"David Knisely" wrote in message
.. .
David Nakamoto wrote:

Remember that if hydrogen is the predominant light emitter, and if it is
excited enough, you will get the three visible wavelength color lines
being
produced; red, green, and blue.

However, the stronger emission lines which help in making M42 look
bluish-green are probably the [OIII] pair at 4959 and 5007 Angstroms. The
one
at 5007 is about 3.4 times as strong as the H-Beta line, not to mention
being
closer to the visual peak sensitiviy of the human eye. The H-Beta line
contributes as well, but for the more greenish cast, the Oxygen lines tend
to
be quite important. Indeed, I greatly prefer the use of the OIII filter
on
M42 over the H-beta filter, as it shows a larger area of nebulosity (I
like
the UHC view the best however, as it takes in all three lines). Clear
skies
to you.

Didn't know Oxygen was so prevalent, spectrum-wise , in M42. I'd of thought
that this was more commonly found in planetary nebula. Live and learn.