APOD: M27: The Dumbbell Nebula (2021 Jul 12)

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APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by APOD Robot » Mon Jul 12, 2021 4:05 am

Image M27: The Dumbbell Nebula

Explanation: What will become of our Sun? The first hint of our Sun's future was discovered inadvertently in 1764. At that time, Charles Messier was compiling a list of diffuse objects not to be confused with comets. The 27th object on Messier's list, now known as M27 or the Dumbbell Nebula, is a planetary nebula, one of the brightest planetary nebulae on the sky -- and visible toward the constellation of the Fox (Vulpecula) with binoculars. It takes light about 1000 years to reach us from M27, featured here in colors emitted by hydrogen and oxygen. We now know that in about 6 billion years, our Sun will shed its outer gases into a planetary nebula like M27, while its remaining center will become an X-ray hot white dwarf star. Understanding the physics and significance of M27 was well beyond 18th century science, though. Even today, many things remain mysterious about planetary nebulas, including how their intricate shapes are created.

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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by RocketRon » Mon Jul 12, 2021 5:55 am

Spectacular photo !

If we are all made of stardust, how many times is this likely to have happened to us before ?

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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by pettygrew » Mon Jul 12, 2021 12:36 pm

Although seeing it in color is cool, what does it look like in a regular photograph?

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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by Chris Peterson » Mon Jul 12, 2021 12:49 pm

pettygrew wrote:
Mon Jul 12, 2021 12:36 pm
Although seeing it in color is cool, what does it look like in a regular photograph?
Not much different. The light from this nebula is dominated by hydrogen and oxygen, so ordinary color images look very similar to H/O narrowband images.
_
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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by orin stepanek » Mon Jul 12, 2021 1:05 pm

M27_Falls_3557.jpg
Just beautiful; Don't want to experience it here on Earth while I am
still here! :mrgreen:

70cec30919aefe50ada3bd8e0e6239e6.jpg
These guys are about to figure it out!🐶 :?
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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by VictorBorun » Mon Jul 12, 2021 1:18 pm

Chris Peterson wrote:
Mon Jul 12, 2021 12:49 pm
pettygrew wrote:
Mon Jul 12, 2021 12:36 pm
Although seeing it in color is cool, what does it look like in a regular photograph?
Not much different. The light from this nebula is dominated by hydrogen and oxygen, so ordinary color images look very similar to H/O narrowband images.
An overlay rotated by 208.1°: and
Last edited by VictorBorun on Mon Jul 12, 2021 1:20 pm, edited 1 time in total.

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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by neufer » Mon Jul 12, 2021 1:20 pm

RocketRon wrote:
Mon Jul 12, 2021 5:55 am

If we are all made of stardust, how many times is this likely to have happened to us before ?
We are all made of ancient supernova stardust not recent planetary nebula stardust.
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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by stefanz » Mon Jul 12, 2021 1:44 pm

Here are two other pictures which show more H-alpha details. In the first one, H-alpha is mapped to orange and [OIII] is mapped to bluish cyan, in the second one H-alpha is mapped to green, [OIII] is mapped to red and [SII] is mapped to blue.

Image Image

Sa Ji Tario

Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by Sa Ji Tario » Mon Jul 12, 2021 2:57 pm

pettygrew wrote:
Mon Jul 12, 2021 12:36 pm
Although seeing it in color is cool, what does it look like in a regular photograph?
To the amateur telescope it would not be different from black and white

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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by Chris Peterson » Mon Jul 12, 2021 3:13 pm

Sa Ji Tario wrote:
Mon Jul 12, 2021 2:57 pm
pettygrew wrote:
Mon Jul 12, 2021 12:36 pm
Although seeing it in color is cool, what does it look like in a regular photograph?
To the amateur telescope it would not be different from black and white
No telescope can make an extended object brighter than it is to the naked eye, and only a handful of extended objects are bright enough to (barely) stimulate human color vision. Which is why virtually all deep sky objects are described as gray faint fuzzies. And it's why questions about the "true" colors of these objects are difficult to answer.
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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by ted01 » Mon Jul 12, 2021 3:42 pm

There appear to be several lines radiating from the white dwarf. What process created them?

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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by stefanz » Mon Jul 12, 2021 3:53 pm

Chris Peterson wrote:
Mon Jul 12, 2021 3:13 pm
No telescope can make an extended object brighter than it is to the naked eye, and only a handful of extended objects are bright enough to (barely) stimulate human color vision. Which is why virtually all deep sky objects are described as gray faint fuzzies. And it's why questions about the "true" colors of these objects are difficult to answer.
Not sure how you meant that.

It's the aim of a telescope to collect more light than than the unaided eye can collect (about factor 1000 with a 250mm telescope and assuming a pupil size of 8mm). But since a telescope also magnifies the area brightness is not increased by that factor. For such small objects (at high magnifications) the area brightness is even reduced compared to the unaided eye (by about factor 10 in the example above at magnification of 100)

Nevertheless, it's just a matter of telescope size in order to see colors. But since the human eye hardly detects H-alpha M27 would look almost completely blue ([OIII] and H-beta emissions). How it looks can be "simulated" with an ordinary and unmodified photo camera, because these cameras are optimized to reproduce what we see.

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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by Chris Peterson » Mon Jul 12, 2021 4:02 pm

stefanz wrote:
Mon Jul 12, 2021 3:53 pm
Chris Peterson wrote:
Mon Jul 12, 2021 3:13 pm
No telescope can make an extended object brighter than it is to the naked eye, and only a handful of extended objects are bright enough to (barely) stimulate human color vision. Which is why virtually all deep sky objects are described as gray faint fuzzies. And it's why questions about the "true" colors of these objects are difficult to answer.
Not sure how you meant that.

It's the aim of a telescope to collect more light than than the unaided eye can collect (about factor 1000 with a 250mm telescope and assuming a pupil size of 8mm). But since a telescope also magnifies the area brightness is not increased by that factor. For such small objects (at high magnifications) the area brightness is even reduced compared to the unaided eye (by about factor 10 in the example above at magnification of 100)

Nevertheless, it's just a matter of telescope size in order to see colors. But since the human eye hardly detects H-alpha M27 would look almost completely blue ([OIII] and H-beta emissions). How it looks can be "simulated" with an ordinary and unmodified photo camera, because these cameras are optimized to reproduce what we see.
I meant it just as I stated it. No telescope of any size can make an object brighter. If it allows you to see a hint of color,
it is only because it covers more retinal cells, which provides more signal. A matter of biology, not optics.
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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by neufer » Mon Jul 12, 2021 5:52 pm

ted01 wrote:
Mon Jul 12, 2021 3:42 pm

There appear to be several lines radiating from the white dwarf. What process created them?
https://en.wikipedia.org/wiki/Helix_Nebula#Knots wrote:
<<Like many nearby planetary nebulae, the Dumbbell contains knots. Its central region is marked by a pattern of dark and bright cusped knots and their associated dark tails. The knots vary in appearance from symmetric objects with tails to rather irregular tail-less objects. Similarly to the Helix Nebula and the Eskimo Nebula, the heads of the knots have bright cusps which are local photoionization fronts.

The knots are the probably result of Rayleigh-Taylor instability. The low density, high expansion velocity ionized inner nebula is accelerating the denser, slowly expanding, largely neutral material which had been shed earlier when the star was on the Asymptotic Giant Branch.

The Helix Nebula was the first planetary nebula discovered to contain cometary knots. Its main ring contains knots of nebulosity, which have now been detected in several nearby planetary nebulae, especially those with a molecular envelope like the Ring nebula and the Dumbbell Nebula. These knots are radially symmetric (from the CS) and are described as "cometary", each centered on a core of neutral molecular gas and containing bright local photoionization fronts or cusps towards the central star and tails away from it. All tails extend away from the Planetary Nebula Nucleus (PNN) in a radial direction. Excluding the tails, each knot is approximately the size of the Solar system, while each of the cusp knots are optically thick due to Lyc photons from the CS. There are about 40,000 cometary knots in the Helix Nebula. The excitation temperature varies across the Helix nebula. The rotational-vibrational temperature ranges from 1800 K in a cometary knot located in the inner region of the nebula are about 2.5'(arcmin) from the CS, and is calculated at about 900 K in the outer region at the distance of 5.6'.>>
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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by stefanz » Mon Jul 12, 2021 9:14 pm

Chris Peterson wrote:
Mon Jul 12, 2021 4:02 pm
I meant it just as I stated it. No telescope of any size can make an object brighter. If it allows you to see a hint of color,
it is only because it covers more retinal cells, which provides more signal. A matter of biology, not optics.
Of course, an object does not emit more photons if we increase the optics (which seems to be what you mean).

Whether we can see colors (that was the question) depends on whether we receive enough photons for the cone cells (responsible for color vision and less sensitive than the rod cells which are responsible for monochromatic vision at low light levels). How much photons we receive depends from the aperture of the optics. Otherwise astronomers could save a lot of money by purchasing the smallest telescopes they find on Ebay.

@neufer and @ted01,
These Cometary knot tails probably cannot be resolved with amateur telescopes (at least not in M27). The larger and not very sharp ray-like structures in H-alpha channel are probably just outpouring gas.

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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by Chris Peterson » Mon Jul 12, 2021 9:34 pm

stefanz wrote:
Mon Jul 12, 2021 9:14 pm
Chris Peterson wrote:
Mon Jul 12, 2021 4:02 pm
I meant it just as I stated it. No telescope of any size can make an object brighter. If it allows you to see a hint of color,
it is only because it covers more retinal cells, which provides more signal. A matter of biology, not optics.
Of course, an object does not emit more photons if we increase the optics (which seems to be what you mean).

Whether we can see colors (that was the question) depends on whether we receive enough photons for the cone cells (responsible for color vision and less sensitive than the rod cells which are responsible for monochromatic vision at low light levels). How much photons we receive depends from the aperture of the optics. Otherwise astronomers could save a lot of money by purchasing the smallest telescopes they find on Ebay.

@neufer and @ted01,
These Cometary knot tails probably cannot be resolved with amateur telescopes (at least not in M27). The larger and not very sharp ray-like structures in H-alpha channel are probably just outpouring gas.
The number of photons a rod or cone receives does not increase with aperture.
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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by neufer » Mon Jul 12, 2021 10:52 pm

Chris Peterson wrote:
Mon Jul 12, 2021 9:34 pm
stefanz wrote:
Mon Jul 12, 2021 9:14 pm
Chris Peterson wrote:
Mon Jul 12, 2021 4:02 pm

I meant it just as I stated it. No telescope of any size can make an object brighter. If it allows you to see a hint of color,
it is only because it covers more retinal cells, which provides more signal. A matter of biology, not optics.
Of course, an object does not emit more photons if we increase the optics (which seems to be what you mean).

Whether we can see colors (that was the question) depends on whether we receive enough photons for the cone cells (responsible for color vision and less sensitive than the rod cells which are responsible for monochromatic vision at low light levels). How much photons we receive depends from the aperture of the optics. Otherwise astronomers could save a lot of money by purchasing the smallest telescopes they find on Ebay.
The number of photons a rod or cone receives does not increase with aperture.
The number of photons one's own pupil receives does, in fact, increase with aperture.
(Duo-pupil binoculars doubles this.)

However, the greater the ratio between a telescope's primary objective diameter
to the diameter of one's own pupil the more those photons get spread out over ones retina such
that: "The number of photons any single rod or cone receives does not increase with aperture."
http://www.seasky.org/astronomy/astronomy-messier.html wrote:
<<Although many deep sky objects can display a wide array of colors in long exposure photographs, the human eye is not sensitive enough to see most of these colors through a small telescope. The[se] photos are presented in black and white to best represent the view that would be seen through a telescope.>>
https://astronomy.stackexchange.com/questions/28837/can-we-see-the-color-of-nebulae wrote:
Can we see the color of nebulae?
sanaris wrote:
<<YES and NO.

YES, you will get "color" of nebulae. Reason of this is that because of very low luminosity for small diameter telescope, your eye is using "grey vision". When luminosity will reach certain level, your eye will start adopt color receptors. Good luminosity levels begin from diameters above 200mm.

NO, that "color" which you may observe does NOT have anything in common with colors which you see everywhere. Because most nebulae emitting in near-infrared and ultra-blue regions, which when mixing, give "greenish" light. But this color is result of your brain mixing different wavelengths, not actual green wave. Some people see it green, others see "pink" etc...>>
adarsha joisa wrote:
<<I have been able to see color in the Orion nebula (M42) with my 16 inch dobsonian from a heavily light polluted city. This is only in the center ([apparent magnitude: 4.0] trapezium area), where I can see a reddish color. Interestingly, it's much more difficult to see this red color from a dark location. Several other people in my astronomy club have also noticed this, and one explanation that I got was that the cone cells are still active in a light polluted city, so I'm able to notice the red color ( I don't know whether it's true). From a good dark location, I've seen color on many planetary nebulae such as the Ghost of Jupiter. You usually see these are greenish in color, if you have a suitably large scope. With my 16 inch, I've seen color in a small number of bright objects (almost always limited to greens), but most objects don't show color. Having observed in instruments ranging from 2 inch to 18 inches, it certainly seems like a larger aperture allows you to see color in a larger number of objects. In my personal experience, I've only been able to notice color in the brighter objects in scopes 16 inches and above.>>
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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by heehaw » Mon Jul 12, 2021 10:58 pm

My experience is that Chris is always right. There is a big difference between point sources and extended sources (surface brightness). Surface brightness does NOT change with distance. Point sources fade in brightness as the square of the distance. Most astronomers know the latter, but NOT the former. Which annoys me big time!

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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by Chris Peterson » Mon Jul 12, 2021 11:18 pm

heehaw wrote:
Mon Jul 12, 2021 10:58 pm
My experience is that Chris is always right.
Thanks. But Art catches me on something occasionally!
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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by zeecatman » Mon Jul 12, 2021 11:24 pm

neufer wrote:
Mon Jul 12, 2021 10:52 pm
https://astronomy.stackexchange.com/questions/28837/can-we-see-the-color-of-nebulae wrote:
Can we see the color of nebulae?
sanaris wrote:
<<YES and NO.

YES, you will get "color" of nebulae. Reason of this is that because of very low luminosity for small diameter telescope, your eye is using "grey vision". When luminosity will reach certain level, your eye will start adopt color receptors. Good luminosity levels begin from diameters above 200mm.

NO, that "color" which you may observe does NOT have anything in common with colors which you see everywhere. Because most nebulae emitting in near-infrared and ultra-blue regions, which when mixing, give "greenish" light. But this color is result of your brain mixing different wavelengths, not actual green wave. Some people see it green, others see "pink" etc...>>
adarsha joisa wrote:
<<I have been able to see color in the Orion nebula (M42) with my 16 inch dobsonian from a heavily light polluted city. This is only in the center ([apparent magnitude: 4.0] trapezium area), where I can see a reddish color. Interestingly, it's much more difficult to see this red color from a dark location. Several other people in my astronomy club have also noticed this, and one explanation that I got was that the cone cells are still active in a light polluted city, so I'm able to notice the red color ( I don't know whether it's true). From a good dark location, I've seen color on many planetary nebulae such as the Ghost of Jupiter. You usually see these are greenish in color, if you have a suitably large scope. With my 16 inch, I've seen color in a small number of bright objects (almost always limited to greens), but most objects don't show color. Having observed in instruments ranging from 2 inch to 18 inches, it certainly seems like a larger aperture allows you to see color in a larger number of objects. In my personal experience, I've only been able to notice color in the brighter objects in scopes 16 inches and above.>>
This discussion is really illuminating to a hobbyist observer! To add my own anecdote to the mix, the vast majority of objects I've observed have been in "grayscale" to my eyes, but through my 8" Newtonian in a suburban area my eyes can pick out blue in the Orion nebula and a vague blueish-green color from the Ghost of Jupiter.

One thought I had while reading this -- when we physically view stars and see faint colors in them, how "true" are the colors we're seeing? I tend to think about the visual double Albireo, and how it's relatively easy to pick out the difference in color between the brighter orange star and dimmer blue star. As someone who sees less red when I stare at Mars through a scope than when I use my naked eyes, the way we process the colors of these distant objects has always been fascinating to me.

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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by Chris Peterson » Mon Jul 12, 2021 11:35 pm

zeecatman wrote:
Mon Jul 12, 2021 11:24 pm
neufer wrote:
Mon Jul 12, 2021 10:52 pm
This discussion is really illuminating to a hobbyist observer! To add my own anecdote to the mix, the vast majority of objects I've observed have been in "grayscale" to my eyes, but through my 8" Newtonian in a suburban area my eyes can pick out blue in the Orion nebula and a vague blueish-green color from the Ghost of Jupiter.

One thought I had while reading this -- when we physically view stars and see faint colors in them, how "true" are the colors we're seeing? I tend to think about the visual double Albireo, and how it's relatively easy to pick out the difference in color between the brighter orange star and dimmer blue star. As someone who sees less red when I stare at Mars through a scope than when I use my naked eyes, the way we process the colors of these distant objects has always been fascinating to me.
It can't be overemphasized that color is, fundamentally, a physiological property, not a physical one. So yeah, what "true color" actually means is a complex question with no simple (or single solution). Our visual system not only builds color perception from the various spectral components, but also from intensity. Which is why we see a pure wavelength (like H-alpha) presented at two different intensities as two completely different colors. And on top of the reality that different people are working with different biological capabilities, there is the issue of experience. Most people who have spent little time looking at stars don't even realize they show colors. To them, they all look white- probably because they expect them to be white. As we learn to observe astronomical objects, we also learn how to perceive colors that we had previously not noticed.

I am quite certain I have seen that blue-green color of OIII on two or three planetary nebulas. I've never seen red, and I'm pretty skeptical of claims that it can be seen. But maybe, a very experienced observer, with very good eyes, under just the right conditions... Suffice to say, your observation that virtually every deep sky object seen at an eyepiece is gray is accurate.
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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by stefanz » Tue Jul 13, 2021 12:37 am

heehaw wrote:
Mon Jul 12, 2021 10:58 pm
My experience is that Chris is always right. There is a big difference between point sources and extended sources (surface brightness). Surface brightness does NOT change with distance. Point sources fade in brightness as the square of the distance. Most astronomers know the latter, but NOT the former. Which annoys me big time!
I think we all agree that neither the intrinsic luminosity, nor the apparent magnitude nor the surface brightness depends on the optic we use for observation.

But the question was whether we can see colors. This depends on the amount of photons we receive. Chris stated more than once that this is independent from aperture. If that would be correct large telescopes would be waste of money.

Whether all light collected by the main mirror (or lens) receives the retina depends on the diameter of the light beam behind the eyepiece. That diameter is the aperture divided by magnification. That means, if the magnification becomes smaller than aperture divided by pupil size point sources become fainter and extended objects become smaller but observed brightness density (photons per area) isn't increased. Above that magnification limit point sources does not change and brightness density is reduced on extended objectets.

But that magnification limit does not mean that color vision is "A matter of biology, not optics", as Chris stated. (BTW, that limit explains why you wont find 50mm binoculars with a magnification of less than 7).

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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by Chris Peterson » Tue Jul 13, 2021 1:37 am

stefanz wrote:
Tue Jul 13, 2021 12:37 am
heehaw wrote:
Mon Jul 12, 2021 10:58 pm
My experience is that Chris is always right. There is a big difference between point sources and extended sources (surface brightness). Surface brightness does NOT change with distance. Point sources fade in brightness as the square of the distance. Most astronomers know the latter, but NOT the former. Which annoys me big time!
I think we all agree that neither the intrinsic luminosity, nor the apparent magnitude nor the surface brightness depends on the optic we use for observation.

But the question was whether we can see colors. This depends on the amount of photons we receive. Chris stated more than once that this is independent from aperture. If that would be correct large telescopes would be waste of money.

Whether all light collected by the main mirror (or lens) receives the retina depends on the diameter of the light beam behind the eyepiece. That diameter is the aperture divided by magnification. That means, if the magnification becomes smaller than aperture divided by pupil size point sources become fainter and extended objects become smaller but observed brightness density (photons per area) isn't increased. Above that magnification limit point sources does not change and brightness density is reduced on extended objectets.

But that magnification limit does not mean that color vision is "A matter of biology, not optics", as Chris stated. (BTW, that limit explains why you wont find 50mm binoculars with a magnification of less than 7).
Visually, the only value in a larger aperture is that you can operate at a higher magnification. Not that you collect more light.

The reason it is a matter of biology and not optics is that the individual rods and cones will never receive a greater flux of photons through a telescope than they will unassisted. So why do we possibly see a little color, and certainly better contrast in a large scope? Because the image covers more of the retina, and the nature of our biology is that if more cells are involved, the brain will do a better job reconstructing the image. This is quite different from how an electronic sensor works, where you can never do better than the capabilities of an individual pixel.
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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by zeecatman » Tue Jul 13, 2021 2:14 am

Chris Peterson wrote:
Mon Jul 12, 2021 11:35 pm
It can't be overemphasized that color is, fundamentally, a physiological property, not a physical one. So yeah, what "true color" actually means is a complex question with no simple (or single solution). Our visual system not only builds color perception from the various spectral components, but also from intensity. Which is why we see a pure wavelength (like H-alpha) presented at two different intensities as two completely different colors. And on top of the reality that different people are working with different biological capabilities, there is the issue of experience. Most people who have spent little time looking at stars don't even realize they show colors. To them, they all look white- probably because they expect them to be white. As we learn to observe astronomical objects, we also learn how to perceive colors that we had previously not noticed.

I am quite certain I have seen that blue-green color of OIII on two or three planetary nebulas. I've never seen red, and I'm pretty skeptical of claims that it can be seen. But maybe, a very experienced observer, with very good eyes, under just the right conditions... Suffice to say, your observation that virtually every deep sky object seen at an eyepiece is gray is accurate.
Thanks for your insight, Chris. It makes sense that the best answer is simply that there IS no answer... As much as I wish there was a way to arrive at some concept of "true" color. Still, I'm glad I'm not alone in seeing that blue-green! I've found it really exciting to see occasional splashes of color here and there in the deep sky through direct observation.

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Re: APOD: M27: The Dumbbell Nebula (2021 Jul 12)

Post by Ann » Tue Jul 13, 2021 6:21 am

Sanaris wrote:

I have been able to see color in the Orion nebula (M42) with my 16 inch dobsonian from a heavily light polluted city. This is only in the center (trapezium area), where I can see a reddish color. Interestingly, it's much more difficult to see this red color from a dark location. Several other people in my astronomy club have also noticed this, and one explanation that I got was that the cone cells are still active in a light polluted city, so I'm able to notice the red color ( I don't know whether it's true). From a good dark location, I've seen color on many planetary nebulae such as the Ghost of Jupiter. You usually see these are greenish in color, if you have a suitably large scope. With my 16 inch, I've seen color in a small number of bright objects (almost always limited to greens), but most objects don't show color. Having observed in instruments ranging from 2 inch to 18 inches, it certainly seems like a larger aperture allows you to see color in a larger number of objects. In my personal experience, I've only been able to notice color in the brighter objects in scopes 16 inches and above.

Disclaimer: I don't have any mathematical relation for aperture v/s color visibility; it appears to depend heavily on the observers' eye.
There is no reason to see any red color in the Orion Nebula. Its brightest part is at the Trapezium, whose dominant star, Theta 1 C Orionis, has a reddened B-V color index of +0.20. That does not correspond to a red color in any sense of the sword!

But it is reasonable that it should sometimes be possible to see a green or blue-green color in planetary nebulas. The surface brightness of planetary nebulas is often high, and the emission from OIII at around 500 nm falls comfortably into the range of greatest color sensitivity in the human eye. Also, the OIII emission in planetary nebulas is often quite "pure", not mixed with yellow and red wavelenghts, which is always the case when it comes to light from hot blue-white stars.

Ann
Color Commentator