  We examine mechanisms that may explain the luminosities and relatively low
temperatures of extended X-ray emission in planetary nebulae. By building a
simple flow structure for the wind from the central star during the proto, and
early, planetary nebulae phase, we estimate the temperature of the X-ray
emitting gas and its total X-ray luminosity. We conclude that in order to
account for the X-ray temperature and luminosity, both the evolution of the
wind from the central star and the adiabatic cooling of the post-shocked wind's
material must be considered. The X-ray emitting gas results mainly from shocked
wind segments that were expelled during the early planetary nebulae phase, when
the wind speed was moderate. Alternatively, the X-ray emitting gas may result
from a collimated fast wind blown by a companion to the central star. Heat
conduction and mixing between hot and cool regions are likely to occur in some
cases and may determine the detailed X-ray morphology of a nebula, but are not
required to explain the basic properties of the X-ray emitting gas.
