In article ,
Odysseus writes:
I'm not sure about the Ring, but in many planetary nebulae, and
occasionally elsewhere, the glow we see is fluorescence:

The visible glow from gaseous nebulae, including ionized hydrogen
regions such as the Orion Nebula, is indeed from the gas, not
reflected starlight. The visible light is not continuum emission at
all wavelengths but rather is concentrated in emission lines at
discrete wavelengths. (There is some continuum emission, too, but
it's weak.) The basic physical processes were worked out in the
1930s. Fluorescence is part of the story (look up "Bowen
fluorescence mechanism"), but a bigger part is collisional excitation
of ions by electrons. The hydrogen and helium lines come from
recombination.

That's in visible light. Emission processes at other wavelengths
differ. In particular, the infrared has continuum emission from
dust. This is not reflected starlight either.

There are "reflection nebulae" and a few other cases where starlight
reflected by dust is important (including the Sun's "F corona"), but
overall such cases are pretty rare.

X-ray and UV emissions from very hot stars excite the surrounding
gas (otherwise cool and invisible) to re-emit down-spectrum in the
visible range.

Basically right, but UV is pretty much the whole story. There aren't
enough X-rays to do much excitation. Stellar temperatures in PN
central stars range from roughly 30000 K to upwards of 100000 K. In
H II regions, stellar temperatures can be somewhat lower, perhaps
down to 15000 K. The UV from the hot stars ionizes nearby gas, and
the various processes lead to emission from the gas.

Modulated starlight, if you will, rather than simply reflected,
and a little like our own aurorae.

I think the aurorae are also collisional excitation by electrons, but
the specific atoms or ions are not all the same as in ionized
nebulae.

Dust is harder to excite than gas.

Yes, in visible light. When dust absorbs visible or UV light, the
dust heats up, and warm dust radiates in the infrared. In
equilibrium, the energy in has to equal the energy out. The net
effect is converting visible or UV light to infrared, so it's the
same sort of outcome, but the physical process is different.

The interior of the vast SNR-bubble whose outer margins form the Veil
Nebula shows noticeably more stars than the surrounding area, apparently
because the region has been 'blown clear'.

In general, star counts can be used to measure extinction: more
stars, less extinction. Of course you have to be sure the true
number of background stars is constant, so the method only works over
limited areas.

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