  We present the result of a study on the expansion properties and internal
kinematics of round/elliptical planetary nebulae of the Milky Way disk, the
halo, and of the globular cluster M15. The purpose of this study is to
considerably enlarge the small sample of nebulae with precisely determined
expansion properties. To this aim, we selected a representative sample of
objects with different evolutionary stages and metallicities and conducted
high-resolution echelle spectroscopy. In most cases, we succeeded in detecting
the weak signals from the outer nebular shell which are attached to the main
line emission from the bright nebular rim. Next to the measurement of the
motion of the rim gas by decomposition of the main line components into
Gaussians, we were able to measure separately, for most objects for the first
time, the gas velocity immediately behind the leading shock of the shell, i.e.
the post-shock velocity. We more than doubled the number of objects for which
the velocities of both rim and shell are known and confirm that the overall
expansion of planetary nebulae is accelerating with time. There are, however,
differences between the expansion behaviour of the shell and the rim. This
observed distinct velocity evolution of both rim and shell is explained by
radiation-hydrodynamics simulations, at least qualitatively. Because of the
time-dependent boundary conditions, a planetary nebula will never evolve into a
simple self-similar expansion. Also the metal-poor objects behave as theory
predicts: The post-shock velocities are higher and the rim flow velocities are
equal or even lower compared to disk objects at similar evolutionary stage. We
detected, for the first time, in some objects an asymmetric expansion
behaviour: The relative expansions between rim and shell appear to be different
for the receding and approaching parts of the nebular envelope.

Solar and Stellar Astrophysics