Blue Stragglers

At the Interface of Stellar Evolution and Stellar Dynamics: The classical fields of stellar evolution and stellar dynamics each have relied heavily on the stars of open clusters. Yet for most of their histories the fields have developed largely independently. Today, investigations at the interface of stellar evolution and stellar dynamics are expanding our understanding of both fields. Stellar dynamicists have long recognized that encounters involving binary stars supply the inevitable energy flow out of the cores of star clusters.  These complex dynamical dances lead to stellar exchanges, binary orbit evolution, close stellar passages, mass transfer, mergers, and stellar collisions, all of which lead to a rich array of new stellar evolution paths and products.

Open clusters are uniquely valuable for study of the interface of stellar dynamics and evolution due to their large primordial binary populations and their accessibility to observation. Detailed observational studies of blue stragglers, sub-subgiants, X-ray sources, and binary populations are defining the products of binary encounters and mapping their evolutionary paths. At the same time, a vigorous theoretical effort is exploring formation and evolution of the entire array of evolutionary products in open clusters.

We have underway an integrated program of observational and theoretical study of open clusters in order to define and understand the interface of stellar evolution and stellar dynamics.Specifically, our work seeks to i) comprehensively define the dynamical states of rich, evolved open clusters with ages from 0.1 to 7 Gyr, with particular attention to their binary populations, ii) characterize the populations of blue stragglers and other anomalous stars in the older clusters, focusing on their binary, rotation and activity distributions; iii) evolve N-body cluster simulations with realistic stellar and binary populations to ages of 7 Gyr, providing distributions of observed diagnostics as a function of age to compare with the observed clusters; and iv) model the rotational evolution of collision and mass transfer products in the cluster simulations to compare with the observed populations of anomalous stars.

Current student work:

  • N. Gosnell, XMM-Newton, “An XMM-Newton Survey of Rich Open Clusters” – Search for X-ray sources in 7 open cluster spanning a range of ages.
  • N. Gosnell, Hubble Space Telescope, 41 orbits, “The Nature of the Binary Companions to the Blue Stragglers in the Old Open Cluster NGC 188” – A search for UV emission from white dwarf companions to blue straggler binaries.


UW-Madison Astronomy Home