An empirical clock to measure the dynamical age of stellar systems
  Blue Straggler Stars (BSS) are among the brightest and more massive stars in
globular clusters (GCs). For this reason they represent an ideal tool to probe
the dynamical evolution of these stellar systems. Here I show, following the
results by Ferraro et al. (2012), that the BSS radial distribution can be used
as a powerful indicator of the cluster dynamical age. In fact on the basis of
their BSS radial distribution shape, GCs can be efficiently grouped in
different families corresponding to the different dynamical stages reached by
the stellar systems. This allows to define a first empirical clock, the
dynamical clock, able to measure the dynamical age of a stellar system from
pure observational quantities.
