Blue Straggler Stars in Globular Clusters: a powerful tool to probe the   internal dynamical evolution of stellar systems
  This chapter presents an overview of the main observational results obtained to date about Blue Straggler Stars (BSSs) in Galactic Globular Clusters (GCs). The BSS specific frequency, radial distribution, chemical composition and rotational properties are presented and discussed in the framework of using this stellar population as probe of GC internal dynamics. In particular, the shape of the BSS radial distribution has been found to be a powerful tracer of the dynamical age of stellar systems, thus allowing the definition of the first empirical "dynamical clock". 
Francesco R. Ferraro, Barbara Lanzoni, Emanuele Dalessandro, Alessio   Mucciarelli and Loredana Lovisi
astro-ph.SR