Apparent color in the orion nebula?

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

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
=============
Email: Remove "ic" from .comic above to respond

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 *
**********************************************

"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.

From my highly light polluted porch (chicago suburbs), the nebula
appears very clearly green.

My experience as well. The other night it was particularly green.

Try this one one for size. Best to do this when the moon is near full because
it involves ruining your dark adaption.

A scope of at least 10 inch preferable:

Step #`1. Find Orion Nebula

Step #2. Stare at the moon with your observing eye through the scope or a
bright light for long enough to be sure that you dark adaptation is gone and
that you are relying on your color vision.

Step #3. Find the Orion Nebula with your other eye and then view it with your
"observing eye" (not dark adapted) eye with a low power eyepiece.

What I seem to see doing this is a smaller but more colorful Orion Nebula.

My hypothesis is that since night vision is essentially greyscale, by bleaching
out the receptors, that greyscale signal no longer overwhelms the color signal
from the cones and if the image is bright enough (low power, large exit pupil)
then one can see the color....

For what its worth...

jon isaacs

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 *
**********************************************

Thanks for all the responses!

Mark

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
Last month I spent two weeks vacation on Great Abaco island in the Bahamas.
Anticipating that I might get darker skies there than I normally experience
in New Jersey, I decided to take a telescope with me to take advantage
of the situation. I had just completed a 10" Dobsonian designed to be
taken apart and nested together compactly. The whole thing occupied less
space than a briefcase and easily fit in my luggage (fortunately I had no
problems with the safety inspections at the airports). Most of the evenings
were cloudy, but I did get three good nights with very dark skies. M42 was
the best I have ever seen it - it was almost as big in extent as in appears in
photographs. I had never before seen any color other than pale greenish hues
in the nebula, but from the dark skies of Great Abaco I got a distinct impression
of a pink color in the outlying parts of the nebula and a definite strong green
in the brighter area around the Trapezium.
Clif

"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.

From my highly light polluted porch (chicago suburbs), the nebula
appears very clearly green.

In my experience:
In an 8" SCT, the central glow of M42 appears grey.
In a 10" F5 Newtonian, the central glow appears grey, with a hint of very
pale green
In a 12.5" F4.8 Newtonian, the central glow appears grey, with a hint of
very pale aqua.
In a 16" F4.5 Newtonian, the central glow appears pale aqua and the area
along the edge appears ever so slightly tinted toward red (looks sorta
pink).

I don't think that it ever appears "green", although I once made that claim.
Supported by the results above, over time I've come to the conclusion that
with the increased brightness of increasing apertures the response of the
eye is most sensitive first in green, then blue and then red. I have no idea
if that tracks with what other science indicates about the response of the
human eye.

A second piece of empirical data would be that when surrounded by red light,
the eye has less trouble reverting back to its former state of dark
adaptation than it does under blue and green respectively (green being the
worst of the three, and second only to white).

Stephen Paul (as always, willing to be wrong)