Radial trends in Galactic globular clusters and their possible origin
  The relaxation time at the half-mass radius of Galactic globular clusters
(GGCs) is typically within a few Gyr. Hence, the majority of GGCs are expected
to be well relaxed systems, given their age is around 12-13 Gyr. So any initial
radial segregation between stars of the same initial mass on the main sequence
(MS), in particular, the progenitors of the present day sub-giant and red-giant
branch (SGB, RGB) stars should already have dissipated. However, a body of
evidence contradicting to these expectations has been accumulated to date. The
paradox could be solved by taking into account the effect of stellar
collisions. They occur at particularly high rate in collapsing nuclei of GGCs
and seem to be mainly responsible for unrelaxed central regions and the radial
segregation observed. We draw attention that actually observed collisional blue
stragglers should be less numerous than their lower-mass counterparts formed
and accumulated at and below the present day MS turnoff. The effect of this is
that MS/SGB/RGB stars of a given luminosity are not of the same mass but fall
in a range of mass.
