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

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Expand view Topic review: APOD: Rigel and the Witch Head Nebula (2022 Jan 24)

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

by Chris Peterson » Tue Jan 25, 2022 5:17 pm

JohnD wrote: Tue Jan 25, 2022 5:13 pm
Chris Peterson wrote: Mon Jan 24, 2022 6:49 pm
JohnD wrote: Mon Jan 24, 2022 6:38 pm

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.
But, Chris......! Speechless! The image shown in the Apod itself looks like a jet, narrow base, expanding tip!
Every jet image I've seen features a tenuous stream that shows lots of shock structures. This appears to be a dense molecular cloud being eroded by stellar winds. Apples and oranges visually.

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

by JohnD » Tue Jan 25, 2022 5:13 pm

Chris Peterson wrote: 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

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.
But, Chris......! Speechless! The image of this nebula shown in the Apod itself looks like a jet, narrow base, expanding tip!

And I'm not alone in not being unable to understand your denial - Johnnydeep and VictorBourne have made similar comments to mine, in particular asking if this could be part of a HerbigHaro. Please, Chris, lead the tutorial, teach us, not just tell us we are wrong!

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

by Chris Peterson » Tue Jan 25, 2022 5:02 pm

VictorBorun wrote: Tue Jan 25, 2022 4:40 pm
neufer wrote: Tue Jan 25, 2022 3:38 pm On the other hand, these same jets also induce vortices whose angular momentum complicates any secondary material collapse into stars etc.
Hence, jets are not the sort of environment I would expect to be:
So, when a giant going blue dwarf sheds some matter in jets they won't become proto-star material until lose their spin by colliding with other bubbles or something.
If the Witch Head Nebula were a pair of jets 10 ky old then the spin would have discouraged any starbirth so far.

By the way, however blue and scattering, the Witch Head Nebula must also emit some Hα red to measure Doppler's shift and to directly (dis)prove the spinning.
Doppler measurements can be made from lots of other emission sources than hydrogen, and can be seen in reflection spectra, as well. Or in non-optical wavelengths.

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

by VictorBorun » Tue Jan 25, 2022 4:40 pm

neufer wrote: Tue Jan 25, 2022 3:38 pm On the other hand, these same jets also induce vortices whose angular momentum complicates any secondary material collapse into stars etc.
Hence, jets are not the sort of environment I would expect to be:
So, when a giant going blue dwarf sheds some matter in jets they won't become proto-star material until lose their spin by colliding with other bubbles or something.
If the Witch Head Nebula were a pair of jets 10 ky old then the spin would have discouraged any starbirth so far.

By the way, however blue and scattering, the Witch Head Nebula must also emit some Hα red to measure Doppler's shift and to directly (dis)prove the spinning.

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

by neufer » Tue Jan 25, 2022 3:38 pm

VictorBorun wrote: Tue Jan 25, 2022 4:58 am
neufer wrote: Tue Jan 25, 2022 4:16 am
VictorBorun wrote: Tue Jan 25, 2022 3:01 am why can't Witch Head Nebula be an aging HH object (10 ky old) in the thick of giant stars' bubble wall?
It is not the sort of environment I would expect:
https://en.wikipedia.org/wiki/IC_2118 wrote:
Candidates for pre-main sequence stars and some classic T-Tauri stars have been found deep within the nebula.
Do I get it right: a star is welcome to emit a pair of jets at 2 moments:
when it's a proto-star of 1 Sun mass using an accretion disk in the core of proto-planets dusty disk to make make a pair of jets of 30 Earth mass
when it's a SuperNova of 100 Suns mass using a shedded disk to make a pair of jets of 3 Suns mass

Now the Witch Head Nebula is rather Super-Nova scale, is not it?
Regardless of scale, jets provide a valuable service in dispersing angular momentum in order to allow for material collapse into: stars, supernova, black holes, etc..

On the other hand, these same jets also induce vortices whose angular momentum complicates any secondary material collapse into stars etc.

Hence, jets are not the sort of environment I would expect to be:
https://en.wikipedia.org/wiki/IC_2118 wrote:
Candidates for pre-main sequence stars and some classic T-Tauri stars that have been found deep within the Witch Head Nebula.

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

by Chris Peterson » Tue Jan 25, 2022 2:22 pm

Ann wrote: Tue Jan 25, 2022 2:12 pm
Chris Peterson wrote: Tue Jan 25, 2022 1:39 pm
Ann wrote: Tue Jan 25, 2022 5:01 am

That's my point, Chris. There is less blue than usual in the Witch Head Nebula in today's APOD. Usually there is more blue there in pictures - because there is more blue there.

Ann
I don't see much blue in other images, either.
You may check out several photos of the Witch Head Nebula here. I find all the images relatively blue, except the fifth image, which is dominated by a very red background. The red background of that image resembles the background of the APOD, and in my opinion, this emphasis on red hydrogen alpha background light (which is not strong near Rigel) alters the color balance of the entire image and risks making the Witch Head Nebula look almost completely non-blue.


The image above is a favorite of mine. We can clearly see that only some parts of the Witch Head Nebula scatters blue light, while other parts appear gray or brown. Nevertheless, to me this nebula, when it looks the way it looks in the picture above, is clearly more blue than red.

If I know you, you remain perfectly unmoved by my arguments! That's okay, because I'm not really trying to convince you, and instead I'm arguing for the benefit of other members of (and visitors of) Starship Asterisk*.

Bear in mind that I have never said that the Witch Head Nebula is very blue. Indeed, and as you can see from the picture I posted, it is not even all blue in the first place. I'd describe the Witch Head Nebula as moderately blue, but in my opinion (and in the opinion of more or less everyone who writes about it on the net) the Witch Head Nebula is indeed more blue than red. However, if a photographer emphasizes the red background of this general area, and brings out red light more than blue light, then there is an increasing likelihood that the Witch Head Nebula is hardly going to look blue at all.

And now I've got nothing more to add!

Ann
I disagree. I would not call that a blue nebula. It is clearly dominated by brown reflection, with just a little bit of blue scatter from the outer shell.

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

by Ann » Tue Jan 25, 2022 2:12 pm

Chris Peterson wrote: Tue Jan 25, 2022 1:39 pm
Ann wrote: Tue Jan 25, 2022 5:01 am
Chris Peterson wrote: Mon Jan 24, 2022 9:21 pm
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).
That's my point, Chris. There is less blue than usual in the Witch Head Nebula in today's APOD. Usually there is more blue there in pictures - because there is more blue there.

Ann
I don't see much blue in other images, either.
You may check out several photos of the Witch Head Nebula here. I find all the images relatively blue, except the fifth image, which is dominated by a very red background. The red background of that image resembles the background of the APOD, and in my opinion, this emphasis on red hydrogen alpha background light (which is not strong near Rigel) alters the color balance of the entire image and risks making the Witch Head Nebula look almost completely non-blue.


The image above is a favorite of mine. We can clearly see that only some parts of the Witch Head Nebula scatters blue light, while other parts appear gray or brown. Nevertheless, to me this nebula, when it looks the way it looks in the picture above, is clearly more blue than red.

If I know you, you remain perfectly unmoved by my arguments! That's okay, because I'm not really trying to convince you, and instead I'm arguing for the benefit of other members of (and visitors of) Starship Asterisk*.

Bear in mind that I have never said that the Witch Head Nebula is very blue. Indeed, and as you can see from the picture I posted, it is not even all blue in the first place. I'd describe the Witch Head Nebula as moderately blue, but in my opinion (and in the opinion of more or less everyone who writes about it on the net) the Witch Head Nebula is indeed more blue than red. However, if a photographer emphasizes the red background of this general area, and brings out red light more than blue light, then there is an increasing likelihood that the Witch Head Nebula is hardly going to look blue at all.

And now I've got nothing more to add!

Ann

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

by Chris Peterson » Tue Jan 25, 2022 1:39 pm

Ann wrote: Tue Jan 25, 2022 5:01 am
Chris Peterson wrote: Mon Jan 24, 2022 9:21 pm
Ann wrote: Mon Jan 24, 2022 9:14 pm

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).
That's my point, Chris. There is less blue than usual in the Witch Head Nebula in today's APOD. Usually there is more blue there in pictures - because there is more blue there.

Ann
I don't see much blue in other images, either.

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

by Ann » Tue Jan 25, 2022 5:01 am

Chris Peterson wrote: 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
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).
That's my point, Chris. There is less blue than usual in the Witch Head Nebula in today's APOD. Usually there is more blue there in pictures - because there is more blue there.

Ann

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

by VictorBorun » Tue Jan 25, 2022 4:58 am

neufer wrote: Tue Jan 25, 2022 4:16 am
VictorBorun wrote: Tue Jan 25, 2022 3:01 am why can't Witch Head Nebula be an aging HH object (10 ky old) in the thick of giant stars' bubble wall?
It is not the sort of environment I would expect:
https://en.wikipedia.org/wiki/IC_2118 wrote: Candidates for pre-main sequence stars and some classic T-Tauri stars have been found deep within the nebula.
Do I get it right: a star is welcome to emit a pair of jets at 2 moments:
when it's a proto-star of 1 Sun mass using an accretion disk in the core of proto-planets dusty disk to make make a pair of jets of 30 Earth mass
when it's a SuperNova of 100 Suns mass using a shedded disk to make a pair of jets of 3 Suns mass

Now the Witch Head Nebula is rather Super-Nova scale, is not it?

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

by neufer » Tue Jan 25, 2022 4:16 am

VictorBorun wrote: Tue Jan 25, 2022 3:01 am
neufer wrote: Tue Jan 25, 2022 2:17 am
https://en.wikipedia.org/wiki/IC_2118 wrote:
favorable circumstances for star formation
well, why can't Witch Head Nebula be an aging HH object (10 ky old) in the thick of giant stars' bubble wall?
It is not the sort of environment I would expect:
https://en.wikipedia.org/wiki/IC_2118 wrote:
Candidates for pre-main sequence stars and some classic T-Tauri stars have been found deep within the nebula.

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

by VictorBorun » Tue Jan 25, 2022 3:01 am

neufer wrote: Tue Jan 25, 2022 2:17 am
https://en.wikipedia.org/wiki/IC_2118 wrote: favorable circumstances for star formation
well, why can't Witch Head Nebula be an aging HH object (10 ky old) in the thick of giant stars' bubble wall?

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

by neufer » Tue Jan 25, 2022 2:17 am

VictorBorun wrote: Tue Jan 25, 2022 12:32 am
johnnydeep wrote: Mon Jan 24, 2022 9:30 pm

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 imageThis 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.
Can Witch Head Nebula be an aging Herbig–Haro object? Say, 10 ky old one?
  • Let's ask the Wizardkipedia ... the Wizardkipedia will know:
https://en.wikipedia.org/wiki/IC_2118 wrote:
Click to play embedded YouTube video.
<<IC 2118 (also known as Witch Head Nebula due to its shape) is an extremely faint reflection nebula believed to be an ancient supernova remnant or gas cloud illuminated by nearby supergiant star Rigel in the constellation of Orion. It lies in the Orion constellation, about 900 light-years from Earth. The nature of the dust particles, reflecting blue light better than red, is a factor in giving the Witch Head its blue color. Radio observations show substantial carbon monoxide emission throughout parts of IC 2118, an indicator of the presence of molecular clouds and star formation in the nebula. In fact candidates for pre-main sequence stars and some classic T-Tauri stars have been found deep within the nebula.

The molecular clouds of IC 2118 are probably juxtaposed to the outer boundaries of the vast Orion-Eridanus bubble, a giant supershell of molecular hydrogen blown by the high mass stars of the Orion OB1 association. As the supershell expands into the interstellar medium, favorable circumstances for star formation occur. IC 2118 is located in one such area. The wind blown appearance and cometary shape of the bright reflection nebula is highly suggestive of a strong association with the high mass luminous stars of Orion OB1. The fact that the heads of the cometary clouds of IC2118 point northeast towards the association is strong support of that relationship.>>
https://en.wikipedia.org/wiki/Orion%E2%80%93Eridanus_Superbubble wrote: The Orion–Eridanus Superbubble or Eridanus Soft X-ray Enhancement is a superbubble located west of the Orion Nebula. The region is formed from overlapping supernova remnants that may be associated with the Orion OB1 stellar association; the bubble is approximately 1200 ly across. It is the nearest superbubble to the Local Bubble containing the Sun, with the respective shock fronts being about 500 ly apart.

The structure was discovered from 21 cm radio observations by Carl Heiles and interstellar optical emission line observations by Reynolds and Ogden in the 1970s.

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

by VictorBorun » Tue Jan 25, 2022 12:32 am

johnnydeep wrote: Mon Jan 24, 2022 9:30 pm 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.
Can Witch Head Nebula be an aging Herbig–Haro object? Say, 10 ky old one?

Much of the wind eating away the cloud in the path of the jets is from Rigel or other Orion's giants, all north-east in the posted picture.
But some wind must blow from the HH object's core, so there should be some skirt-shape features giving away the direction of the wind from the core and so the position of the core along the joined path of the pair of the jets.
Attachments
Rigel and the Witch Head Nebula..jpg

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

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.

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

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).

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

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.

Ann

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

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

Gah! So much to read here today!

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

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.


Ann

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

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.

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

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.

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

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).

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

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.)

Ann

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

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.

Ann

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

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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