsdam and the Leibniz Institute for Astrophysics Potsdam, Germany. This paper includes data collected by the Kepler mission. Kepler was competitively selected as the tenth Discovery mission. Funding for the Kepler mission is provided by the NASA Science Mission directorate. Some or all of the data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX13AC07G and by other grants and contracts. Spectroscopic observations of NGC 6819 with the Hectochelle spectrograph were obtained at the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona.

Author information


  1. Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA

    • Søren Meibom
    •  & David W. Latham
  2. Leibniz Institute for Astrophysics, An der Sternwarte 16, 14482 Potsdam, Germany

    • Sydney A. Barnes
  3. Space Science Institute, 4750 Walnut Street #205, Boulder, Colorado 80301, USA

    • Sydney A. Barnes
  4. Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA

    • Imants Platais
  5. Center for Exoplanets and Habitable Worlds, The Pennsylvania State University, University Park, Pennsylvania 16802, USA

    • Ronald L. Gilliland
  6. Department of Astronomy, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA

    • Robert D. Mathieu


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S.M. is the Principal Investigator for The Kepler Cluster Study and led the planning and execution of the study; the determination and validation of rotation periods from the Kepler data; the membership, binarity and vsin(i) survey in NGC 6819 with the Hectochelle spectrograph on the MMT telescope; and the writing of this paper. S.A.B. is a Co-Investigator on The Kepler Cluster Study and participated in planning the study, evaluated the light curves for periodicity, performed the gyrochronology analysis and age determination for the stars, and collaborated closely with S.M. in writing the paper. I.P. is a Co-Investigator on The Kepler Cluster Study and contributed to the selection of Kepler targets in NGC 6819 with proper-motion membership information and to the analysis of crowding and contamination in the vicinity of target stars from deep, high-spatial-resolution images of this star cluster. R.L.G. is a Co-Investigator on The Kepler Cluster Study and contributed to the selection of Kepler targets in NGC 6819 with analysis of crowding and contamination in deep, high-spatial-resolution images. He carried out analysis of alternative methods for extracting light curves from raw Kepler data. D.W.L. participated in the NGC 6819 radial-velocity membership and binarity surveys, and led the preparation of the Kepler Input Catalog. R.D.M. is the Principal Investigator of the WIYN Open Cluster Study, which contributed radial-velocity measurements to the membership and binary star surveys in NGC 6819.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Søren Meibom.

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