APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

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APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by APOD Robot » Mon Jan 24, 2022 5:05 am

Image Rigel and the Witch Head Nebula

Explanation: By starlight this eerie visage shines in the dark, a crooked profile evoking its popular name, the Witch Head Nebula. In fact, this entrancing telescopic portrait gives the impression that the witch has fixed her gaze on Orion's bright supergiant star Rigel. More formally known as IC 2118, the Witch Head Nebula spans about 50 light-years and is composed of interstellar dust grains reflecting Rigel's starlight. The blue color of the Witch Head Nebula and of the dust surrounding Rigel is caused not only by Rigel's intense blue starlight but because the dust grains scatter blue light more efficiently than red. The same physical process causes Earth's daytime sky to appear blue, although the scatterers in Earth's atmosphere are molecules of nitrogen and oxygen. Rigel, the Witch Head Nebula, and gas and dust that surrounds them lie about 800 light-years away.

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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by Ann » Mon Jan 24, 2022 6:42 am

Witch Head Nebula outline APOD January 24 2022.png
The Witch Head Nebula. Sorry I made her smile!

I'm always glad to see a nice color image of (all or a part of) Orion. But as a Color Commentator (and a crazy lover of all things blue), I can't help noticing the the rather non-blue color of the Witch Head Nebula in the APOD. Why is the Witch Head so lacking of azure hues in this APOD, in view of the fact that the Witch Head Nebula is a reflection nebula lit up by the blue light of B-type supergiant Rigel?

For comparison, let's look at the possibly best ever picture of Orion, Stanislav Volskiy's 212-hour exposure, and then let's zoom in on his portrait of Rigel and the Witch Head Nebula:

Witch Head detail from 212 hour exposure of Orion.png
The Witch Head Nebula and Rigel from Stanislav Volskiy's image.

As you can see, the Witch Head is clearly bluer in Stanislav Volksiy's image than in José Mtanous'. What's the reason for the difference?

I'd say that the ratio between blue light and red Hα-light is different in Stanislav Volskiy's and José Mtanous' images. Mtanous has emphasized the red H-alpha emission by relying, I think, relatively heavily on an Hα filter. Volskiy, on the other hand, spent 212 hours to bring out, among other details in Orion, a blue "arc" of reflection nebulosity that seems to rise from Rigel and stretch almost all the way towards Orion's Belt.

I don't get the impression that there is a shortage of H-alpha in Stanislav Volskiy's image, because there is a lot of bright red light there. But his ultra-long exposure gave the blue light of Rigel the chance to shine and bounce back from dust particles in a way that we hardly ever see.

It took 212 hours for Stanislav Volskiy to bring out all that blue reflection nebulosity around Rigel, including the Witch Head Nebula. We can't expect many photographers to make such a Herculean effort. It is a lot easier to use an Hα filter to bring out extra amounts of the red light of ionized hydrogen in the vicinity of hot and not-quite-so-hot stars and reflection nebulas. And the red light is there, too, so José Mtanous is not lying to us.

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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by JohnD » Mon Jan 24, 2022 12:19 pm

Sorry to ride my hobby horse again, but this nebula looks neither 'creepy' nor like a witch. It looks like a jet, a truly enormous jet from some star early in its formation. Which star might that be?
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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by orin stepanek » Mon Jan 24, 2022 1:49 pm

RigelWitchHead_Mtanous_960.jpg
I can't say I have seen Rigel & the Witch Head photographed together
before! I'm sure they were; I just don't remember it! :oops:
Beautiful photo
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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by Chris Peterson » Mon Jan 24, 2022 1:52 pm

JohnD wrote: Mon Jan 24, 2022 12:19 pm Sorry to ride my hobby horse again, but this nebula looks neither 'creepy' nor like a witch.
Your pareidolia is broken. Maybe you should have that looked at! ;-)
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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by neufer » Mon Jan 24, 2022 2:21 pm

Chris Peterson wrote: Mon Jan 24, 2022 1:52 pm
JohnD wrote: Mon Jan 24, 2022 12:19 pm

Sorry to ride my hobby horse again, but this nebula looks neither 'creepy' nor like a witch.
Your pareidolia is broken. Maybe you should have that looked at! ;-)
https://en.wikipedia.org/wiki/Hobby_horse_(toy) wrote:
<<A hobby horse (or hobby-horse) is a child's toy horse. Children played at riding a wooden hobby horse made of a straight stick with a small horse's head (of wood or stuffed fabric), and perhaps reins, attached to one end. The bottom end of the stick sometimes had a small wheel or wheels attached. This toy was also sometimes known as a cock horse (as in the nursery rhyme Ride a cock horse to Banbury Cross) or stick horse.>>
Last edited by neufer on Mon Jan 24, 2022 4:03 pm, edited 1 time in total.
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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by neufer » Mon Jan 24, 2022 2:23 pm

Ann wrote: Mon Jan 24, 2022 6:42 am
As a Color Commentator (and a crazy lover of all things blue), I can't help noticing the the rather non-blue color of the Witch Head Nebula in the APOD. Why is the Witch Head so lacking of azure hues in this APOD, in view of the fact that the Witch Head Nebula is a reflection nebula lit up by the blue light of B-type supergiant Rigel?
https://en.wikipedia.org/wiki/Fairy_with_Turquoise_Hair wrote:
<<The Fairy with Turquoise Hair (often simply referred to as The Blue Fairy, La Fata Turchina) is a character in the 1883 Italian book The Adventures of Pinocchio by Carlo Collodi, repeatedly appearing at critical moments in Pinocchio's wanderings to admonish the little wooden puppet to avoid bad or risky behavior. Although the naïvely willful marionette initially resists her good advice, he later comes to follow her instruction.>>
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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by Chris Peterson » Mon Jan 24, 2022 2:39 pm

Ann wrote: Mon Jan 24, 2022 6:42 am But as a Color Commentator (and a crazy lover of all things blue), I can't help noticing the the rather non-blue color of the Witch Head Nebula in the APOD. Why is the Witch Head so lacking of azure hues in this APOD, in view of the fact that the Witch Head Nebula is a reflection nebula lit up by the blue light of B-type supergiant Rigel?
Dust grains are red to brown. The light that reaches us from dusty nebulas comes from two very different processes: reflection and scatter. The color of the reflected light is dominated by the absorption characteristics of the dust, modified slightly by the color of the illuminating source(s). In general, that color will be brown. The color of the scattered light is determined by the size and density of the dust particles, but will generally be blue.

Different nebulas will range widely in color based on the balance of these two processes. Most reflection nebulas are brown, because most are too dense for efficient scattering. It's common to see reflection nebulas as primarily brown where the dust is dense, and blue around the edges where it is not.
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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by Ann » Mon Jan 24, 2022 2:39 pm

neufer wrote: Mon Jan 24, 2022 2:21 pm
Ann wrote: Mon Jan 24, 2022 6:42 am
As a Color Commentator (and a crazy lover of all things blue), I can't help noticing the the rather non-blue color of the Witch Head Nebula in the APOD. Why is the Witch Head so lacking of azure hues in this APOD, in view of the fact that the Witch Head Nebula is a reflection nebula lit up by the blue light of B-type supergiant Rigel?
https://en.wikipedia.org/wiki/Fairy_with_Turquoise_Hair wrote:
<<The Fairy with Turquoise Hair (often simply referred to as The Blue Fairy, La Fata Turchina) is a character in the 1883 Italian book The Adventures of Pinocchio by Carlo Collodi, repeatedly appearing at critical moments in Pinocchio's wanderings to admonish the little wooden puppet to avoid bad or risky behavior. Although the naïvely willful marionette initially resists her good advice, he later comes to follow her instruction.>>
Hey, Art, did you have to rub it in twice? :wink:

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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by JohnD » Mon Jan 24, 2022 2:45 pm

Chris Peterson wrote: Mon Jan 24, 2022 1:52 pm
JohnD wrote: Mon Jan 24, 2022 12:19 pm Sorry to ride my hobby horse again, but this nebula looks neither 'creepy' nor like a witch.
Your pareidolia is broken. Maybe you should have that looked at! ;-)
That is most patronising, Chris, and unlike you!
Come on, astronomer, is this a stellar jet, and if so where from?

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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by Chris Peterson » Mon Jan 24, 2022 2:51 pm

JohnD wrote: Mon Jan 24, 2022 2:45 pm
Chris Peterson wrote: Mon Jan 24, 2022 1:52 pm
JohnD wrote: Mon Jan 24, 2022 12:19 pm Sorry to ride my hobby horse again, but this nebula looks neither 'creepy' nor like a witch.
Your pareidolia is broken. Maybe you should have that looked at! ;-)
That is most patronising, Chris, and unlike you!
Come on, astronomer, is this a stellar jet, and if so where from?
I see nothing here that makes me think it is a stellar jet of any kind. Just another dusty region shaped by star formation and stellar winds.
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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by Ann » Mon Jan 24, 2022 2:52 pm

Chris Peterson wrote: Mon Jan 24, 2022 2:39 pm
Ann wrote: Mon Jan 24, 2022 6:42 am But as a Color Commentator (and a crazy lover of all things blue), I can't help noticing the the rather non-blue color of the Witch Head Nebula in the APOD. Why is the Witch Head so lacking of azure hues in this APOD, in view of the fact that the Witch Head Nebula is a reflection nebula lit up by the blue light of B-type supergiant Rigel?
Dust grains are red to brown. The light that reaches us from dusty nebulas comes from two very different processes: reflection and scatter. The color of the reflected light is dominated by the absorption characteristics of the dust, modified slightly by the color of the illuminating source(s). In general, that color will be brown. The color of the scattered light is determined by the size and density of the dust particles, but will generally be blue.

Different nebulas will range widely in color based on the balance of these two processes. Most reflection nebulas are brown, because most are too dense for efficient scattering. It's common to see reflection nebulas as primarily brown where the dust is dense, and blue around the edges where it is not.
Thanks for explaining the difference between reflection and scattering, Chris.

I'm not arguing, but I do note that the large majority of Witch Head Nebula images available on the net make the Witch Head look bluer than it does in José Mtanous' image. That doesn't mean that I think that the Witch Head is a very blue nebula; rather I think that the Witch Head is a nebula with relatively muted and mixed colors, which however as a nebula is probably more blue than red. By that I mean that two equally long exposures, one through a broadband blue and the other through a broadband red filter, would most likely detect more blue than red light.


I consider the above picture of the Witch Head Nebula by Mario Cogo a good and balanced portrait of the "true optical" color of the Witch Head Nebula.

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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by Chris Peterson » Mon Jan 24, 2022 2:55 pm

Ann wrote: Mon Jan 24, 2022 2:52 pm
Chris Peterson wrote: Mon Jan 24, 2022 2:39 pm
Ann wrote: Mon Jan 24, 2022 6:42 am But as a Color Commentator (and a crazy lover of all things blue), I can't help noticing the the rather non-blue color of the Witch Head Nebula in the APOD. Why is the Witch Head so lacking of azure hues in this APOD, in view of the fact that the Witch Head Nebula is a reflection nebula lit up by the blue light of B-type supergiant Rigel?
Dust grains are red to brown. The light that reaches us from dusty nebulas comes from two very different processes: reflection and scatter. The color of the reflected light is dominated by the absorption characteristics of the dust, modified slightly by the color of the illuminating source(s). In general, that color will be brown. The color of the scattered light is determined by the size and density of the dust particles, but will generally be blue.

Different nebulas will range widely in color based on the balance of these two processes. Most reflection nebulas are brown, because most are too dense for efficient scattering. It's common to see reflection nebulas as primarily brown where the dust is dense, and blue around the edges where it is not.
Thanks for explaining the difference between reflection and scattering, Chris.

I'm not arguing, but I do note that the large majority of Witch Head Nebula images available on the net make the Witch Head look bluer than it does in José Mtanous' image. That doesn't mean that I think that the Witch Head is a very blue nebula; rather I think that the Witch Head is a nebula with relatively muted and mixed colors, which however as a nebula is probably more blue than red. By that I mean that two equally long exposures, one through a broadband blue and the other through a broadband red filter, would most likely detect more blue than red light.

As always, of course, processing is everything. There's really no such thing as "real" color in objects which are too dim for us to perceive color.
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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by JohnD » Mon Jan 24, 2022 3:02 pm

Chris Peterson wrote: Mon Jan 24, 2022 2:51 pm
JohnD wrote: Mon Jan 24, 2022 2:45 pm
Chris Peterson wrote: Mon Jan 24, 2022 1:52 pm

Your pareidolia is broken. Maybe you should have that looked at! ;-)
That is most patronising, Chris, and unlike you!
Come on, astronomer, is this a stellar jet, and if so where from?
I see nothing here that makes me think it is a stellar jet of any kind. Just another dusty region shaped by star formation and stellar winds.
No?
HH 46/47, MHO 2147 (featured as an APoD only three days ago), HH111, HH34 etc.etc etc. etc All Herbig Haro objects, but if this isn't an HH, what marks it out as different? Sure, it's shaped by stellar winds and adjacent stars, why not?

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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by Chris Peterson » Mon Jan 24, 2022 3:11 pm

JohnD wrote: Mon Jan 24, 2022 3:02 pm
Chris Peterson wrote: Mon Jan 24, 2022 2:51 pm
JohnD wrote: Mon Jan 24, 2022 2:45 pm

That is most patronising, Chris, and unlike you!
Come on, astronomer, is this a stellar jet, and if so where from?
I see nothing here that makes me think it is a stellar jet of any kind. Just another dusty region shaped by star formation and stellar winds.
No?
HH 46/47, MHO 2147 (featured as an APoD only three days ago), HH111, HH34 etc.etc etc. etc All Herbig Haro objects, but if this isn't an HH, what marks it out as different? Sure, it's shaped by stellar winds and adjacent stars, why not?
Sorry, I see no resemblance between those things and this dusty nebula. I see no evidence of a jet here.
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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by Ann » Mon Jan 24, 2022 5:23 pm

I want to return to the question of why the Witch Head Nebula looks so relatively red in today's APOD. My answer is that the photographer probably relied rather heavily on an Hα filter.


Let's look at Stanislav Volskiy's amazing portrait of Orion again. I haven't searched very diligently at all, but I haven't managed to find out what filters he used for his amazing image. So I'm going to have a guess: I think it was either LRGB filters (for luminosity, red, green and blue filters) or, more likely (I think) HαRGB filters.


RGB filters are typically broadband filters, which is to say that they detect a wide range of wavelengths. The image above shows you the sensitivity of a particular set of RGB filters: The B filter detects wavelengths from 350 to 550 nm, the G filter detects wavelengths from 440 to 640 nm, and the R filter detects wavelengths from 550 to 750 nm.

Blackbody curve for the Sun Betelgeuse and Rigel New Jersey Science and Technology University.png
Approximate blackbody curves for the Sun (at 6000 K), Betelgeuse
(at 3000 K) and Rigel (at 12,000 K). New Jersey Science and Technology University

Starlight is broadband light, because stars emit a huge number of wavelengths. In the picture at left you can see an illustration of all the wavelengths emitted by the Sun between 400 nm and 700 nm. In the picture at right, you can see the blackbody curves of stars of different temperatures, which show us where their emission peaks. The emission of a star like the Sun peaks at wavelengths a little shorter (or bluer) than 500 nm, a star like Betelgeuse peaks in the invisible infrared part of the spectrum (and Betelgeuse emits very little blue light), whereas the blackbody curve of a star like Rigel peaks in the invisible ultraviolet. Note that Rigel emits more visible blue than visible red light, but Rigel still emits a lot of red light, too.

In the picture at right, I have tried to show that the wavelengths scattered by dust from a star like Rigel is not just blue light, but green, violet and ultraviolet as well. (Yes, red and yellow light is also scattered by dust, but a lot less efficiently than shortwave light.)

My point is that starlight is broadband light, and reflection nebulas are also lit up by (scattered) broadband light. Therefore the best way of photographing the light from stars and reflection nebulas is, in my opinion, to use broadband filters.

Yes, but the light from emission nebulas is something else entirely! Because emission nebulas (except planetary nebulas) are typically extremely strongly dominated by a very narrow red wavelength, hydrogen alpha, at 656.281 nm. The spectrum of an emission nebula typically looks like this:


The hydrogen alpha wavelength is almost exactly the same as the NII emission wavelength, and these two (almost coincident) wavelengths typically dominate emission nebulas completely. The green OIII wavelength is sometimes strong, but, except in planetary nebulas, it is almost never dominant.

What does it mean that "a single red wavelength" typically dominates the light from emission nebulas so completely? Let's look at two versions of the Lagoon Nebula, one that shows hydrogen alpha light only, and one broadband RGB image where Hα has been used to enhance the luminosity and the red hues of the image:

Lagoon Nebula in Hα and RGB Ignacio Diaz Bobillo.png
The Lagoon Nebula in RGB and Hα. Image: Ignacio Diaz Bobillo

You can see that the Hα image is much "flatter" in hue and "all red". The RGB+Hα image, by contrast, shows various shades of pink, because the very red Hα is being diluted by green OIII, bluish cyan Hβ and scattered blue starlight. Note however the dull color of the "wing" to the left of the "bright Lagoon Nebula proper". The light here is probably all Hα. The RGB+Hα image is nicer-looking, if you ask me, but the "Hα only" image has captured almost as much nebular light as the RGB+Hα image.

The point I'm trying to make is that Hα is almost always very bright in an emission nebula. By using an Hα filter, an astrophotographer can bring out huge amounts of Hα and lots and lots of details in an emission nebula, because it doesn't take so long to get a good Hα exposure. A good broadband B filter exposure takes much longer.

Another point of mine is that it is sometimes "too easy" to bring out a lot of narrowband Hα at the expense of broadband filters. Then again, you can't lie with an Hα filter: Where there is no Hα, no Hα filter will detect any Hα.

That point has been dramatically demonstrated by Alistair Symon in his portraits of the Milky Way:

Milky Way from Cygnus to Scorpius Alistair Symon.png
The Milky Way from Cygnus to Scorpius. Image: Alistair Symon.

In the image above, Cygnus is at upper right and the tiny little Antares and Rho Ophiuchi nebula complex is at lower left. So why is the area around Cygnus so extremely red? It's because there is so much Hα light in Cygnus. Yes, but look at the long stretch of Milky Way to the left of Cygnus in this image - Sagitta, Vulpecula, Aquila, Scutum - which seems to be almost completely lacking in red. Why is there no red there? D'uh. Because there is so little Hα light there that is not hidden by dust in the Milky Way. An Hα filter there would not bring out what is not there.

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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by Ann » Mon Jan 24, 2022 5:43 pm

Chris Peterson wrote: Mon Jan 24, 2022 2:55 pm
Ann wrote: Mon Jan 24, 2022 2:52 pm
Chris Peterson wrote: Mon Jan 24, 2022 2:39 pm

Dust grains are red to brown. The light that reaches us from dusty nebulas comes from two very different processes: reflection and scatter. The color of the reflected light is dominated by the absorption characteristics of the dust, modified slightly by the color of the illuminating source(s). In general, that color will be brown. The color of the scattered light is determined by the size and density of the dust particles, but will generally be blue.

Different nebulas will range widely in color based on the balance of these two processes. Most reflection nebulas are brown, because most are too dense for efficient scattering. It's common to see reflection nebulas as primarily brown where the dust is dense, and blue around the edges where it is not.
Thanks for explaining the difference between reflection and scattering, Chris.

I'm not arguing, but I do note that the large majority of Witch Head Nebula images available on the net make the Witch Head look bluer than it does in José Mtanous' image. That doesn't mean that I think that the Witch Head is a very blue nebula; rather I think that the Witch Head is a nebula with relatively muted and mixed colors, which however as a nebula is probably more blue than red. By that I mean that two equally long exposures, one through a broadband blue and the other through a broadband red filter, would most likely detect more blue than red light.

As always, of course, processing is everything. There's really no such thing as "real" color in objects which are too dim for us to perceive color.
Are you saying that there is no way to determine the amount of red and blue light that is being scattered by a reflection nebula? (Assuming, of course, that we start out by agreeing what we mean by "red" and "blue" light.)

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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by Chris Peterson » Mon Jan 24, 2022 6:14 pm

Ann wrote: Mon Jan 24, 2022 5:23 pm Let's look at Stanislav Volskiy's amazing portrait of Orion again. I haven't searched very diligently at all, but I haven't managed to find out what filters he used for his amazing image. So I'm going to have a guess: I think it was either LRGB filters (for luminosity, red, green and blue filters) or, more likely (I think) HαRGB filters.
A properly processed HαRGB image shouldn't be any redder than a simple RGB image, it should just show clearer structure in the red areas of the image. One thing that could make for a redder image is shooting in LRGB without using an L filter. That's because an L filter is designed to cut out near IR, which silicon detectors are very sensitive to. If you shoot your luminance channel without any filters, you will end up with a lot more red signal, which, depending on how you process, can show up as a boost in the red channel (there are different ways of constructing the final RGB image when working with four input channels).
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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by JohnD » Mon Jan 24, 2022 6:38 pm

Chris Peterson wrote: Mon Jan 24, 2022 3:11 pm
JohnD wrote: Mon Jan 24, 2022 3:02 pm
Chris Peterson wrote: Mon Jan 24, 2022 2:51 pm

I see nothing here that makes me think it is a stellar jet of any kind. Just another dusty region shaped by star formation and stellar winds.
No?
HH 46/47, MHO 2147 (featured as an APoD only three days ago), HH111, HH34 etc.etc etc. etc All Herbig Haro objects, but if this isn't an HH, what marks it out as different? Sure, it's shaped by stellar winds and adjacent stars, why not?
Sorry, I see no resemblance between those things and this dusty nebula. I see no evidence of a jet here.
OK, Chris, yiou are the Astronomer. Please teach me! Why is this "dusty", and not a jet? It is certainly jet shaped.

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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by Chris Peterson » Mon Jan 24, 2022 6:49 pm

JohnD wrote: Mon Jan 24, 2022 6:38 pm
Chris Peterson wrote: Mon Jan 24, 2022 3:11 pm
JohnD wrote: Mon Jan 24, 2022 3:02 pm

No?
HH 46/47, MHO 2147 (featured as an APoD only three days ago), HH111, HH34 etc.etc etc. etc All Herbig Haro objects, but if this isn't an HH, what marks it out as different? Sure, it's shaped by stellar winds and adjacent stars, why not?
Sorry, I see no resemblance between those things and this dusty nebula. I see no evidence of a jet here.
OK, Chris, yiou are the Astronomer. Please teach me! Why is this "dusty", and not a jet? It is certainly jet shaped.
I disagree that it is "jet shaped". It looks like a hundred other dense molecular clouds. It does not have the tenuous look of a jet.
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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by Ann » Mon Jan 24, 2022 8:15 pm

JohnD wrote: Mon Jan 24, 2022 6:38 pm
Chris Peterson wrote: Mon Jan 24, 2022 3:11 pm
JohnD wrote: Mon Jan 24, 2022 3:02 pm

No?
HH 46/47, MHO 2147 (featured as an APoD only three days ago), HH111, HH34 etc.etc etc. etc All Herbig Haro objects, but if this isn't an HH, what marks it out as different? Sure, it's shaped by stellar winds and adjacent stars, why not?
Sorry, I see no resemblance between those things and this dusty nebula. I see no evidence of a jet here.
OK, Chris, yiou are the Astronomer. Please teach me! Why is this "dusty", and not a jet? It is certainly jet shaped.

John,

Jets are often bipolar and flare out at the ends. We see no sign of that in the Witch Head Nebula. Many jets are also quite straight and narrow, and we can often see, or guess, that they are powered by a central engine.


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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by johnnydeep » Mon Jan 24, 2022 9:04 pm

Gah! So much to read here today!
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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by Ann » Mon Jan 24, 2022 9:14 pm

Chris Peterson wrote: Mon Jan 24, 2022 2:55 pm
Ann wrote: Mon Jan 24, 2022 2:52 pm
Chris Peterson wrote: Mon Jan 24, 2022 2:39 pm

Dust grains are red to brown. The light that reaches us from dusty nebulas comes from two very different processes: reflection and scatter. The color of the reflected light is dominated by the absorption characteristics of the dust, modified slightly by the color of the illuminating source(s). In general, that color will be brown. The color of the scattered light is determined by the size and density of the dust particles, but will generally be blue.

Different nebulas will range widely in color based on the balance of these two processes. Most reflection nebulas are brown, because most are too dense for efficient scattering. It's common to see reflection nebulas as primarily brown where the dust is dense, and blue around the edges where it is not.
Thanks for explaining the difference between reflection and scattering, Chris.

I'm not arguing, but I do note that the large majority of Witch Head Nebula images available on the net make the Witch Head look bluer than it does in José Mtanous' image. That doesn't mean that I think that the Witch Head is a very blue nebula; rather I think that the Witch Head is a nebula with relatively muted and mixed colors, which however as a nebula is probably more blue than red. By that I mean that two equally long exposures, one through a broadband blue and the other through a broadband red filter, would most likely detect more blue than red light.

As always, of course, processing is everything. There's really no such thing as "real" color in objects which are too dim for us to perceive color.
Chris, I asked you before if you think that reflection nebulas do display real colors in such a way that they scatter more blue than red light our way, or more red than blue light. You haven't answered.

Well, my old catalog, Sky Catalogue 2000.0, Volume 2, answered me on page 297. Here it lists IC 2118 (which is another name for the Witch Head Nebula), and describes its color as "B", for blue. This catalog distinguishes between different levels of saturation for blue nebulas: all the Pleiades nebulas are listed as "VB", for very blue. vdB 24 is listed as "MB", for moderately blue. There are also nebulas that are listed as "I" for intermediate (between red and blue), or R, or VR. I couldn't immediately spot a nebula that was listed as MR, for moderately red.

Anyway, Chris. I'd say that the Witch Head Nebula is blue, although it seems all right to me to call it moderately blue.

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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by Chris Peterson » Mon Jan 24, 2022 9:21 pm

Ann wrote: Mon Jan 24, 2022 9:14 pm
Chris Peterson wrote: Mon Jan 24, 2022 2:55 pm
Ann wrote: Mon Jan 24, 2022 2:52 pm

Thanks for explaining the difference between reflection and scattering, Chris.

I'm not arguing, but I do note that the large majority of Witch Head Nebula images available on the net make the Witch Head look bluer than it does in José Mtanous' image. That doesn't mean that I think that the Witch Head is a very blue nebula; rather I think that the Witch Head is a nebula with relatively muted and mixed colors, which however as a nebula is probably more blue than red. By that I mean that two equally long exposures, one through a broadband blue and the other through a broadband red filter, would most likely detect more blue than red light.

As always, of course, processing is everything. There's really no such thing as "real" color in objects which are too dim for us to perceive color.
Chris, I asked you before if you think that reflection nebulas do display real colors in such a way that they scatter more blue than red light our way, or more red than blue light. You haven't answered.
I thought I did answer that above. The answer is neither. Or both, if you prefer. Scattering requires a low density dust cloud. A high density cloud is opaque, and there is almost no scattering. Such a cloud reflects light, and is therefore brown (unsaturated red). A low density cloud doesn't reflect much, but is an efficient scatterer. So it appears blue.

I don't see much blue in the Witch Head. Mostly I see reflected light, fairly unsaturated brown (like a warm gray).
Chris

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Re: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

Post by johnnydeep » Mon Jan 24, 2022 9:30 pm

Ann wrote: Mon Jan 24, 2022 8:15 pm
JohnD wrote: Mon Jan 24, 2022 6:38 pm
Chris Peterson wrote: Mon Jan 24, 2022 3:11 pm

Sorry, I see no resemblance between those things and this dusty nebula. I see no evidence of a jet here.
OK, Chris, yiou are the Astronomer. Please teach me! Why is this "dusty", and not a jet? It is certainly jet shaped.

John,

Jets are often bipolar and flare out at the ends. We see no sign of that in the Witch Head Nebula. Many jets are also quite straight and narrow, and we can often see, or guess, that they are powered by a central engine.


Ann
This jet (bottom image) from a young star bears a passing structural resemblance to the Witch Head Nebula:


The image is from https://esahubble.org/images/opo9524a/, and it suggests the jet is monodirectional:
Bottom image

This view of a three trillion mile-long jet called HH-47 reveals a very complicated jet pattern that indicates the star (hidden inside a dust cloud near the left edge of the image) might be wobbling, possibly caused by the gravitational pull of a companion star.
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