  Fast stellar winds can sweep up ambient media and form bubbles. The evolution
of a bubble is largely controlled by the content and physical conditions of the
shocked fast wind in its interior. This hot gas was not clearly observed until
the recent advent of Chandra and XMM-Newton X-ray observatories. To date,
diffuse X-ray emission has been unambiguously detected from two circumstellar
bubbles blown by WR stars, four planetary nebulae, and two superbubbles blown
by young clusters. Model fits to the X-ray spectra show that the circumstellar
bubbles are dominated by hot gas with low temperatures (<= 3x10^6 K), while the
interstellar bubbles contain significant fractions of hotter gas (>= 5x10^6 K).
In all cases, large discrepancies in the X-ray luminosity are found between
observations and conventional models of bubbles. Future theoretical models of
bubbles need to re-examine the validity of heat conduction and take into
account realistic microscopic processes such as mass loading from dense
clumps/knots and turbulent mixing. Chandra ACIS-S observation of NGC 6888 will
shed light on these astrophysical processes.

Astrophysics