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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.