The "dynamical clock": dating the internal dynamical evolution of star
  clusters with Blue Straggler Stars
  We discuss the observational properties of a special class of objects (the
so-called "Blue Straggler Stars", BSSs) in the framework of using this stellar
population as probe of the dynamical processes occurring in high-density
stellar systems. Indeed, the shape of the BSS radial distribution and their
level of central concentration are powerful tracers of the stage of dynamical
evolution reached by the host cluster since formation. Hence, they can be used
as empirical chronometers able to measure the dynamical age of stellar systems.
In addition, the presence of a double BSS sequence in the color-magnitude
diagram is likely the signature of the most extreme dynamical process occurring
in globular cluster life: the core collapse event. Such a feature can therefore
be used to reveal the occurrence of this process and, for the first time, even
date it.
