Exotic populations in Globular Clusters: Blue Stragglers as tracers of
  the internal dynamical evolution of stellar systems
  In this paper I present an overview of the main observational properties of a
special class of exotic objects (the so-called Blue Straggler Stars, BSSs) in
Galactic Globular Clusters (GCs). The BSS specific frequency and their radial
distribution are 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 evolution
of stellar systems, thus allowing the definition of an empirical "clock" able
to measure the dynamical age of stellar aggregates from pure observational
properties.
