Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Letter
  • Published:

A mass transfer origin for blue stragglers in NGC 188 as revealed by half-solar-mass companions


In open star clusters, where all members formed at about the same time, blue straggler stars are typically observed to be brighter and bluer than hydrogen-burning main-sequence stars, and therefore should already have evolved into giant stars and stellar remnants. Correlations between blue straggler frequency and cluster binary star fraction1, core mass2 and radial position3 suggest that mass transfer or mergers in binary stars dominates the production of blue stragglers in open clusters. Analytic models4,5, detailed observations6 and sophisticated N-body simulations7, however, argue in favour of stellar collisions. Here we report that the blue stragglers in long-period binaries in the old8 (7 × 109-year) open cluster NGC 188 have companions with masses of about half a solar mass, with a surprisingly narrow mass distribution. This conclusively rules out a collisional origin, as the collision hypothesis predicts a companion mass distribution with significantly higher masses. Mergers in hierarchical triple stars9 are marginally permitted by the data, but the observations do not favour this hypothesis. The data are highly consistent with a mass transfer origin for the long-period blue straggler binaries in NGC 188, in which the companions would be white dwarfs of about half a solar mass.

This is a preview of subscription content, access via your institution

Access options

Buy this article

Prices may be subject to local taxes which are calculated during checkout

Figure 1: Companion mass distribution for the 12 blue straggler binaries in NGC 188 with periods of order 1,000 d.
Figure 2: Cumulative distribution of mass functions of the NGC 188 blue straggler binaries with periods of order 1,000 d.
Figure 3: Distributions of binary orbital elements for the blue stragglers in the NGC 188 N -body model.

Similar content being viewed by others


  1. Sollima, A. The evolution of the binary population in globular clusters: a full analytical computation. Mon. Not. R. Astron. Soc. 388, 307–322 (2008)

    Article  ADS  CAS  Google Scholar 

  2. Knigge, C., Leigh, N. & Sills, A. A binary origin for ‘blue stragglers’ in globular clusters. Nature 457, 288–290 (2009)

    Article  ADS  CAS  Google Scholar 

  3. Ferraro, F. R. et al. HST observations of blue straggler stars in the core of the globular cluster M3. Astron. Astrophys. 324, 915–928 (1997)

    ADS  Google Scholar 

  4. Leonard, P. J. T. The implications of the binary properties of the M67 blue stragglers. Astron. Astrophys. 470, 521–527 (1996)

    ADS  Google Scholar 

  5. Leigh, N. & Sills, A. An analytic technique for constraining the dynamical origins of multiple star systems containing merger products. Mon. Not. R. Astron. Soc. 410, 2370–2384 (2011)

    Article  ADS  Google Scholar 

  6. Sandquist, E. L. Blue stragglers in low-luminosity star clusters. Astrophys. J. 635, L73–L76 (2005)

    Article  ADS  CAS  Google Scholar 

  7. Hurley, J. R., Pols, O. R., Aarseth, S. J. & Tout, C. A. A complete N-body model of the old open cluster M67. Mon. Not. R. Astron. Soc. 363, 293–314 (2005)

    Article  ADS  CAS  Google Scholar 

  8. Sarajedini, A., von Hippel, T., Kozhurina-Platais, V. & Demarque, P. WIYN Open Cluster Study. II. UBVRI CCD photometry of the open cluster NGC 188. Astron. J. 118, 2894–2907 (1999)

    Article  ADS  Google Scholar 

  9. Perets, H. B. & Fabrycky, D. C. On the triple origin of blue stragglers. Astrophys. J. 697, 1048–1056 (2009)

    Article  ADS  Google Scholar 

  10. Mathieu, R. D. & Geller, A. M. A binary star fraction of 76 per cent and unusual orbit parameters for the blue stragglers of NGC 188. Nature 462, 1032–1035 (2009)

    Article  ADS  CAS  Google Scholar 

  11. Mathieu, R. D. in Stellar Clusters and Associations: Convection, Rotation, and Dynamos (eds Pallavicini, R., Micela, G. & Sciortino, S. ) 517–531 (Astron. Soc. Pacif. Conf. Ser. 198, ASP, 2000)

    Google Scholar 

  12. Geller, A. M., Mathieu, R. D., Harris, H. C. & McClure, R. D. WIYN Open Cluster Study. XXXVI. Spectroscopic binary orbits in NGC 188. Astron. J. 137, 3743–3760 (2009)

    Article  ADS  Google Scholar 

  13. Mazeh, T. & Goldberg, D. On the study of the mass ratio of spectroscopic binaries. Astrophys. J. 394, 592–598 (1992)

    Article  ADS  Google Scholar 

  14. McCrea, W. H. Extended main-sequence of some stellar clusters. Mon. Not. R. Astron. Soc. 128, 147–155 (1964)

    Article  ADS  Google Scholar 

  15. Hurley, J. R., Tout, C. A. & Pols, O. R. Evolution of binary stars and the effect of tides on binary populations. Mon. Not. R. Astron. Soc. 329, 897–928 (2002)

    Article  ADS  Google Scholar 

  16. Belczynski, K. et al. Compact object modeling with the StarTrack population synthesis code. Astrophys. J. Suppl. Ser. 174, 223–260 (2008)

    Article  ADS  CAS  Google Scholar 

  17. Chen, X. & Han, Z. Mass transfer from a giant star to a main-sequence companion and its contribution to long-orbital-period blue stragglers. Mon. Not. R. Astron. Soc. 387, 1416–1430 (2008)

    Article  ADS  CAS  Google Scholar 

  18. Hurley, J. R. & Shara, M. M. The promiscuous nature of stars in clusters. Astrophys. J. 570, 184–189 (2002)

    Article  ADS  Google Scholar 

  19. Soker, N. Eccentric orbits of close companions to asymptotic giant branch stars. Astron. Astrophys. 357, 557–560 (2000)

    ADS  Google Scholar 

  20. Bonačić Marinović, A. A., Glebbeek, E. & Pols, O. R. Orbital eccentricities of binary systems with a former AGB star. Astron. Astrophys. 480, 797–805 (2008)

    Article  ADS  Google Scholar 

  21. Sepinsky, J. F., Willems, B., Kalogera, V. & Rasio, F. A. Interacting binaries with eccentric orbits. II. Secular orbital evolution due to non-conservative mass transfer. Astrophys. J. 702, 1387–1392 (2009)

    Article  ADS  Google Scholar 

  22. Carney, B. W., Latham, D. W. & Laird, J. B. Metal-poor field blue stragglers: more evidence for mass transfer. Astron. J. 129, 466–479 (2005)

    Article  ADS  CAS  Google Scholar 

  23. Carney, B. W., Latham, D. W. & Laird, J. B. A survey of proper-motion stars. VIII - On the Galaxy’s third population. Astron. J. 97, 423–430 (1989)

    Article  ADS  Google Scholar 

  24. Ivanova, N. in Multiple Stars Across the H-R Diagram (eds Hubrig, S. Petr-Gotzens, M. & Tokovinin, A. ). 101–106 (2008)

    Book  Google Scholar 

  25. Tokovinin, A., Thomas, S., Sterzik, M. & Udry, S. Tertiary companions to close spectroscopic binaries. Astron. Astrophys. 450, 681–693 (2006)

    Article  ADS  Google Scholar 

  26. Tokovinin, A. A. MSC - a catalogue of physical multiple stars. Astron. Astrophys. Suppl. Ser. 124, 75–84 (1997)

    Article  ADS  Google Scholar 

  27. Robin, A. C., Reylé, C., Derrière, S. & Picaud, S. A synthetic view on structure and evolution of the Milky Way. Astron. Astrophys. 409, 523–540 (2003)

    Article  ADS  Google Scholar 

  28. Hurley, J. R., Pols, O. R. & Tout, C. A. Comprehensive analytic formulae for stellar evolution as a function of mass and metallicity. Mon. Not. R. Astron. Soc. 315, 543–569 (2000)

    Article  ADS  CAS  Google Scholar 

  29. Marigo, P. et al. Evolution of asymptotic giant branch stars. II. Optical to far-infrared isochrones with improved TP-AGB models. Astron. Astrophys. 482, 883–905 (2008)

    Article  ADS  CAS  Google Scholar 

  30. Kroupa, P. On the variation of the initial mass function. Mon. Not. R. Astron. Soc. 322, 231–246 (2001)

    Article  ADS  Google Scholar 

Download references


We thank the staff of the WIYN Observatory and the many graduate and undergraduate students who have assisted in observing NGC 188. Thanks to J. Hurley, A. Sills, N. Leigh, R. Taam and H. Perets for their comments and suggestions. Both A.M.G. and R.D.M. were visiting astronomers at Kitt Peak National Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy under cooperative agreement with the National Science Foundation. The WIYN Observatory is a joint facility of the University of Wisconsin–Madison, Indiana University, Yale University and the US National Optical Astronomy Observatories. This work was funded by the US National Science Foundation grant AST-0908082 to the University of Wisconsin–Madison, the Wisconsin Space Grant Consortium and the Lindheimer Fellowship at Northwestern University.

Author information

Authors and Affiliations



Both A.M.G. and R.D.M. obtained spectra of the NGC 188 blue stragglers using the WIYN telescope. A.M.G. analysed these data and constructed the N-body model of the cluster. A.M.G. and R.D.M. contributed equally to the writing of the manuscript.

Corresponding author

Correspondence to Aaron M. Geller.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains a Supplementary Discussion and Supplementary References. (PDF 262 kb)

PowerPoint slides

Rights and permissions

Reprints and permissions

About this article

Cite this article

Geller, A., Mathieu, R. A mass transfer origin for blue stragglers in NGC 188 as revealed by half-solar-mass companions. Nature 478, 356–359 (2011).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

This article is cited by


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing