Mark De Smet wrote:
From my highly light polluted porch (chicago suburbs), the nebula
appears very clearly green. (I am not using any filters) None of the
pictures I have seen show it as green.

Is this the real color? (visible wavelengths) Or perhaps is it that my
eye is just more sensitive to green and not getting enough light to see
the other parts of the spectrum? Or possibly just a funny result of
light pollution?

Stellar nurseries like M42 are also called H II regions, a reference to the
fact that hydrogen is the most abundant element in these nebulae. However,
oxygen is the strongest light source. Ultraviolet radiation from hot, young
stars causes the surrounding gases to fluoresce and glow. Most of the light
comes from doubly-ionized oxygen, O III. O III emissions are strongest at about
501-nm, which falls within the green portion of the visible light spectrum.
This is close to the eye's peak sensitivity when full or paritally
dark-adapted. As a result, bright emission nebulae are often seen as green.

In color CCD and photographic images, reds and blues are also prominent. The
red is produced mostly by hydrogen (H alpha and H beta) and Nitrogen (N II).
The blue is mostly scattered star light. This is the reflection component of
the nebular glow. Just as our sky looks blue due to the scattering of blue
light (Rayleigh scattering), reflection nebulae look blue for the same reason.

Some visual observers report seeing reds and other colors in bright emission
nebulae. But yours is a common experience. I often see M42 as a deep emerald
green from my dark sky site.

Regards,

Bill Ferris
"Cosmic Voyage: The Online Resource for Amateur Astronomers"
URL: http://www.cosmic-voyage.net
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