Apparent color in the orion nebula?

It can appear greenish. Part of this is the human eye response. If enough
photons hit the eye to activate the color receptors, the eye is tuned to see
greens better than reds. Also, I've heard where many people perceive some
shades of grey as greenish, but this is hearsay.

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. Due to the way the eye responds to colors
when the lighting is bright enough, your eye will favor seeing greenish
shades. I've also seen this effect in a lot of diffuse nebula images with
my single-shot color camera, whose CCD has a spectral response that mimics
the eye's.

Through a 60-inch Cassegrain, I've seen shade of just about every color in
the Orion nebula except perhaps purple, but even there the colors were not
bold, but shades of colors, more like pastels.

--- Dave

"Mark De Smet" wrote in message
...

I recently got an orion XT10 IS, and although I have not had a chance to
get to a dark site, I did have a chance to take a peek at the orion
nebula from my porch. Having only seen it before in lower quality and
much smaller apperature scopes, it always appeared as a grey fuzzy.

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?

Mark

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.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
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"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.

David Nakamoto wrote:

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.

I didn't know the Oxygen lines were quite this strong either until I was doing
a little research for a talk I was doing on nebula filters and picked up "THE
COLOURS OF THE STARS" by David Malin and Paul Murdin. P. 97 had the relative
intensities of bright spectral lines and showed that the forbidden [OIII]
doublet was quite strong. In most planetary nebulae, they are even stronger,
with the 4959 Angstrom line often being three times the brightness of the
H-Beta line and the 5007 Angstrom line often being *eight* times as strong as
H-Beta. There are a few exceptions to this (NGC 40 and Campbell's Hydrogen
Star for a couple of examples), but by in large, the OIII lines really shine
out in emission nebulae. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************