  Blue straggler stars are more massive than the average star in globular
clusters, as they originate from the merger of two stars. Consequently, they
experience dynamical friction, progressively sinking to the cluster center.
Recently, several indicators of the degree of dynamical relaxation of a
globular cluster have been proposed, based on the observed radial distribution
of blue straggler stars. The most successful is the Alessandrini indicator, or
A+ for short, which is the integral of the cumulative distribution of the blue
straggler stars minus that of a lighter reference population. A+ correlates
with the dynamical age of a cluster both in realistic simulations and in
observations. Here I calculate the temporal dependence of the A+ indicator
analytically in a simplified model of the evolution of the blue straggler star
distribution under dynamical friction only.
