APOD: Alnitak and the Flame Nebula (2021 Apr 12)

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APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by APOD Robot » Mon Apr 12, 2021 4:06 am

Image Alnitak and the Flame Nebula

Explanation: What lights up the Flame Nebula? Fifteen hundred light years away towards the constellation of Orion lies a nebula which, from its glow and dark dust lanes, appears, on the left, like a billowing fire. But fire, the rapid acquisition of oxygen, is not what makes this Flame glow. Rather the bright star Alnitak, the easternmost star in the Belt of Orion visible on the far left, shines energetic light into the Flame that knocks electrons away from the great clouds of hydrogen gas that reside there. Much of the glow results when the electrons and ionized hydrogen recombine. The featured picture of the Flame Nebula (NGC 2024) was taken across three visible color bands with detail added by a long duration exposure taken in light emitted only by hydrogen. The Flame Nebula is part of the Orion Molecular Cloud Complex, a star-forming region that includes the famous Horsehead Nebula.

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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by neufer » Mon Apr 12, 2021 4:40 am

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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by Ann » Mon Apr 12, 2021 6:36 am


Here's what I wonder about when it comes to the Flame Nebula. Brilliant hot blue star Alnitak is located, apparently, to one side of it. But the side of the Flames Nebula that is closest to Alnitak doesn't seem to be lit up.

Compare the appearance of the Flame Nebula with the appearance of the dust lanes that are closest to the ionizing O7.5III star HD 164492A (the Theta1 C Orionis of the Trifid cluster) inside the Trifid Nebula. The part of the dust lane that is closest to the hot star is lit up.


I'd say that the Flame Nebula is lit up from inside as well as from the outside. I don't question the fact that Alnitak is pumping energy into the Flame Nebula, although I do wonder where the yellow color comes from. Yellow emission nebulas, anyone? They are not common.

Anyway, energy is being produced from inside the Flame Nebula, too, as many new stars are being born there.
Wikipedia wrote:

At the center of the Flame Nebula is a cluster of newly formed stars, 86% of which have circumstellar disks. X-ray observations by the Chandra X-ray Observatory show several hundred young stars, out of an estimated population of 800 stars. X-ray and infrared images indicate that the youngest stars are concentrated near the center of the cluster.
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Last edited by Ann on Mon Apr 12, 2021 12:25 pm, edited 2 times in total.
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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by Knight of Clear Skies » Mon Apr 12, 2021 7:10 am

That's a striking rendition of the Flame Nebula, looks more like a painting. Unfortunately it's been flipped left-right.

Yes Ann, I've often wondered about the yellow colour of the Flame Nebula. I'm guessing dust has something to do with it?
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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by VictorBorun » Mon Apr 12, 2021 8:07 am

Knight of Clear Skies wrote:
Mon Apr 12, 2021 7:10 am
That's a striking rendition of the Flame Nebula, looks more like a painting. Unfortunately it's been flipped left-right.

Yes Ann, I've often wondered about the yellow colour of the Flame Nebula. I'm guessing dust has something to do with it?
Can the Flame Nebula reside in front of Alnitak? Backlight should take a warmer color after penetrating dust.

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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by VictorBorun » Mon Apr 12, 2021 8:35 am

Knight of Clear Skies wrote:
Mon Apr 12, 2021 7:10 am
That's a striking rendition of the Flame Nebula, looks more like a painting. Unfortunately it's been flipped left-right.
I had to rotate by 174,5°after the flip:

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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by Ann » Mon Apr 12, 2021 9:53 am

VictorBorun wrote:
Mon Apr 12, 2021 8:35 am
Knight of Clear Skies wrote:
Mon Apr 12, 2021 7:10 am
That's a striking rendition of the Flame Nebula, looks more like a painting. Unfortunately it's been flipped left-right.
I had to rotate by 174,5°after the flip:
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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by orin stepanek » Mon Apr 12, 2021 11:50 am

Flame_ARO_960.jpg

Flame > Very nice 8-)

longcat.jpg

Long duration? :-D

flame_optical.jpg

I like this view; besides it shows the horsehead! :wink:
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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by Ann » Mon Apr 12, 2021 12:58 pm

VictorBorun wrote:
Mon Apr 12, 2021 8:07 am
Knight of Clear Skies wrote:
Mon Apr 12, 2021 7:10 am
That's a striking rendition of the Flame Nebula, looks more like a painting. Unfortunately it's been flipped left-right.

Yes Ann, I've often wondered about the yellow colour of the Flame Nebula. I'm guessing dust has something to do with it?
Can the Flame Nebula reside in front of Alnitak? Backlight should take a warmer color after penetrating dust.
I don't see how dust could make an intrinsically red emission nebula yellower.


Emission nebulas shine in optical light mostly because of four emission lines of hydrogen. Of these, hydrogen alpha at 656.2 nm, is by far the brightest. 656.2 nm corresponds to a deep red color.

Yes, but an emission nebula will also emit light at a few other wavelengths, of which hydrogen beta is the most important. Hydrogen beta emits light at 486.1 nm, corresponding to a blue color.

When light from a hydrogen-rich emission nebulas passes through dust, the shorter wavelengths will be dimmed or lost, while the longest one will be affected the least.

Look at nebulas NGC 3603 and NGC 3576 in the picture at left. They look like they existed side by side as a "double nebula". But in fact NGC 3603 is located thousands of light-years behind NGC 3576 - probably some 14,000 light-years behind!

And look at the color of these two nebulas. NGC 3576 is visibly more "pink" or "magenta" than NGC 3603, which is more "pure red". That's because all the dust that is located between these two nebulas has robbed NGC 3603 of much of its blue and violet light, leaving almost only the red component.


So I don't think that dust can make an intrinsically red (or pink, or magenta) emission nebula yellow. However, an embedded region of star formation may well produce quite a lot of blue light, which may then be reddened to a fairly deep yellow hue because of lots of intervening dust.

I guess Alnitak does inject energy into the Flame Nebula, but I see no visible signs of it happening.

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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by VictorBorun » Mon Apr 12, 2021 2:02 pm

Ann wrote:
Mon Apr 12, 2021 12:58 pm
So I don't think that dust can make an intrinsically red (or pink, or magenta) emission nebula yellow. However, an embedded region of star formation may well produce quite a lot of blue light, which may then be reddened to a fairly deep yellow hue because of lots of intervening dust.
I guess Alnitak does inject energy into the Flame Nebula, but I see no visible signs of it happening.
Ann
I meant like can Alnitak be backlighting the Flame Nebula with full thermal spectrum, and can there be a tiny angle of scattering the ray Alnitak — the Flame Nebula — Earth, and can such scattering make the thermal spectrum "warmer", i.e. prefer longer waves over the shorter ones like the skyshine at small angles from the sun at horizon.
Last edited by VictorBorun on Mon Apr 12, 2021 7:01 pm, edited 1 time in total.

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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by Chris Peterson » Mon Apr 12, 2021 2:12 pm

Ann wrote:
Mon Apr 12, 2021 12:58 pm
VictorBorun wrote:
Mon Apr 12, 2021 8:07 am
Knight of Clear Skies wrote:
Mon Apr 12, 2021 7:10 am
That's a striking rendition of the Flame Nebula, looks more like a painting. Unfortunately it's been flipped left-right.

Yes Ann, I've often wondered about the yellow colour of the Flame Nebula. I'm guessing dust has something to do with it?
Can the Flame Nebula reside in front of Alnitak? Backlight should take a warmer color after penetrating dust.
I don't see how dust could make an intrinsically red emission nebula yellower.
Why not? Many nebulas have an apparent color that is the product of a combination of both emitted and reflected light. Shifting the balance towards the latter is very likely to shift the color towards yellow, both by washing out (desaturating) the emission lines, and by reflecting the intrinsic color of the star (possibly reddened or blued by the scattering angles involved).

I'm not suggesting that's necessarily what's going on with this nebula, but it's certainly something we see with others.
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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by Knight of Clear Skies » Mon Apr 12, 2021 3:31 pm

Do we know if Alnitak is in front of or behind the nebula?
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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by Uncle Jeff » Mon Apr 12, 2021 3:50 pm

Today's pic has a blue nebula in the upper-left corner that looks a little like a jellyfish. Is part of the same complex, or is it nearer/farther? Does it have a name?

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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by Ann » Mon Apr 12, 2021 4:33 pm

Chris Peterson wrote:
Mon Apr 12, 2021 2:12 pm
Ann wrote:
Mon Apr 12, 2021 12:58 pm
VictorBorun wrote:
Mon Apr 12, 2021 8:07 am


Can the Flame Nebula reside in front of Alnitak? Backlight should take a warmer color after penetrating dust.
I don't see how dust could make an intrinsically red emission nebula yellower.
Why not? Many nebulas have an apparent color that is the product of a combination of both emitted and reflected light. Shifting the balance towards the latter is very likely to shift the color towards yellow, both by washing out (desaturating) the emission lines, and by reflecting the intrinsic color of the star (possibly reddened or blued by the scattering angles involved).

I'm not suggesting that's necessarily what's going on with this nebula, but it's certainly something we see with others.

Chris, I know that you have said that the yellow Antares nebula is at least partly an emission nebula. I don't think that the yellow Antares nebula is as close to an O-type giant star as the Flame nebula is (apparently) close to supergiant star Alnitak. Also, I don't think star formation is going on inside the actual yellow nebula. Instead, star formation in this region is going on north of the yellow nebula, behind or adjacent to the very dark dust lanes that are visible there.

To me, the Flame Nebula is primarily a site of star formation that is lit up by a cluster of newborn stars inside. The Antares nebula is, to me, primarily a dust cloud immersing and reflecting the light of red supergiant star Antares. Perhaps hot blue stars in the vicinity are pumping energy into these two yellow nebulas, but I think that no or few visible signs of this can be seen.

When it comes to the Antares nebula, I think Antares is luminous enough on its own to light up a bright yellow reflection nebula.

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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by Chris Peterson » Mon Apr 12, 2021 5:02 pm

Ann wrote:
Mon Apr 12, 2021 4:33 pm
Chris Peterson wrote:
Mon Apr 12, 2021 2:12 pm
Ann wrote:
Mon Apr 12, 2021 12:58 pm


I don't see how dust could make an intrinsically red emission nebula yellower.
Why not? Many nebulas have an apparent color that is the product of a combination of both emitted and reflected light. Shifting the balance towards the latter is very likely to shift the color towards yellow, both by washing out (desaturating) the emission lines, and by reflecting the intrinsic color of the star (possibly reddened or blued by the scattering angles involved).

I'm not suggesting that's necessarily what's going on with this nebula, but it's certainly something we see with others.
Chris, I know that you have said that the yellow Antares nebula is at least partly an emission nebula. I don't think that the yellow Antares nebula is as close to an O-type giant star as the Flame nebula is (apparently) close to supergiant star Alnitak. Also, I don't think star formation is going on inside the actual yellow nebula. Instead, star formation in this region is going on north of the yellow nebula, behind or adjacent to the very dark dust lanes that are visible there.

To me, the Flame Nebula is primarily a site of star formation that is lit up by a cluster of newborn stars inside. The Antares nebula is, to me, primarily a dust cloud immersing and reflecting the light of red supergiant star Antares. Perhaps hot blue stars in the vicinity are pumping energy into these two yellow nebulas, but I think that no or few visible signs of this can be seen.

When it comes to the Antares nebula, I think Antares is luminous enough on its own to light up a bright yellow reflection nebula.
To be clear, I'm not assessing any particular nebula, just pointing out that there are reasonable conditions under which dust could make a hydrogen emission nebula shift in color towards the yellow.
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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by Ann » Mon Apr 12, 2021 6:06 pm

Uncle Jeff wrote:
Mon Apr 12, 2021 3:50 pm
Today's pic has a blue nebula in the upper-left corner that looks a little like a jellyfish. Is part of the same complex, or is it nearer/farther? Does it have a name?

That's reflection nebula IC 432. It looks particularly good in today's APOD. Above you can see it and its neighbor, IC 431, photographed by Rick J at Cloudy Nights.

I recommend that you read Rick J's interesting writeup about these nebulas and the star illuminating nebula IC 432, although I'd suggest you don't take Rick J's discussion of the Hipparcos parallaxes and thus distances to these nebulas and their stars too seriously. At the distances of these nebulas, the Hipparcos parallaxes are simply not reliable.

Instead, Gaia has measured the parallax of V901 Ori (the star in the nebula you asked about), and found its parallax to be, reliably, 2.2859 milliarcseconds, mas, with an uncertainty of plus/minus 0.0562 mas. I'll leave it to someone else to figure out how far away that puts V901 Ori.

As for Alnitak, it has been unreliably measured by Hipparcos, which found its parallax to be 4.43 plus/minus 0.64 mas, which, according to my software, Guide, puts Alnitak at a distance of 740 plus/minus 110 light-years. If the parallax of Alnitak is indeed 4.43 plus/minus 0.64 mas, then it is indeed located in front of IC 432 and V901 Ori. But you really can't trust Hipparcos when parallaxes get this small.

Rick J wrote that the red component seen behind the blue reflection nebulosity of IC 432 is a part of the red hydrogen emission of the Flame Nebula. Well, it could be, but I because the spectral class of V901 Ori is B2, I think this star might just be borderline hot enough to ionize its own emission nebula.

So in my opinion, the red component behind the blue swirls of reflection nebulosity of V901 Ori (whose swirliness might possibly be due to what Rick J called "magnetic braking of {the star's} rotation period"), just might be caused by the ultraviolet photons coming from V901 Ori itself.

Of course I could be wrong.

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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by Ann » Mon Apr 12, 2021 6:44 pm

Chris Peterson wrote:
Mon Apr 12, 2021 2:12 pm
Ann wrote:
Mon Apr 12, 2021 12:58 pm
VictorBorun wrote:
Mon Apr 12, 2021 8:07 am


Can the Flame Nebula reside in front of Alnitak? Backlight should take a warmer color after penetrating dust.
I don't see how dust could make an intrinsically red emission nebula yellower.
Why not? Many nebulas have an apparent color that is the product of a combination of both emitted and reflected light. Shifting the balance towards the latter is very likely to shift the color towards yellow, both by washing out (desaturating) the emission lines, and by reflecting the intrinsic color of the star (possibly reddened or blued by the scattering angles involved).

I'm not suggesting that's necessarily what's going on with this nebula, but it's certainly something we see with others.
Chris, this will be my last post to you on this particular topic in this particular thread. But as you know, I take an incredible interest in colors, particularly in stars and galaxies but also in nebulas. And I have never heard or read about a yellow-looking emission nebula.

I made a quick googling of "yellow emission nebula" and came up absolutely empty.

You said the shifting of the apparent color of an emission nebula from red towards yellow is "certainly something we see with" other nebulas. But I've never read or heard about it, and I've never seen a picture of a yellow-looking emission nebula.

You don't seem to be able to point me to a particular emission nebula whose apparent color has been shifted to the yellow. In other words, you want me to take your word for the possibility of dust-yellowing red emission nebulas, although you can't strengthen your case by pointing to examples.

Obviously I can't say that you are wrong, but I don't necessarily believe that you are right, either.

However, I will say this to everyone else out there:

Chris is our resident astronomer. And he knows math. I'm no astronomer, and I don't know math.

I'm just a complete color nerd.

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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by VictorBorun » Mon Apr 12, 2021 7:11 pm

so Alnitak has a large parallax and therefore is close to us and the Flame Nebula has a zero or small parallax and can never be backlighted from Alnitak.

Now the Flame Nebula can not be backlighted from the stars within at a small angle; we get to see all angles from 0 to 90°.

And how can it be yellow then? It must have:
this color like an ink of a specific chemistry then, getting yellow to amber to brown to black as it gets thicker.

Or can it be backlighted anyhow by some obscured bright stars a few ly behind the Flame Nebula to keep all the scattering angles small? This I doubt: there would be then some paler yellow parts at larger angles at the edges, but in fact we see no global pattern of a paler yellow edge at a large angler.

Or can a nebula shine yellow like a flame by dust particles in a hot media of >2700°C ? Not so bright as a bonfire, but paler, with fewer dust particles? If there is a UV stellar source to warm the media? This I doubt: there would be then some cooler parts glowing like red ashes, but all we see is slightly darker and oranger parts, nothing close to dark red ashes.

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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by Ann » Mon Apr 12, 2021 8:37 pm

VictorBorun wrote:
Mon Apr 12, 2021 7:11 pm
so Alnitak has a large parallax and therefore is close to us and the Flame Nebula has a zero or small parallax and can never be backlighted from Alnitak.

Now the Flame Nebula can not be backlighted from the stars within at a small angle; we get to see all angles from 0 to 90°.

And how can it be yellow then? It must have:
this color like an ink of a specific chemistry then
or
be backlighted anyhow by some obscured bright stars a few ly behind the Flame Nebula to keep all the scattering angles small

Or can a nebula shine yellow like a flame by dust particles in a hot media of >2700°C ? Not so bright as a bonfire, but paler, with fewer dust particles? If there is a UV stellar source to warm the media?
No, Alnitak has a small parallax. We just don't know how small it is, because Hipparcos, which measured its parallax, is not reliable when it comes to small parallaxes.

Give Hipparcos a nearby object with a large parallax, like red giant star Pollux, one of the bright stars of Gemini. Hipparcos measured its parallax to be 96.54 plus/minus 0.27 milliarcseconds. Note how small the margin of error is, compared with the parallax itself. That's why the distance estimate is so reliable. The Hipparcos parallax of Pollux puts Pollux at a distance of 33.785 plus/minus 0.094 light-years. That's reliable.

But when Hipparcos said that the parallax of Alnitak is 4.43 plus/minus 0.64 mas, and the distance to Alnitak therefore is 740 plus/minus 110 light-years, then that is far less reliable than it sounds. If you ask me, that Hipparcos Parallax is so unreliable that Alnitak could easily be either 500 or 1,000 light-years away. Probably not. But with Pollux, we can know that it's about 33 or 34 light-years away.

As for the Flame Nebula, I'm not sure that Gaia, reliable Gaia, can measure the parallax of its embedded infant stars at all, because they are probably only seen at infrared wavelengths. And I don't think it's at all possible for Gaia to measure the distances to extended sources like nebulas.

And I know that Gaia can't measure the parallax of Alnitak, because Alnitak is too bright for Gaia.

Can the Flame Nebula be "backlighted" by Alnitak? Yes, I guess that is possible. I don't know how we can find out, though.

Would a nebula shine yellow if it is at a temperature of >2700°C? Well, a star that shone at around 2700°C (or better yet, K) would look a deep dark red. Actually, though, it would more probable look a rather deep orange, and that in itself would look quite red compared with the pale unsaturated colors of other stars.

Emission nebulas, though, shine by emitting specific, narrow wavelenghts, and I don't think the temperature of the gas- and dust cloud itself typically determines the specific wavelengths being emitted.

But I could be wrong. Ask the math people!

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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by Chris Peterson » Mon Apr 12, 2021 11:04 pm

Ann wrote:
Mon Apr 12, 2021 6:44 pm
Chris Peterson wrote:
Mon Apr 12, 2021 2:12 pm
Ann wrote:
Mon Apr 12, 2021 12:58 pm


I don't see how dust could make an intrinsically red emission nebula yellower.
Why not? Many nebulas have an apparent color that is the product of a combination of both emitted and reflected light. Shifting the balance towards the latter is very likely to shift the color towards yellow, both by washing out (desaturating) the emission lines, and by reflecting the intrinsic color of the star (possibly reddened or blued by the scattering angles involved).

I'm not suggesting that's necessarily what's going on with this nebula, but it's certainly something we see with others.
Chris, this will be my last post to you on this particular topic in this particular thread. But as you know, I take an incredible interest in colors, particularly in stars and galaxies but also in nebulas. And I have never heard or read about a yellow-looking emission nebula.

I made a quick googling of "yellow emission nebula" and came up absolutely empty.
There are a huge number of nebulas that have both emission lines from ionized gas as well as reflections from surrounding stars. They show up on APOD all the time.
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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by neufer » Tue Apr 13, 2021 2:42 am

Chris Peterson wrote:
Mon Apr 12, 2021 2:12 pm
Ann wrote:
Mon Apr 12, 2021 12:58 pm
VictorBorun wrote:
Mon Apr 12, 2021 8:07 am


Can the Flame Nebula reside in front of Alnitak? Backlight should take a warmer color after penetrating dust.
I don't see how dust could make an intrinsically red emission nebula yellower.
Why not? Many nebulas have an apparent color that is the product of a combination of both emitted and reflected light. Shifting the balance towards the latter is very likely to shift the color towards yellow, both by washing out (desaturating) the emission lines, and by reflecting the intrinsic color of the star (possibly reddened or blued by the scattering angles involved).

I'm not suggesting that's necessarily what's going on with this nebula, but it's certainly something we see with others.
It seems to me that we dealt with this sort of situation just a couple of weeks ago:
This seems to be an artistic choice to tweak the Hubble
Hydrogen green to a nearby yellow with a natural reddish tinge:

http://asterisk.apod.com/viewtopic.php? ... 75#p311975

(It certainly seems like an apt selection for a Flame Nebula, at least.)
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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by Chris Peterson » Tue Apr 13, 2021 4:18 am

neufer wrote:
Tue Apr 13, 2021 2:42 am
Chris Peterson wrote:
Mon Apr 12, 2021 2:12 pm
Ann wrote:
Mon Apr 12, 2021 12:58 pm


I don't see how dust could make an intrinsically red emission nebula yellower.
Why not? Many nebulas have an apparent color that is the product of a combination of both emitted and reflected light. Shifting the balance towards the latter is very likely to shift the color towards yellow, both by washing out (desaturating) the emission lines, and by reflecting the intrinsic color of the star (possibly reddened or blued by the scattering angles involved).

I'm not suggesting that's necessarily what's going on with this nebula, but it's certainly something we see with others.
It seems to me that we dealt with this sort of situation just a couple of weeks ago:
This seems to be an artistic choice to tweak the Hubble
Hydrogen green to a nearby yellow with a natural reddish tinge:

http://asterisk.apod.com/viewtopic.php? ... 75#p311975

(It certainly seems like an apt selection for a Flame Nebula, at least.)
Well, sure. When you're using a false color palette, there's are lots of choices, both practical and aesthetic. But this is an image that is processed using a pretty normal set of RGB data, so presumably intended to give a true color approximation.
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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by Ann » Tue Apr 13, 2021 5:00 am

Chris Peterson wrote:
Mon Apr 12, 2021 11:04 pm
Ann wrote:
Mon Apr 12, 2021 6:44 pm
Chris Peterson wrote:
Mon Apr 12, 2021 2:12 pm

Why not? Many nebulas have an apparent color that is the product of a combination of both emitted and reflected light. Shifting the balance towards the latter is very likely to shift the color towards yellow, both by washing out (desaturating) the emission lines, and by reflecting the intrinsic color of the star (possibly reddened or blued by the scattering angles involved).

I'm not suggesting that's necessarily what's going on with this nebula, but it's certainly something we see with others.
Chris, this will be my last post to you on this particular topic in this particular thread. But as you know, I take an incredible interest in colors, particularly in stars and galaxies but also in nebulas. And I have never heard or read about a yellow-looking emission nebula.

I made a quick googling of "yellow emission nebula" and came up absolutely empty.
There are a huge number of nebulas that have both emission lines from ionized gas as well as reflections from surrounding stars. They show up on APOD all the time.
Chris, I'm breaking my promise to you. I apologize.

You said:
There are a huge number of nebulas that have both emission lines from ionized gas as well as reflections from surrounding stars. They show up on APOD all the time.
They do. But they are not yellow. Not in RGB photography.


The Flaming Star Nebula is a mixture of emission and reflection nebulosity. The Flaming Star Nebula is red from hydrogen emission and blue from dust reflection.

The Trapezium region in the Orion Nebula is a yellow. David Malin described it as such, and I trust him. He said that the yellow color of the Trapezium region is due to a mixture of red H-Alpha emission and green OIII emission. There is probably some reflection nebulosity involved as well, but it doesn't affect the color much.

ChristmasTree-ConeNebula-CumeadaObservatoryDSA-net1100[1].jpg
Fox Fur, Unicorn, and Christmas Tree.
Photo: Miguel Claro (TWAN, Dark Sky Alqueva).

The small reflection nebula vdB 101 (seen at upper center in the Blue Horsehead image) is yellow because it is a yellow reflection nebula, surrounding a K-type star. The yellow light of this star is being scattered by dust.

The NGC 2264 nebula, which contains the Cone Nebula, the Fox Fur Nebula, and the Christmas Tree Cluster, displays a few hints of yellow. The yellow flecks correspond to regions of embedded star formation.

In short: Yes, there are nebulas that are yellow. But the yellow color is never caused by the presence of dust causing "blue-ing" of a red emission nebula.

Now I have nothing to add.

Ann
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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by Knight of Clear Skies » Tue Apr 13, 2021 6:38 am

Ann wrote:
Mon Apr 12, 2021 12:58 pm

Look at nebulas NGC 3603 and NGC 3576 in the picture at left. They look like they existed side by side as a "double nebula". But in fact NGC 3603 is located thousands of light-years behind NGC 3576 - probably some 14,000 light-years behind!

And look at the color of these two nebulas. NGC 3576 is visibly more "pink" or "magenta" than NGC 3603, which is more "pure red". That's because all the dust that is located between these two nebulas has robbed NGC 3603 of much of its blue and violet light, leaving almost only the red component.
Thanks Ann, nicely illustrated.
Ann wrote: So I don't think that dust can make an intrinsically red (or pink, or magenta) emission nebula yellow. However, an embedded region of star formation may well produce quite a lot of blue light, which may then be reddened to a fairly deep yellow hue because of lots of intervening dust.
I was thinking along similar lines. Perhaps the Flame Nebula would show more blue reflection nebulosity from the right perspective, but it's attenuated by dust along our line of sight, between us and the nebula. Or maybe the density of dust in the nebula itself is just right to block most of the blue light, and it will become more blue as it evolves. Am I right in thinking the Flame is at an intermediate stage between the Horsehead (early star formation) and M42 (where most active star formation has ceased, and the gases and dust are being ejected into space)?
Caradon Observatory, Cornwall, UK.

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Re: APOD: Alnitak and the Flame Nebula (2021 Apr 12)

Post by Chris Peterson » Tue Apr 13, 2021 7:14 am

Ann wrote:
Tue Apr 13, 2021 5:00 am
Chris Peterson wrote:
Mon Apr 12, 2021 11:04 pm
Chris, I'm breaking my promise to you. I apologize.

You said:
There are a huge number of nebulas that have both emission lines from ionized gas as well as reflections from surrounding stars. They show up on APOD all the time.
In short: Yes, there are nebulas that are yellow. But the yellow color is never caused by the presence of dust causing "blue-ing" of a red emission nebula.
I'm not quite sure what that last thing is, but it doesn't sound much like what I'm describing. All I'm talking about is what happens in most nebulas that are illuminated by a combination of emission and reflection.
Chris

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