  As star clusters are expected to form with low star formation efficiencies,
the gas in the cluster is expelled quickly and early in their development: the
star cluster "pops." This leads to an unbound stellar system, evolving in the
Galactic potential. Previous N-body simulations have demonstrated the existence
of a stepped number density distribution of cluster stars after popping, both
in vertical position and vertical velocity, with a passing resemblance to a
Christmas tree. Using numerical and analytical methods, we investigate the
source of this structure, which arises due to the phase mixing of the
out-of-equilibrium stellar system, determined entirely by the background
analytic potential. Considering only the vertical motions, we construct a
theoretical model to describe the time evolution of the phase space
distribution of stars in a Miyamoto-Nagai disk potential and a full Milky-Way
type potential comprising bulge, halo and disk components, which is then
compared with N-body simulations. Using our theoretical model, we investigate
the possible observational signatures and the feasibility of detection.
