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.

A binary origin for ‘blue stragglers’ in globular clusters


Blue stragglers in globular clusters are abnormally massive stars that should have evolved off the stellar main sequence long ago. There are two known processes that can create these objects: direct stellar collisions1 and binary evolution2. However, the relative importance of these processes has remained unclear. In particular, the total number of blue stragglers found in a given cluster does not seem to correlate with the predicted collision rate3,4, providing indirect support for the binary-evolution model. Yet the radial distributions of blue stragglers in many clusters are bimodal, with a dominant central peak5,6,7: this has been interpreted as an indication that collisions do dominate blue straggler production, at least in the high-density cluster cores7,8. Here we report that there is a clear, but sublinear, correlation between the number of blue stragglers found in a cluster core and the total stellar mass contained within it. From this we conclude that most blue stragglers, even those found in cluster cores, come from binary systems. The parent binaries, however, may themselves have been affected by dynamical encounters. This may be the key to reconciling all of the seemingly conflicting results found to date.

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

Access options

Rent or buy this article

Prices vary by article type



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

Figure 1: The observed number of core blue stragglers versus the number expected from single–single collisions.
Figure 2: The observed number of core blue stragglers versus the estimated core mass.
Figure 3: Results of fitting the generalized model to the observed blue straggler number.


  1. Hills, J. G. & Day, C. A. Stellar collisions in globular clusters. Astrophys. Lett. 17, 87–93 (1976)

    ADS  Google Scholar 

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

    Article  ADS  Google Scholar 

  3. Piotto, G. et al. Relative frequencies of blue stragglers in galactic globular clusters: Constraints for the formation mechanisms. Astrophys. J. 604, L109–L112 (2004)

    Article  ADS  CAS  Google Scholar 

  4. Leigh, N., Sills, A. & Knigge, C. Where the blue stragglers roam: Searching for a link between formation and environment. Astrophys. J. 661, 210–221 (2007); erratum 678, 564–566 (2008)

    Article  ADS  CAS  Google Scholar 

  5. Ferraro, F. R. et al. Blue stragglers in the Galactic globular clusters M3: Evidence for two populations. Astron. J. 106, 2324–2334 (1993)

    Article  ADS  Google Scholar 

  6. Ferraro, F. R. et al. Discovery of another peculiar radial distribution of blue stragglers in globular clusters: The case of 47 Tucanae. Astrophys. J. 603, 127–134 (2004)

    Article  ADS  Google Scholar 

  7. Mapelli, M. et al. The radial distribution of blue straggler stars and the nature of their progenitors. Mon. Not. R. Astron. Soc. 373, 361–368 (2006)

    Article  ADS  Google Scholar 

  8. Mapelli, M. et al. The contribution of primordial binaries to the blue straggler population in 47 Tucanae. Astrophys. J. 605, L29–L32 (2004)

    Article  ADS  Google Scholar 

  9. Benz, W. & Hills, J. G. Three-dimensional hydrodynamical simulations of stellar collisions. I - Equal-mass main-sequence stars. Astrophys. J. 323, 614–628 (1987)

    Article  ADS  Google Scholar 

  10. Leonard, P. J. T. & Livio, M. The rotational rates of blue stragglers produced by physical stellar collisions. Astrophys. J. 447, L121–L123 (1995)

    Article  ADS  Google Scholar 

  11. Sills, A. P., Bailyn, C. D. & Demarque, P. Are blue stragglers mixed during collisions? Astrophys. J. 455, L163–L165 (1995)

    ADS  Google Scholar 

  12. Sandquist, E. L., Bolte, M. & Hernquist, L. Composition mixing during blue straggler formation and evolution. Astrophys. J. 477, 335–345 (1997)

    Article  ADS  CAS  Google Scholar 

  13. Sills, A. & Bailyn, C. D. The distribution of collisionally induced blue stragglers in the color-magnitude diagram. Astrophys. J. 513, 428–441 (1999)

    Article  ADS  CAS  Google Scholar 

  14. Ferraro, F. R. et al. Discovery of carbon/oxygen-depleted blue straggler stars in 47 Tucanae: The chemical signature of a mass transfer formation process. Astrophys. J. 647, L53–L56 (2006)

    Article  ADS  CAS  Google Scholar 

  15. Piotto, G. et al. HST color-magnitude diagrams of 74 galactic globular clusters in the HST F439W and F555W bands. Astron. Astrophys. 391, 945–965 (2002)

    Article  ADS  Google Scholar 

  16. Sollima, A., Beccari, G., Ferraro, F. R., Fusi Pecci, F. & Sarajedini, A. The fraction of binary systems in the core of 13 low-density Galactic globular clusters. Mon. Not. R. Astron. Soc. 380, 781–791 (2007)

    Article  ADS  Google Scholar 

  17. Milone, A. P., Piotto, G., Bedin, L. R. & Sarajedini, A. in XXI Century Challenges for Stellar Evolution (eds Cassisi, S & Salaris, M.) 623–627 (Società Astronomica Italiana, 2008)

    Google Scholar 

  18. Sollima, A., Lanzoni, B., Beccari, G., Ferraro, F. R. & Fusi Pecci, F. The correlation between blue straggler and binary fractions in the core of Galactic globular clusters. Astron. Astrophys. 481, 701–704 (2008)

    Article  ADS  Google Scholar 

  19. Davies, M. B., Piotto, G. & de Angeli, F. Blue straggler production in globular clusters. Mon. Not. R. Astron. Soc. 349, 129–134 (2004)

    Article  ADS  Google Scholar 

  20. Knigge, C. et al. A blue straggler binary with three progenitors in the core of a globular cluster? Astrophys. J. 641, 281–287 (2006)

    Article  ADS  CAS  Google Scholar 

  21. Knigge, C. et al. Stellar exotica in 47 Tucanae. Astrophys. J. 683, 1006–1030 (2008)

    Article  ADS  CAS  Google Scholar 

  22. Gehrels, N. Confidence limits for small numbers of events in astrophysical data. Astrophys. J. 303, 336–346 (1986)

    Article  ADS  CAS  Google Scholar 

  23. Leonard, P. J. T. Stellar collisions in globular clusters and the blue straggler problem. Astron. J. 98, 217–226 (1989)

    Article  ADS  Google Scholar 

  24. Harris, W. E. A catalog of parameters for globular clusters in the Milky Way. Astron. J. 112, 1487–1488 (1996)

    Article  ADS  Google Scholar 

  25. McLaughlin, D. E. & van der Marel, R. P. Resolved massive star clusters in the Milky Way and its satellites: Brightness profiles and a catalog of fundamental parameters. Astrophys. J. Suppl. Ser. 161, 304–360 (2005)

    Article  ADS  CAS  Google Scholar 

  26. Pryor, C. & Meylan, G. in Structure and Dynamics of Globular Clusters (eds Djorgovski, S. G. & Meylan, G.) 357–371 (Astronomical Society of the Pacific, 1993)

    Google Scholar 

  27. Webbink, R. F. in Dynamics of Star Clusters (eds Goodman, J. & Hut, P.) 541–577 (Reidel, 1985)

    Google Scholar 

  28. Sills, A. et al. Evolution of stellar collision products in globular clusters. I. Head-on collisions. Astrophys. J. 487, 290–303 (1997)

    Article  ADS  Google Scholar 

  29. Sills, A., Faber, J. A., Lombardi, J. C., Rasio, F. A. & Warren, A. R. Evolution of stellar collision products in globular clusters. II. Off-axis collisions. Astrophys. J. 548, 323–334 (2001)

    Article  ADS  Google Scholar 

  30. Verbunt, F. W. M. Observational evidence for origin of stellar exotica in globular clusters. Highlights Astron. 14, 440–441 (2007)

    ADS  Google Scholar 

Download references


Research support for C.K. was provided by the UK Science and Technology Facilities Council. A.S. and N.L. are supported by the Natural Sciences and Engineering Research Council of Canada. C.K. would like to thank T. Maccarone for discussions.

Author information

Authors and Affiliations


Corresponding author

Correspondence to Christian Knigge.

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Cite this article

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

Download citation

  • Received:

  • Accepted:

  • 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