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.

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

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

I've heard people say that as you age, colors change.

Growing up, I spent a lot of time on the orion nebula. I had a really nice
Criterion Dynascope 6". I remember the nebula being remarkably green,
almost bright green. Then I didn't do much viewing for 10 years and it has
virtually changed colors. It can only be one of 3 things:

1. The color has changed
2. My ability to see color has changed
3. There's something wrong with one of my scopes.

My guess the orion hasn't changed color much in the last 25 years. I'm
pretty confident it has in the last 15.
I know my eyesight has changed significantly during that time (and I stare
at computers all day long, I'm sure that doesn't help).
Scopes have only gotten better although I'd put that Dynascope up against
most in it's class.

My best guess is #2 is right.

Mike.


"Jon Isaacs" wrote in message
...
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

In article , Jon Isaacs wrote:
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

Possibly related: twilight colors of bright comets?

I remember several times, for Hyakutake, Hale-Bopp and
once for the binocular comet Lee in spring of '99,
that in twilight the coma would look distinctly blue-greenish.
But looking at the same comets in full darkness, I wouldn't
get that impression, neither naked-eye nor through binoculars
or a telescope. Wonder if the background twilight gives the same
effect you're pointing out here.

Stuart

Thanks for all the responses!

Mark