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

[Knight of Clear Skies]

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.

I've been following the thread with interest and I don't see any real disagreement between what you and Ann are saying.

[Chris Peterson]

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.

I've been following the thread with interest and I don't see any real disagreement between what you and Ann are saying.

Maybe not. I just feel like there's an artificial distinction being created between reflection nebulas and emission nebulas. It's not a binary divide. There are some nebulas, of course, which we really only see by their emission lines, and some that we only see by reflection. But I think the majority of nebulas (particularly dusty ones) have a combination of reflection and emission, which significantly alters the apparent colors of the latter.

[Ann]

Do we know if Alnitak is in front of or behind the nebula?

I should have paid more attention to this smart question.

Alnitak and the Flame Nebula. Credit: Team ARO.

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Yellow sunset on the Earth.

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We are familiar with the fact that the right combination of sunsets and clouds on the Earth can produce yellow skies and yellow clouds, when the Sun is low in the sky and behind the clouds. Perhaps something similar just might be going on with Alnitak and the Flame Nebula.

WARNING!!

[amateur speculation]Just maybe, Alnitak might be at the right angle and distance behind the Flame Nebula to shine a lot of its own visible light into this relatively compact cloud of gas and dust. Alnitak's violet and blue light can't penetrate the dust enough to make it out the other side, but maybe the yellow, orange and red light can. And yes, Alnitak produces a lot of yellow, orange and red light too - it just produces more blue and violet light.

I can easily imagine that all the light of Alnitak being scattered away inside the Flame Nebula, so that the Flame Nebula really ought to look "all dark". But first of all, it is indeed possible to imagine some of the red and yellow light penetrating all the way. And second, the Flame Nebula produces its own mostly blue light from inside - because it is forming new stars - and this inner light, too, only partly makes it through the dust of the Flame Nebula. Again, it would be the yellow and red light that would make it through.

If Alnitak is sufficiently close to the Flame Nebula, it really ought to ionize it, too. But perhaps the Flame Nebula really does show low to moderate amounts of hydrogen ionization. This might deepen the color of the Flame Nebula and make it redder.[/amateur speculation]

Ah, speculation, speculation, speculation!

Ann

[Ann]

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.

I've been following the thread with interest and I don't see any real disagreement between what you and Ann are saying.

Let me try to explain what my objection is.

APOD Robot wrote:

(...) 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.

What is being described here is the process of ionization. An ionized cloud of hydrogen, intermixed with some dust, will emit visible, primarily red light of ionized hydrogen in the Balmer series. The caption talks about the Flame Nebula as if it was a red emission nebula. But the Flame Nebula is a yellow nebula.

The hydrogen emission spectrum is the Balmer series. Image: ttsz / Getty Images

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Interstellar dust reddening. faculty.virginia.edu

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An ionized cloud of primarily hydrogen will emit primarily red light (from hydrogen alpha, at 656 nm), mixed with some blue and violet light, making the total light of ionized hydrogen reddish-pink or magenta.

But if the pink or magenta light from a hydrogen nebula passes through considerable amounts of dust, the blue and violet light will be filtered away, leaving only the red light from hydrogen alpha.

Look at the picture of the dust cloud at right. Mostly white stars shine behind it. The dust cloud gets thicker and thicker towards the middle, and as the white light of the stars behind tries to penetrate, more and more of the light is scattered away by the dust. First only the violet light disappears, making the starlight pale yellow. Then the blue light disappears, making the starlight yellow-orange. Then the green and yellow light disappears, making the starlight a deep shade of orange. Finally, even the red light is filtered away, and the center of the nebula is completely dark.

Here's my point. Dust makes starlight redder. Yellow is a redder shade than white. Orange is a redder shade than yellow. Red is a redder shade than orange.

The Antares and Rho Ophiuchi region. Photo: Adam Block.

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Let's look at Adam Block's picture of the Antares and Rho Ophiuchi region again and consider what the different colors mean.

There are two pink nebulas here. They are emission nebulas which shine because of ionized hydrogen. The one at right surrounds multiple star Sigma Scorpii, a binary B1 star, hot enough to ionize hydrogen and make it glow pink. The pink nebula at bottom right is ionized by B0-type star Tau Scorpii.

There is also the large yellow nebula. In my opinion, this nebula is primarily a reflection nebula, scattering the orange light of mighty red supergiant star Antares in a cloud of yellow. However, other factors may contribute to the appearance of the yellow nebula, such as dust reddening (certainly in the more orange parts of the nebula) and some mixing with scattered red hydrogen alpha light.

There are two blue nebulas here. The small one surrounds star 22 Scorpii, a star of spectral type B3. This star is not hot enough to ionize hydrogen (to any important extent), but it produces a lot of blue light, which is being "forward-scattered" by dust. The large blue nebula is the Rho Ophiuchi nebula, surrounding multiple star Rho Ophiuchi star of spectral class B2. This star is borderline hot enough to produce a (relativelhy small and faint) emission nebula, but we don't see one. Don't ask me why.

I believe that Rho Ophiuchi is the largest blue reflection nebula in the sky. One reason might be that so much of the blue light of Rho Ophiuci and its two companions has been scattered away from the stars themselves, making the stars themselves appear yellowish, according to one observer. I don't believe that. But if we compare Rho Ophiuchi with 22 Scorpii , we can see that 22 Scorpii has "retained" a lot more of its blue light (i.e., seems to emit much bluer light our way) than Rho Ophiuchi. Perhaps the conditions for scattering blue light in all directions are particularly favorable in the vicinity of Rho Ophiuchi, but not so much near 22 Scorpii.

Finally, we can see a lot of very dark dust in the Rho Ophiuchi region. Much of this dark dust is impenetrable to optical light. Stars behind it can't be seen at optical wavelengths.

To conclude:

My objections have been that the caption of today's APOD described the yellow Flame Nebula as an emission nebula, but a red emission nebula can't become yellow because of dust. I have never seen a single instance of this in all the thousands of astronomical images I have looked at.

Emission nebulosity may contribute to the color of the Flame Nebula. But the Flame Nebula can't be a red emission nebula whose color has become bluer because of dust.

I have tried to explain what I think may be the reason for the highly unusual color of the Flame Nebula. I believe light from Alnitak has been filtered though the Flame Nebula, allowing only some yellow and red light to make it through, and I think that the yellow color has been enhanced by embedded star formation in the Flame Nebula.

Ann

[Chris Peterson]

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.

I've been following the thread with interest and I don't see any real disagreement between what you and Ann are saying.

Let me try to explain what my objection is.

APOD Robot wrote:

(...) 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.

What is being described here is the process of ionization. An ionized cloud of hydrogen, intermixed with some dust, will emit visible, primarily red light of ionized hydrogen in the Balmer series. The caption talks about the Flame Nebula as if it was a red emission nebula. But the Flame Nebula is a yellow nebula.

The hydrogen emission spectrum is the Balmer series. Image: ttsz / Getty Images

---

Interstellar dust reddening. faculty.virginia.edu

---

An ionized cloud of primarily hydrogen will emit primarily red light (from hydrogen alpha, at 656 nm), mixed with some blue and violet light, making the total light of ionized hydrogen reddish-pink or magenta.

But if the pink or magenta light from a hydrogen nebula passes through considerable amounts of dust, the blue and violet light will be filtered away, leaving only the red light from hydrogen alpha.

Look at the picture of the dust cloud at right. Mostly white stars shine behind it. The dust cloud gets thicker and thicker towards the middle, and as the white light of the stars behind tries to penetrate, more and more of the light is scattered away by the dust. First only the violet light disappears, making the starlight pale yellow. Then the blue light disappears, making the starlight yellow-orange. Then the green and yellow light disappears, making the starlight a deep shade of orange. Finally, even the red light is filtered away, and the center of the nebula is completely dark.

Here's my point. Dust makes starlight redder. Yellow is a redder shade than white. Orange is a redder shade than yellow. Red is a redder shade than orange.

The Antares and Rho Ophiuchi region. Photo: Adam Block.

---

Let's look at Adam Block's picture of the Antares and Rho Ophiuchi region again and consider what the different colors mean.

There are two pink nebulas here. They are emission nebulas which shine because of ionized hydrogen. The one at right surrounds multiple star Sigma Scorpii, a binary B1 star, hot enough to ionize hydrogen and make it glow pink. The pink nebula at bottom right is ionized by B0-type star Tau Scorpii.

There is also the large yellow nebula. In my opinion, this nebula is primarily a reflection nebula, scattering the orange light of mighty red supergiant star Antares in a cloud of yellow. However, other factors may contribute to the appearance of the yellow nebula, such as dust reddening (certainly in the more orange parts of the nebula) and some mixing with scattered red hydrogen alpha light.

There are two blue nebulas here. The small one surrounds star 22 Scorpii, a star of spectral type B3. This star is not hot enough to ionize hydrogen (to any important extent), but it produces a lot of blue light, which is being "forward-scattered" by dust. The large blue nebula is the Rho Ophiuchi nebula, surrounding multiple star Rho Ophiuchi star of spectral class B2. This star is borderline hot enough to produce a (relativelhy small and faint) emission nebula, but we don't see one. Don't ask me why.

I believe that Rho Ophiuchi is the largest blue reflection nebula in the sky. One reason might be that so much of the blue light of Rho Ophiuci and its two companions has been scattered away from the stars themselves, making the stars themselves appear yellowish, according to one observer. I don't believe that. But if we compare Rho Ophiuchi with 22 Scorpii , we can see that 22 Scorpii has "retained" a lot more of its blue light (i.e., seems to emit much bluer light our way) than Rho Ophiuchi. Perhaps the conditions for scattering blue light in all directions are particularly favorable in the vicinity of Rho Ophiuchi, but not so much near 22 Scorpii.

Finally, we can see a lot of very dark dust in the Rho Ophiuchi region. Much of this dark dust is impenetrable to optical light. Stars behind it can't be seen at optical wavelengths.

To conclude:

My objections have been that the caption of today's APOD described the yellow Flame Nebula as an emission nebula, but a red emission nebula can't become yellow because of dust. I have never seen a single instance of this in all the thousands of astronomical images I have looked at.

Emission nebulosity may contribute to the color of the Flame Nebula. But the Flame Nebula can't be a red emission nebula whose color has become bluer because of dust.

I have tried to explain what I think may be the reason for the highly unusual color of the Flame Nebula. I believe light from Alnitak has been filtered though the Flame Nebula, allowing only some yellow and red light to make it through, and I think that the yellow color has been enhanced by embedded star formation in the Flame Nebula.

Ann

To be clear (again) my comments were not directed toward the Flame Nebula. Only towards explaining why it is so common to see yellow in nebulas, due to the combination of emitted and reflected light.

The Flame Nebula is certainly a hydrogen emission nebula, producing abundant red light. Were that not the case, it would not have been possible to include a channel imaged in narrowband Ha.

Most hydrogen emission nebulas emit very little light outside of the primary Ha line, so most are dominated by red, not the weaker lines that also require higher ionization energies.

[neufer]

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:

• https://apod.nasa.gov/apod/ap210322.html

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.

Alnitak and the Flame Nebula

Explanation: 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.

[johnnydeep]

Oh dear - I miss checking APOD for one day and come back to a voluminous and colorful discussion of the colors of nebulae!

[Chris Peterson]

It seems to me that we dealt with this sort of situation just a couple of weeks ago:

• https://apod.nasa.gov/apod/ap210322.html

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.

Alnitak and the Flame Nebula

Explanation: 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.

Which doesn't mean it's not true color, as such images are either constructed by adding the Ha to the red channel, or to all of the channels... in both cases leaving the colors relatively unaffected. It just adds detail that would otherwise be difficult to capture.

[Ann]

To be clear (again) my comments were not directed toward the Flame Nebula. Only towards explaining why it is so common to see yellow in nebulas, due to the combination of emitted and reflected light.

The Flame Nebula is certainly a hydrogen emission nebula, producing abundant red light. Were that not the case, it would not have been possible to include a channel imaged in narrowband Ha.

It is not common to see yellow in nebulas. It is particularly not common to see large nebulas that are "all yellow". Apart from the Antares Nebula, I can't think of a single example.

I concede the fact that the Flame Nebula is also an emission nebula. It makes sense that it should be, if it is so close to hot bright star Alnitak. Therefore, the Flame Nebula is really orange-colored rather than red.

What causes the "yellowing" of the red emission nebula has not been explained, but I assume it has to do with the embedded star cluster.

Ann

[Chris Peterson]

To be clear (again) my comments were not directed toward the Flame Nebula. Only towards explaining why it is so common to see yellow in nebulas, due to the combination of emitted and reflected light.

The Flame Nebula is certainly a hydrogen emission nebula, producing abundant red light. Were that not the case, it would not have been possible to include a channel imaged in narrowband Ha.

It is not common to see yellow in nebulas. It is particularly not common to see large nebulas that are "all yellow". Apart from the Antares Nebula, I can't think of a single example.

I concede the fact that the Flame Nebula is also an emission nebula. It makes sense that it should be, if it is so close to hot bright star Alnitak. Therefore, the Flame Nebula is really orange-colored rather than red.

What causes the "yellowing" of the red emission nebula has not been explained, but I assume it has to do with the embedded star cluster.

Ann

Your eyes are different from mine. I see few examples of dusty nebulas that don't show yellow. (And keep in mind that a great deal depends upon processing. The actual colors of objects we see imaged are almost always far less saturated than they are presented. You only have to look at the unrealistically saturated star colors in this image to see that. Nothing wrong with doing this, it's just pushing things to make the different colors more apparent to our eyes. But this nebula is hardly bright yellow. It is a warm white, which appears strongly yellow only when the saturation is pushed.)

[MartinNi]

Appeal to ...the APOD robot ?

Every now and then, images shown on APOD are flipped... but why ?
This time, even on the homepage of Team ARO the "enhanced image" of NGC 2024 is flipped (and rotated 90 degress), that's a pity.
But then, one could get the impression, that people responsible for publication of APOD images don't have an idea, how the imaged constellation, scenery, object etc. of nature really looks like !
Isn't an image showing an enhanced vision of reality ? What one could see with own eyes, if they were capable of beeing sensible enough ?

No problem for an image, if the orientation is not kind of "North is up". But a mirrored image is NOT acceptable at all.
And SHOULD not, if we really care on showing "what's going on somewhere", when making an image of whatever scene is appreciating.
PLEASE: take care, showing images NOT flipped.

Many thanks, and kind regards
Martin

[neufer]

Every now and then, images shown on APOD are flipped... but why ?
This time, even on the homepage of Team ARO the "enhanced image" of NGC 2024 is flipped (and rotated 90 degress), that's a pity.
But then, one could get the impression, that people responsible for publication of APOD images don't have an idea, how the imaged constellation, scenery, object etc. of nature really looks like !
Isn't an image showing an enhanced vision of reality ? What one could see with own eyes, if they were capable of beeing sensible enough ?

No problem for an image, if the orientation is not kind of "North is up". But a mirrored image is NOT acceptable at all.
And SHOULD not, if we really care on showing "what's going on somewhere", when making an image of whatever scene is appreciating.
PLEASE: take care, showing images NOT flipped.

Click to play embedded YouTube video.

It almost ought to be outlawed.

It's not fair.

[Ann]

I see few examples of dusty nebulas that don't show yellow. (And keep in mind that a great deal depends upon processing. The actual colors of objects we see imaged are almost always far less saturated than they are presented. You only have to look at the unrealistically saturated star colors in this image to see that. Nothing wrong with doing this, it's just pushing things to make the different colors more apparent to our eyes. But this nebula is hardly bright yellow. It is a warm white, which appears strongly yellow only when the saturation is pushed.)

David Malin, who took great pains determining the RGB hue of deep space objects (if not necessarily their degree of saturation), described the Flame Nebula as a yellow nebula and a mysterious oddity.

Ann

[Chris Peterson]

I see few examples of dusty nebulas that don't show yellow. (And keep in mind that a great deal depends upon processing. The actual colors of objects we see imaged are almost always far less saturated than they are presented. You only have to look at the unrealistically saturated star colors in this image to see that. Nothing wrong with doing this, it's just pushing things to make the different colors more apparent to our eyes. But this nebula is hardly bright yellow. It is a warm white, which appears strongly yellow only when the saturation is pushed.)

David Malin, who took great pains determining the RGB hue of deep space objects (if not necessarily their degree of saturation), described the Flame Nebula as a yellow nebula and a mysterious oddity.

Saturation is the main issue here, I think. That said, the only thing that seems to make this nebula unusual is the size and expanse of the color, not its presence.

But if it really is a mystery, there is no reason for it to be. You can't easily figure out what causes color from any RGB image. You need a targeted set of narrowband data, or better, spectroscopic data.

[Ann]

I see few examples of dusty nebulas that don't show yellow. (And keep in mind that a great deal depends upon processing. The actual colors of objects we see imaged are almost always far less saturated than they are presented. You only have to look at the unrealistically saturated star colors in this image to see that. Nothing wrong with doing this, it's just pushing things to make the different colors more apparent to our eyes. But this nebula is hardly bright yellow. It is a warm white, which appears strongly yellow only when the saturation is pushed.)

David Malin, who took great pains determining the RGB hue of deep space objects (if not necessarily their degree of saturation), described the Flame Nebula as a yellow nebula and a mysterious oddity.

Saturation is the main issue here, I think. That said, the only thing that seems to make this nebula unusual is the size and expanse of the color, not its presence.

But if it really is a mystery, there is no reason for it to be. You can't easily figure out what causes color from any RGB image. You need a targeted set of narrowband data, or better, spectroscopic data.

I think David Malin found the Flame Nebula odd because of the "size and expanse of" its yellow color. Now that is unusual in nebulas.

Thanks for the tip of the targeted set of narrowband data or spectroscopic data. No can do, however.

Ann

[neufer]

A selection of dégustation dishes and wines.

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David Malin, who took great pains determining the RGB hue of deep space objects (if not necessarily their degree of saturation), described the Flame Nebula as a yellow nebula and a mysterious oddity.

Saturation is the main issue here, I think. That said, the only thing that seems to make this nebula unusual is the size and expanse of the color, not its presence. But if it really is a mystery, there is no reason for it to be. You can't easily figure out what causes color from any RGB image. You need a targeted set of narrowband data, or better, spectroscopic data.

• Dégustation is the main issue here, I think.

[Chris Peterson]

David Malin, who took great pains determining the RGB hue of deep space objects (if not necessarily their degree of saturation), described the Flame Nebula as a yellow nebula and a mysterious oddity.

Saturation is the main issue here, I think. That said, the only thing that seems to make this nebula unusual is the size and expanse of the color, not its presence.

But if it really is a mystery, there is no reason for it to be. You can't easily figure out what causes color from any RGB image. You need a targeted set of narrowband data, or better, spectroscopic data.

I think David Malin found the Flame Nebula odd because of the "size and expanse of" its yellow color. Now that is unusual in nebulas.

Thanks for the tip of the targeted set of narrowband data or spectroscopic data. No can do, however.

Collecting such data is perfectly possible as an amateur project. But my thinking was that if this is really a mysterious object, the data ought to be out there somewhere already, probably collected professionally and published.

[PhilPaige]

Saturation is the main issue here, I think. That said, the only thing that seems to make this nebula unusual is the size and expanse of the color, not its presence.

But if it really is a mystery, there is no reason for it to be. You can't easily figure out what causes color from any RGB image. You need a targeted set of narrowband data, or better, spectroscopic data.

I think David Malin found the Flame Nebula odd because of the "size and expanse of" its yellow color. Now that is unusual in nebulas.

Thanks for the tip of the targeted set of narrowband data or spectroscopic data. No can do, however.

Collecting such data is perfectly possible as an amateur project. But my thinking was that if this is really a mysterious object, the data ought to be out there somewhere already, probably collected professionally and published.

In general, everything is logical, I would assume the same!

[Ann]

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.

You were right, Chris, and I was wrong.

Red emission nebulas dominated by hydrogen emit not only red hydrogen alpha at 656 nm, but also bluish hydrogen beta at 486 nm. The presence of dust in or in front of an emission nebula could definitely shift the balance between hydrogen alpha and hydrogen beta, dampening the hydrogen beta and taking away the "magenta aspect" of the nebula. The "de-magenta-ing" of the nebula could in itself make the nebula look more orange. And as you said, the presence of dust will make the ionizing star(s) look more yellow, which could certainly contribute to an orange appearance of the nebula.

Ann