I review recent studies of the emission-line regions in Orion and
M 17. Both have similar geometries, a bubble of hot shocked gas
surrounding the central star cluster, with H+, H0, and H2
regions, often referred to as H II regions, PDRs, and molecular
clouds, forming successive shells on the surface of a molecular
cloud. The magnetic fields in the H0 regions have been measured
with 21 cm Zeeman polarization and are found to be 1 – 2 dex
stronger than the field in the diffuse ISM. The regions appear to
be in rough hydrostatic equilibrium. The H+ region is pushed
away from the star cluster by starlight radiation pressure. Since
most starlight is in ionizing radiation, most of its outward push
will act on the H+ region and then on to the H0 region. The
magnetic pressure in the H0 region balances the momentum in
starlight and together they set the location of the H0 region.
The picture is that, when the star cluster formed, it created a
bubble of ionized gas which expanded and compressed surrounding
H0 and H2 regions. The magnetic field was amplified until its
pressure was able to support the momentum in starlight. This offers
a great simplification in understanding the underlying physics that
establishes parameters for PDR models.