APOD: The Seagull Nebula (2024 Mar 13)

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APOD: The Seagull Nebula (2024 Mar 13)

Post by APOD Robot » Wed Mar 13, 2024 4:06 am

Image The Seagull Nebula

Explanation: A broad expanse of glowing gas and dust presents a bird-like visage to astronomers from planet Earth, suggesting its popular moniker: the Seagull Nebula. This portrait of the cosmic bird covers a 1.6-degree wide swath across the plane of the Milky Way, near the direction of Sirius, the alpha star of the constellation of the Big Dog (Canis Major). Of course, the region includes objects with other catalog designations: notably NGC 2327, a compact, dusty emission and reflection nebula with an embedded massive star that forms the bird's head. Dominated by the reddish glow of atomic hydrogen, the complex of gas and dust clouds with bright young stars spans over 100 light-years at an estimated 3,800 light-year distance.

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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by Ann » Wed Mar 13, 2024 7:01 am


That's an interesting and very red portrait of the Seagull Nebula.

I can't complain. The Seagull Nebula is very red, since it is heavily dominated by 656 nm Hydrogen Alpha, which is of course this color, ███.

Only one other color is immediately obvious in the APOD apart form the red Hydrogen Alpha, and that is pale blue. At first I thought that the APOD might be a two-filter image, constructed from from a red Hα filter and a cyan-green OIII filter, mapped as blue, but that is not fully consistent with the appearance of the image. I found just such an Hα/OIII picture of the Seagull Nebula on the net, and as you can see, it doesn't look like the APOD:


In narrowband images of nebulas, such as the Seagull Nebula picture by Simon D., you "play down" the Hydrogen Alpha and "enhance" other emission lines, such as OIII. That is why Simon D.:s image looks so very blue compared with the APOD. Note however that the bluest areas in Simon D.:s image look faintly bluish in the APOD as well, which makes me wonder if a bit of OIII has been used for the APOD, too. Note that both the APOD and Simon D.:s image show the bow shock of the blue star FN CMa (at center right) very clearly.


Not all Seagull Nebula images show any blue nebulosity below the red ridge (or the bow shock of FN CMa):


So I'd say we are probably seeing some OIII in the APOD after all. Of course, Gianni Lacroce, the APOD photographer, has brought out "a ton of Hα", too!


I'd like to point out two stars to you:

APOD 13 March 2024 detail annotated.png
Blue FN CMA with its bow shock reminds me of Mary Poppins with her umbrella!
I'll talk about W CMa later.


Of course, while Mary Poppis used her umbrella to descend from the sky, hot blue B-type FN CMa is pushing its way ahead through gas and dust forming an umbrella-shaped bow shock in front of it.


As for red-orange W CMa, it's a rare and remarkable carbon star. Its small Gaia parallax puts it a distance of ~2,250 light-years. And it is remarkably red. Its B-V index is not that much different from that of Betelgeuse, but W CMa is four magnitudes brighter in R than in B. That's a lot. Its absolute V (yellow-green) luminosity is some 900 times the Sun, but in R (red), it is more than 4,000 times brighter than the Sun. And in infrared J, corresponding to a wavelength of 1220 nm, it is almost 60,000 times brighter than the Sun! And in infrared K, corresponding to 2190 nm, it is 170,000 times brighter than the Sun!

Wikipedia wrote about carbon stars like W CMa:

A carbon star (C-type star) is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen.[1] The two elements combine in the upper layers of the star, forming carbon monoxide, which consumes most of the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere and a strikingly ruby red appearance.

W CMA is a sooty star! That's why it's so red. It is self-reddened by particles in its own atmosphere, just like an extra red sunset.


Clearly W CMa needs a chimney sweep! (Speaking of Mary Poppins...)

Click to play embedded YouTube video.

Ann
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Last edited by Ann on Thu Mar 14, 2024 4:16 am, edited 1 time in total.
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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by Christian G. » Wed Mar 13, 2024 4:56 pm

Ann wrote: Wed Mar 13, 2024 7:01 am
That's an interesting and very red portrait of the Seagull Nebula.

I can't complain. The Seagull Nebula is very red, since it is heavily dominated by 656 nm Hydrogen Alpha, which is of course this color, ███.

Only one other color is immediately obvious apart form the red Hydrogen Alpha, and that is pale blue. At first I thought that the APOD might be a two-filter image, constructed from from a red Hα filter and a cyan-green OIII filter, mapped as blue, but that is not fully consistent with the appearance of the image. I found just such an Hα/OIII picture of the Seagull Nebula on the net, and as you can see, it doesn't look like the APOD:


In narrowband images of nebulas, such as the Seagull Nebula picture by Simon D., your "play down" the Hydrogen Alpha and "enhance" other emission lines, such as OIII. That is why Simon D.:s image looks so very blue compared with the APOD. Note however that the bluest areas in Simon D.:s image look faintly bluish in the APOD as well, which makes me wonder if a bit of OIII has been used for the APOD, too. Note that both the APOD and Simon D.:s image show the bow shock of the blue star FN CMa (at center right) very clearly.


Not all Seagull Nebula images show any blue nebulosity below the red ridge (or the bow shock of FN CMa):


So I'd say we are probably seeing some OIII in the APOD after all. Of course, Gianni Lacroce, the APOD photographer, has brought out "a ton of Hα", too!


I'd like to point out two stars to you:

APOD 13 March 2024 detail annotated.png
Blue FN CMA with its bow shock reminds me of Mary Poppins with her umbrella!
I'll talk about W CMa later.


Of course, while Mary Poppis used her umbrella to descend from the sky, hot blue B-type FN CMa is pushing its way ahead through gas and dust forming an umbrella-shaped bow shock in front of it.


As for red-orange W CMa, it's a rare and remarkable carbon star. Its small Gaia parallax puts it a distance of ~2,250 light-years. And it is remarkably red. Its B-V index is not that much different from that of Betelgeuse, but W CMa is four magnitudes brighter in R than in B. That's a lot. Its absolute V (yellow-green) luminosity is some 900 times the Sun, but in R (red), it is more than 4,000 times brighter than the Sun. And in infrared J, corresponding to a wavelength of 1220 nm, it is almost 60,000 times brighter than the Sun! And in infrared K, corresponding to 2190 nm, it is 170,000 times brighter than the Sun!

Wikipedia wrote about carbon stars like W CMa:

A carbon star (C-type star) is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen.[1] The two elements combine in the upper layers of the star, forming carbon monoxide, which consumes most of the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere and a strikingly ruby red appearance.

W CMA is a sooty star! It needs a chimney sweep! (Speaking of Mary Poppins...)

Click to play embedded YouTube video.

Ann
Very informative and enjoyable to read, Ann, especially the part on W CMa, a favorite of mine in this area of the sky!

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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by johnnydeep » Wed Mar 13, 2024 7:15 pm

So, are the two small round red emission regions in the upper right likely powered by the stars seemingly at their centers, or are the stars' central locations coincidental?
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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by AVAO » Wed Mar 13, 2024 10:04 pm

Ann wrote: Wed Mar 13, 2024 7:01 am
That's an interesting and very red portrait of the Seagull Nebula.

I can't complain. The Seagull Nebula is very red, since it is heavily dominated by 656 nm Hydrogen Alpha, which is of course this color, ███.

Only one other color is immediately obvious apart form the red Hydrogen Alpha, and that is pale blue. At first I thought that the APOD might be a two-filter image, constructed from from a red Hα filter and a cyan-green OIII filter, mapped as blue, but that is not fully consistent with the appearance of the image. I found just such an Hα/OIII picture of the Seagull Nebula on the net, and as you can see, it doesn't look like the APOD:


In narrowband images of nebulas, such as the Seagull Nebula picture by Simon D., your "play down" the Hydrogen Alpha and "enhance" other emission lines, such as OIII. That is why Simon D.:s image looks so very blue compared with the APOD. Note however that the bluest areas in Simon D.:s image look faintly bluish in the APOD as well, which makes me wonder if a bit of OIII has been used for the APOD, too. Note that both the APOD and Simon D.:s image show the bow shock of the blue star FN CMa (at center right) very clearly.


Not all Seagull Nebula images show any blue nebulosity below the red ridge (or the bow shock of FN CMa):


So I'd say we are probably seeing some OIII in the APOD after all. Of course, Gianni Lacroce, the APOD photographer, has brought out "a ton of Hα", too!


I'd like to point out two stars to you:

APOD 13 March 2024 detail annotated.png
Blue FN CMA with its bow shock reminds me of Mary Poppins with her umbrella!
I'll talk about W CMa later.


Of course, while Mary Poppis used her umbrella to descend from the sky, hot blue B-type FN CMa is pushing its way ahead through gas and dust forming an umbrella-shaped bow shock in front of it.


As for red-orange W CMa, it's a rare and remarkable carbon star. Its small Gaia parallax puts it a distance of ~2,250 light-years. And it is remarkably red. Its B-V index is not that much different from that of Betelgeuse, but W CMa is four magnitudes brighter in R than in B. That's a lot. Its absolute V (yellow-green) luminosity is some 900 times the Sun, but in R (red), it is more than 4,000 times brighter than the Sun. And in infrared J, corresponding to a wavelength of 1220 nm, it is almost 60,000 times brighter than the Sun! And in infrared K, corresponding to 2190 nm, it is 170,000 times brighter than the Sun!

Wikipedia wrote about carbon stars like W CMa:

A carbon star (C-type star) is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen.[1] The two elements combine in the upper layers of the star, forming carbon monoxide, which consumes most of the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere and a strikingly ruby red appearance.

W CMA is a sooty star! It needs a chimney sweep! (Speaking of Mary Poppins...)

Click to play embedded YouTube video.

Ann
ThanX Ann

I like your explanation!
... and I like the region in IR too. (Yeaaa - 500 Posts ;- )

Jac
Click to view full size image 1 or image 2
jac berne (flickr)

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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by Ann » Thu Mar 14, 2024 4:30 am

Christian G. wrote: Wed Mar 13, 2024 4:56 pm
Very informative and enjoyable to read, Ann, especially the part on W CMa, a favorite of mine in this area of the sky!
Thanks, Christian, glad you enjoyed it! :D

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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by Ann » Thu Mar 14, 2024 5:05 am

johnnydeep wrote: Wed Mar 13, 2024 7:15 pm So, are the two small round red emission regions in the upper right likely powered by the stars seemingly at their centers, or are the stars' central locations coincidental?
Good question, Johnny. As a rule of thumb, if you see a reasonably bright star that is centrally placed in a nebula, you should assume that the star has something to do with creating that nebula.

The stars in question are GU CMa (the brighter star in the larger nebula) and FZ CMa. Both are spectral class B2. Normally we don't expect stars of class B2 to be able to ionize an emission nebula. But consider the Iris Nebula, a reflection nebula that is illuminated by B2V star HD 200775. Most pictures of this nebula shows it as being all blue, but a few bring out hints of pink from hydrogen alpha in this blue nebula:


The nebulas around GU CMA and FZ CMa have designations vdB 88 and vdB 90. The van den Bergh catalog lists reflection nebulas. I did a quick search, and several pictures show vdB 90 as bluish, but most "reasonably RGB" images show vdB 88 as reddish. Therefore, the nebula around GU CMa is definitely an emission nebula as well as a reflection nebula. I don't know why, because with a spectral class B2 central star, ionization should not be so obvious. Naturally vdB 88 must have an emission nebula designation apart from its van den Bergh designation. So, probably, does vdB 90.


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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by Ann » Thu Mar 14, 2024 5:10 am

AVAO wrote: Wed Mar 13, 2024 10:04 pm
ThanX Ann

I like your explanation!
... and I like the region in IR too. (Yeaaa - 500 Posts ;- )

Jac
Click to view full size image 1 or image 2
jac berne (flickr)
Thanks Jac, I love your imagehovers! :D Great visible and great infrared portraits of the Seagull Nebula!

And congratulations on your 500 posts!

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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by johnnydeep » Thu Mar 14, 2024 2:12 pm

Ann wrote: Thu Mar 14, 2024 5:05 am
johnnydeep wrote: Wed Mar 13, 2024 7:15 pm So, are the two small round red emission regions in the upper right likely powered by the stars seemingly at their centers, or are the stars' central locations coincidental?
Good question, Johnny. As a rule of thumb, if you see a reasonably bright star that is centrally placed in a nebula, you should assume that the star has something to do with creating that nebula.

...
Ann
Thanks. I suppose the rule is that sometimes the simplest explanation is probably the right one. But I'm still not completely convinced in this case.

There's also another very tiny "nebula" near the seagull's head, that appears to have a star at its center, but there are also two other similarly bright stars at the edges of this nebula:

nebula near head of seagull nebula.jpg
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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by Ann » Thu Mar 14, 2024 2:47 pm

johnnydeep wrote: Thu Mar 14, 2024 2:12 pm
Ann wrote: Thu Mar 14, 2024 5:05 am
johnnydeep wrote: Wed Mar 13, 2024 7:15 pm So, are the two small round red emission regions in the upper right likely powered by the stars seemingly at their centers, or are the stars' central locations coincidental?
Good question, Johnny. As a rule of thumb, if you see a reasonably bright star that is centrally placed in a nebula, you should assume that the star has something to do with creating that nebula.

...
Ann
Thanks. I suppose the rule is that sometimes the simplest explanation is probably the right one. But I'm still not completely convinced in this case.

There's also another very tiny "nebula" near the seagull's head, that appears to have a star at its center, but there are also two other similarly bright stars at the edges of this nebula:

My software identifies the position of this nebula as something it calls PK223-2.1 diffuse nebula. I googled it and found this page. It said that PK 223-2.1 is a small and faint planetary nebula.

That makes sense. We do expect the ionizing star (make that white dwarf) to be at the center of its planetary nebula, but we also expect the ionizing white dwarf to be faint.

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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by johnnydeep » Thu Mar 14, 2024 3:54 pm

Ann wrote: Thu Mar 14, 2024 2:47 pm
johnnydeep wrote: Thu Mar 14, 2024 2:12 pm
Ann wrote: Thu Mar 14, 2024 5:05 am

Good question, Johnny. As a rule of thumb, if you see a reasonably bright star that is centrally placed in a nebula, you should assume that the star has something to do with creating that nebula.

...
Ann
Thanks. I suppose the rule is that sometimes the simplest explanation is probably the right one. But I'm still not completely convinced in this case.

There's also another very tiny "nebula" near the seagull's head, that appears to have a star at its center, but there are also two other similarly bright stars at the edges of this nebula:

My software identifies the position of this nebula as something it calls PK223-2.1 diffuse nebula. I googled it and found this page. It said that PK 223-2.1 is a small and faint planetary nebula.

That makes sense. We do expect the ionizing star (make that white dwarf) to be at the center of its planetary nebula, but we also expect the ionizing white dwarf to be faint.

Ann
Thanks. So in this case, the central star is related to the nebula and the two periphery stars are - probably - not.
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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by Chris Peterson » Thu Mar 14, 2024 4:03 pm

Ann wrote: Thu Mar 14, 2024 2:47 pm
johnnydeep wrote: Thu Mar 14, 2024 2:12 pm
Ann wrote: Thu Mar 14, 2024 5:05 am

Good question, Johnny. As a rule of thumb, if you see a reasonably bright star that is centrally placed in a nebula, you should assume that the star has something to do with creating that nebula.

...
Ann
Thanks. I suppose the rule is that sometimes the simplest explanation is probably the right one. But I'm still not completely convinced in this case.

There's also another very tiny "nebula" near the seagull's head, that appears to have a star at its center, but there are also two other similarly bright stars at the edges of this nebula:

My software identifies the position of this nebula as something it calls PK223-2.1 diffuse nebula. I googled it and found this page. It said that PK 223-2.1 is a small and faint planetary nebula.

That makes sense. We do expect the ionizing star (make that white dwarf) to be at the center of its planetary nebula, but we also expect the ionizing white dwarf to be faint.

Ann
It is a compact HII region. The identification as a PN was erroneous, and has been corrected in current catalogs.
Chris

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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by Ann » Thu Mar 14, 2024 4:14 pm

Chris Peterson wrote: Thu Mar 14, 2024 4:03 pm
It is a compact HII region. The identification as a PN was erroneous, and has been corrected in current catalogs.
Thanks for the correction, Chris. However, seeing that the HII region is compact, I'd still say that it's highly likely that the ionizing star is the central one.

After all, what other compact HII regions are known where the ionizing star or stars are located at the periphery of the nebula?

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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by Chris Peterson » Thu Mar 14, 2024 4:18 pm

Ann wrote: Thu Mar 14, 2024 4:14 pm
Chris Peterson wrote: Thu Mar 14, 2024 4:03 pm
It is a compact HII region. The identification as a PN was erroneous, and has been corrected in current catalogs.
Thanks for the correction, Chris. However, seeing that the HII region is compact, I'd still say that it's highly likely that the ionizing star is the central one.

After all, what other compact HII regions are known where the ionizing star or stars are located at the periphery of the nebula?

Ann
Oh, it's almost certain that the region is being ionized by that star. The important point is that we're not seeing material blown off at the end of its life, just ionization of that huge HII region around the Seagull.
Chris

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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by johnnydeep » Thu Mar 14, 2024 5:02 pm

Chris Peterson wrote: Thu Mar 14, 2024 4:18 pm
Ann wrote: Thu Mar 14, 2024 4:14 pm
Chris Peterson wrote: Thu Mar 14, 2024 4:03 pm
It is a compact HII region. The identification as a PN was erroneous, and has been corrected in current catalogs.
Thanks for the correction, Chris. However, seeing that the HII region is compact, I'd still say that it's highly likely that the ionizing star is the central one.

After all, what other compact HII regions are known where the ionizing star or stars are located at the periphery of the nebula?

Ann
Oh, it's almost certain that the region is being ionized by that star. The important point is that we're not seeing material blown off at the end of its life, just ionization of that huge HII region around the Seagull.
Is it at all unusual that the ionization should appear to be so spherical? I assume whatever gas cloud the star is embedded in is not spherical and that the gas likely extends invisibly and asymmetrically all around that star. But I guess the ionization power only extends so far...
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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by Chris Peterson » Thu Mar 14, 2024 6:00 pm

johnnydeep wrote: Thu Mar 14, 2024 5:02 pm
Chris Peterson wrote: Thu Mar 14, 2024 4:18 pm
Ann wrote: Thu Mar 14, 2024 4:14 pm

Thanks for the correction, Chris. However, seeing that the HII region is compact, I'd still say that it's highly likely that the ionizing star is the central one.

After all, what other compact HII regions are known where the ionizing star or stars are located at the periphery of the nebula?

Ann
Oh, it's almost certain that the region is being ionized by that star. The important point is that we're not seeing material blown off at the end of its life, just ionization of that huge HII region around the Seagull.
Is it at all unusual that the ionization should appear to be so spherical? I assume whatever gas cloud the star is embedded in is not spherical and that the gas likely extends invisibly and asymmetrically all around that star. But I guess the ionization power only extends so far...
I don't know. But there's a whole body of literature on compact and ultracompact and hypercompact HII regions, and some of the pictures show pretty spherical objects. So I guess it's not that rare.
Chris

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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by johnnydeep » Thu Mar 14, 2024 6:40 pm

Chris Peterson wrote: Thu Mar 14, 2024 6:00 pm
johnnydeep wrote: Thu Mar 14, 2024 5:02 pm
Chris Peterson wrote: Thu Mar 14, 2024 4:18 pm

Oh, it's almost certain that the region is being ionized by that star. The important point is that we're not seeing material blown off at the end of its life, just ionization of that huge HII region around the Seagull.
Is it at all unusual that the ionization should appear to be so spherical? I assume whatever gas cloud the star is embedded in is not spherical and that the gas likely extends invisibly and asymmetrically all around that star. But I guess the ionization power only extends so far...
I don't know. But there's a whole body of literature on compact and ultracompact and hypercompact HII regions, and some of the pictures show pretty spherical objects. So I guess it's not that rare.
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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by Ann » Thu Mar 14, 2024 6:42 pm

johnnydeep wrote: Thu Mar 14, 2024 5:02 pm
Chris Peterson wrote: Thu Mar 14, 2024 4:18 pm
Ann wrote: Thu Mar 14, 2024 4:14 pm

Thanks for the correction, Chris. However, seeing that the HII region is compact, I'd still say that it's highly likely that the ionizing star is the central one.

After all, what other compact HII regions are known where the ionizing star or stars are located at the periphery of the nebula?

Ann
Oh, it's almost certain that the region is being ionized by that star. The important point is that we're not seeing material blown off at the end of its life, just ionization of that huge HII region around the Seagull.
Is it at all unusual that the ionization should appear to be so spherical? I assume whatever gas cloud the star is embedded in is not spherical and that the gas likely extends invisibly and asymmetrically all around that star. But I guess the ionization power only extends so far...
Interesting question. There are in fact several relatively spherical emission nebulas that are apparently ionized by a single star:


The nebulas I just showed you are ionized by a single star, or by a very small and very compact group of stars. A famous example of a spherical nebula that is ionized by a cluster of several O-type stars is the Rosette Nebula.


Young nebulas like the Cocoon Nebula and the Trifid Nebula are more compact, and older nebulas like IC 1396, the Rosette Nebula and the lambda Orionis Nebula are more spread out and diffuse.

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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by johnnydeep » Thu Mar 14, 2024 6:52 pm

Ann wrote: Thu Mar 14, 2024 6:42 pm
johnnydeep wrote: Thu Mar 14, 2024 5:02 pm
Chris Peterson wrote: Thu Mar 14, 2024 4:18 pm

Oh, it's almost certain that the region is being ionized by that star. The important point is that we're not seeing material blown off at the end of its life, just ionization of that huge HII region around the Seagull.
Is it at all unusual that the ionization should appear to be so spherical? I assume whatever gas cloud the star is embedded in is not spherical and that the gas likely extends invisibly and asymmetrically all around that star. But I guess the ionization power only extends so far...
Interesting question. There are in fact several relatively spherical emission nebulas that are apparently ionized by a single star:

...

The nebulas I just showed you are ionized by a single star, or by a very small and very compact group of stars. A famous example of a spherical nebula that is ionized by a cluster of several O-type stars is the Rosette Nebula.

...

Young nebulas like the Cocoon Nebula and the Trifid Nebula are more compact, and older nebulas like IC 1396, the Rosette Nebula and the lambda Orionis Nebula are more spread out and diffuse.

Ann
Thanks. Intuitively, then, since stars - or compact groups of stars - naturally radiate equally spherically, it only makes sense that, provided their gas cloud cocoon is relatively uniformly dense in all directions, the ionization "power" would also extend equally far spherically. One exception might be a star plowing through a gas cloud that would leave a "trail" of ionization? But I'm sure Chris will tell us that the ionization effect is short lived and we'd still most likely see only a spherical nebula!
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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by Ann » Thu Mar 14, 2024 7:54 pm

johnnydeep wrote: Thu Mar 14, 2024 6:52 pm
Ann wrote: Thu Mar 14, 2024 6:42 pm
johnnydeep wrote: Thu Mar 14, 2024 5:02 pm

Is it at all unusual that the ionization should appear to be so spherical? I assume whatever gas cloud the star is embedded in is not spherical and that the gas likely extends invisibly and asymmetrically all around that star. But I guess the ionization power only extends so far...
Interesting question. There are in fact several relatively spherical emission nebulas that are apparently ionized by a single star:

...

The nebulas I just showed you are ionized by a single star, or by a very small and very compact group of stars. A famous example of a spherical nebula that is ionized by a cluster of several O-type stars is the Rosette Nebula.

...

Young nebulas like the Cocoon Nebula and the Trifid Nebula are more compact, and older nebulas like IC 1396, the Rosette Nebula and the lambda Orionis Nebula are more spread out and diffuse.

Ann
Thanks. Intuitively, then, since stars - or compact groups of stars - naturally radiate equally spherically, it only makes sense that, provided their gas cloud cocoon is relatively uniformly dense in all directions, the ionization "power" would also extend equally far spherically. One exception might be a star plowing through a gas cloud that would leave a "trail" of ionization? But I'm sure Chris will tell us that the ionization effect is short lived and we'd still most likely see only a spherical nebula!
I've never seen a runaway star leaving a trail of ionized gas behind. Take a look at this runaway star near the Tarantula Nebula. It doesn't seem to leave a trail behind, and neither does runaway star AE Aurigae:

1567218568006-heic1008a[1].jpg
A runaway star in the Tarantula Nebula. Credit: NASA, ESA, J. Walsh (ST-ECF), ESO,
Acknowledgments: J. Alves (Calar Alto, Spain), B. Vandame, and Y. Beletski (ESO).
Processing by B. Fosbury (ST-ECF).

But red giant runaway star Mira is shedding material as it runs, and it is leaving a 13 light-year-long invisible ultraviolet tail behind.


The way I understand it, it is the friction between the gases shed by Mira and the interstellar medium that creates the ultraviolet tail. But I have never seen a red hydrogen alpha tail behind a runaway star.

Ann
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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by johnnydeep » Thu Mar 14, 2024 11:34 pm

Ann wrote: Thu Mar 14, 2024 7:54 pm
johnnydeep wrote: Thu Mar 14, 2024 6:52 pm
Ann wrote: Thu Mar 14, 2024 6:42 pm

Interesting question. There are in fact several relatively spherical emission nebulas that are apparently ionized by a single star:

...

The nebulas I just showed you are ionized by a single star, or by a very small and very compact group of stars. A famous example of a spherical nebula that is ionized by a cluster of several O-type stars is the Rosette Nebula.

...

Young nebulas like the Cocoon Nebula and the Trifid Nebula are more compact, and older nebulas like IC 1396, the Rosette Nebula and the lambda Orionis Nebula are more spread out and diffuse.

Ann
Thanks. Intuitively, then, since stars - or compact groups of stars - naturally radiate equally spherically, it only makes sense that, provided their gas cloud cocoon is relatively uniformly dense in all directions, the ionization "power" would also extend equally far spherically. One exception might be a star plowing through a gas cloud that would leave a "trail" of ionization? But I'm sure Chris will tell us that the ionization effect is short lived and we'd still most likely see only a spherical nebula!
I've never seen a runaway star leaving a trail of ionized gas behind. Take a look at this runaway star near the Tarantula Nebula. It doesn't seem to leave a trail behind, and neither does runaway star AE Aurigae:

1567218568006-heic1008a[1].jpg
A runaway star in the Tarantula Nebula. Credit: NASA, ESA, J. Walsh (ST-ECF), ESO,
Acknowledgments: J. Alves (Calar Alto, Spain), B. Vandame, and Y. Beletski (ESO).
Processing by B. Fosbury (ST-ECF).

But red giant runaway star Mira is shedding material as it runs, and it is leaving a 13 light-year-long invisible ultraviolet tail behind.


The way I understand it, it is the friction between the gases shed by Mira and the interstellar medium that creates the ultraviolet tail. But I have never seen a red hydrogen alpha tail behind a runaway star.

Ann
Hmm, so I suppose the effect of that friction - that is, the heating of the gas - is more long lived than any ionization effect on the gas?
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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by Chris Peterson » Thu Mar 14, 2024 11:48 pm

johnnydeep wrote: Thu Mar 14, 2024 11:34 pm Hmm, so I suppose the effect of that friction - that is, the heating of the gas - is more long lived than any ionization effect on the gas?
It's a peculiar case. The bow shock (in front of Mira) creates a heated plasma, which flows backwards and interacts with the cool ISM, resulting in a far UV emission from H2. That is unusual; most emissions we see come from atomic hydrogen, not molecular hydrogen.
Chris

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Re: APOD: The Seagull Nebula (2024 Mar 13)

Post by johnnydeep » Fri Mar 15, 2024 7:23 pm

Chris Peterson wrote: Thu Mar 14, 2024 11:48 pm
johnnydeep wrote: Thu Mar 14, 2024 11:34 pm Hmm, so I suppose the effect of that friction - that is, the heating of the gas - is more long lived than any ionization effect on the gas?
It's a peculiar case. The bow shock (in front of Mira) creates a heated plasma, which flows backwards and interacts with the cool ISM, resulting in a far UV emission from H2. That is unusual; most emissions we see come from atomic hydrogen, not molecular hydrogen.
Cool. Very interesting!
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"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}