  Using the Hubble Space Telescope and WFPC2 we have imaged the central 20pc of
the giant H II region 30 Doradus nebula in three different emission lines. The
images allow us to study the nebula with a physical resolution that is within a
factor of two of that typical of ground based observations of Galactic H II
regions. Most of the emission within 30 Dor is confined to a thin zone located
between the hot interior of the nebula and surrounding dense molecular
material. This zone appears to be directly analogous to the photoionized
photoevaporative flows that dominate emission from small, nearby H II regions.
The dynamical effects of the photoevaporative flow can be seen. The ram
pressure in the photoevaporative flow, derived from thermal pressure at the
surface of the ionization front, is found to balance with the pressure in the
interior of the nebula derived from previous x-ray observations. By analogy
with the comparison of ground and HST images of M16 we infer that the same
sharply stratified structure seen in HST images of M16 almost certainly
underlies the observed structure in 30 Dor. 30 Doradus is a crucial case
because it allows us to bridge the gap between nearby H II regions and the
giant H II regions seen in distant galaxies. The real significance of this
result is that it demonstrates that the physical understanding gained from
detailed study of photoevaporative interfaces in nearby H II regions can be
applied directly to interpretation of giant H II regions.
