by Ann » Tue Apr 13, 2021 10:22 am
Knight of Clear Skies wrote: ↑Tue Apr 13, 2021 7:20 am
Chris Peterson wrote: ↑Tue Apr 13, 2021 7:14 am
I'm not quite sure what that last thing is, but it doesn't sound much like what I'm describing. All I'm talking about is what happens in most nebulas that are illuminated by a combination of emission and reflection.
I've been following the thread with interest and I don't see any real disagreement between what you and Ann are saying.
Let me try to explain what my objection is.
APOD Robot wrote:
(...) the bright star Alnitak, the easternmost star in the Belt of Orion visible on the far left, shines energetic light into the Flame that knocks electrons away from the great clouds of hydrogen gas that reside there. Much of the glow results when the electrons and ionized hydrogen recombine.
What is being described here is the process of ionization. An ionized cloud of hydrogen, intermixed with some dust, will emit visible, primarily red light of ionized hydrogen in the Balmer series.
The caption talks about the Flame Nebula as if it was a red emission nebula. But the Flame Nebula is a yellow nebula.
An ionized cloud of primarily hydrogen will emit primarily red light (from hydrogen alpha, at 656 nm), mixed with some blue and violet light, making the total light of ionized hydrogen reddish-pink or magenta.
But if the pink or magenta light from a hydrogen nebula passes through considerable amounts of dust, the blue and violet light will be filtered away, leaving only the red light from hydrogen alpha.
Look at the picture of the dust cloud at right. Mostly white stars shine behind it. The dust cloud gets thicker and thicker towards the middle, and as the white light of the stars behind tries to penetrate, more and more of the light is scattered away by the dust. First only the violet light disappears, making the starlight pale yellow. Then the blue light disappears, making the starlight yellow-orange. Then the green and yellow light disappears, making the starlight a deep shade of orange. Finally, even the red light is filtered away, and the center of the nebula is completely dark.
Here's my point. Dust makes starlight redder. Yellow is a redder shade than white. Orange is a redder shade than yellow. Red is a redder shade than orange.
Let's look at Adam Block's picture of the Antares and Rho Ophiuchi region again and consider what the different colors mean.
There are two pink nebulas here. They are emission nebulas which shine because of ionized hydrogen. The one at right surrounds multiple star Sigma Scorpii, a binary B1 star, hot enough to ionize hydrogen and make it glow pink. The pink nebula at bottom right is ionized by B0-type star Tau Scorpii.
There is also the large yellow nebula. In my opinion, this nebula is primarily a reflection nebula, scattering the orange light of mighty red supergiant star Antares in a cloud of yellow. However, other factors may contribute to the appearance of the yellow nebula, such as dust reddening (certainly in the more orange parts of the nebula) and some mixing with scattered red hydrogen alpha light.
There are two blue nebulas here. The small one surrounds star 22 Scorpii, a star of spectral type B3. This star is not hot enough to ionize hydrogen (to any important extent), but it produces a lot of blue light, which is being "forward-scattered" by dust. The large blue nebula is the Rho Ophiuchi nebula, surrounding multiple star Rho Ophiuchi star of spectral class B2. This star is borderline hot enough to produce a (relativelhy small and faint) emission nebula, but we don't see one. Don't ask me why.
I believe that Rho Ophiuchi is the largest blue reflection nebula in the sky. One reason might be that so much of the blue light of Rho Ophiuci and its two companions has been scattered away from the stars themselves, making the stars themselves appear yellowish, according to one observer. I don't believe that. But if we compare Rho Ophiuchi with 22 Scorpii , we can see that 22 Scorpii has "retained" a lot more of its blue light (i.e., seems to emit much bluer light our way) than Rho Ophiuchi. Perhaps the conditions for scattering blue light in all directions are particularly favorable in the vicinity of Rho Ophiuchi, but not so much near 22 Scorpii.
Finally, we can see a lot of very dark dust in the Rho Ophiuchi region. Much of this dark dust is impenetrable to optical light. Stars behind it can't be seen at optical wavelengths.
To conclude:
My objections have been that the caption of today's APOD described the yellow Flame Nebula as an emission nebula, but a red emission nebula can't become yellow because of dust. I have never seen a single instance of this in all the thousands of astronomical images I have looked at.
Emission nebulosity may
contribute to the color of the Flame Nebula. But the Flame Nebula can't be a red emission nebula whose color has become bluer because of dust.
I have tried to explain what I think may be the reason for the highly unusual color of the Flame Nebula. I believe light from Alnitak has been filtered though the Flame Nebula, allowing only some yellow and red light to make it through, and I think that the yellow color has been enhanced by embedded star formation in the Flame Nebula.
Ann
[quote="Knight of Clear Skies" post_id=312449 time=1618298407 user_id=143544]
[quote="Chris Peterson" post_id=312448 time=1618298050 user_id=117706]
I'm not quite sure what that last thing is, but it doesn't sound much like what I'm describing. All I'm talking about is what happens in most nebulas that are illuminated by a combination of emission and reflection.
[/quote]
I've been following the thread with interest and I don't see any real disagreement between what you and Ann are saying.
[/quote]
Let me try to explain what my objection is.
[quote][url=https://asterisk.apod.com/viewtopic.php?p=312401#p312401]APOD Robot[/url] wrote:
(...) the bright star Alnitak, the easternmost star in the Belt of Orion visible on the far left, shines energetic light into the Flame that knocks electrons away from the great clouds of hydrogen gas that reside there. Much of the glow results when the electrons and ionized hydrogen recombine.[/quote]
What is being described here is the process of ionization. An ionized cloud of hydrogen, intermixed with some dust, will emit visible, primarily red light of ionized hydrogen in the Balmer series. [b][color=#FF0000]The caption talks about the Flame Nebula as if it was a red emission nebula.[/color][/b] [b][color=#FFBF00]But the Flame Nebula is a yellow nebula.[/color][/b]
[float=left][img3="The hydrogen emission spectrum is the Balmer series. Image:
ttsz / Getty Images"]https://www.thoughtco.com/thmb/22wOCjzYVkhBZc_jsfxAeGO5COU=/1732x1732/filters:fill(auto,1)/GettyImages-1096547948-35b3799817ca4b2fa06888893ef4a348.jpg[/img3][/float] [float=right][img3="Interstellar dust reddening. faculty.virginia.edu"]https://faculty.virginia.edu/skrutskie/images/globule.jpg[/img3][/float]
[clear][/clear]
An ionized cloud of primarily hydrogen will emit primarily red light (from hydrogen alpha, at 656 nm), mixed with some blue and violet light, making the total light of ionized hydrogen reddish-pink or magenta.
But if the pink or magenta light from a hydrogen nebula passes through considerable amounts of dust, the blue and violet light will be filtered away, leaving only the red light from hydrogen alpha.
Look at the picture of the dust cloud at right. Mostly white stars shine behind it. The dust cloud gets thicker and thicker towards the middle, and as the white light of the stars behind tries to penetrate, more and more of the light is scattered away by the dust. First only the violet light disappears, making the starlight pale yellow. Then the blue light disappears, making the starlight yellow-orange. Then the green and yellow light disappears, making the starlight a deep shade of orange. Finally, even the red light is filtered away, and the center of the nebula is completely dark.
Here's my point. Dust makes starlight redder. Yellow is a redder shade than white. Orange is a redder shade than yellow. Red is a redder shade than orange.
[float=left][img3="The Antares and Rho Ophiuchi region. Photo: Adam Block. "]https://upload.wikimedia.org/wikipedia/commons/thumb/4/46/Antares_and_Rho_Ophiuchi_by_Adam_Block.jpg/652px-Antares_and_Rho_Ophiuchi_by_Adam_Block.jpg[/img3][/float]
[clear][/clear]
Let's look at Adam Block's picture of the Antares and Rho Ophiuchi region again and consider what the different colors mean.
There are two pink nebulas here. They are emission nebulas which shine because of ionized hydrogen. The one at right surrounds multiple star Sigma Scorpii, a binary B1 star, hot enough to ionize hydrogen and make it glow pink. The pink nebula at bottom right is ionized by B0-type star Tau Scorpii.
There is also the large yellow nebula. In my opinion, this nebula is primarily a reflection nebula, scattering the orange light of mighty red supergiant star Antares in a cloud of yellow. However, other factors may contribute to the appearance of the yellow nebula, such as dust reddening (certainly in the more orange parts of the nebula) and some mixing with scattered red hydrogen alpha light.
There are two blue nebulas here. The small one surrounds star 22 Scorpii, a star of spectral type B3. This star is not hot enough to ionize hydrogen (to any important extent), but it produces a lot of blue light, which is being "forward-scattered" by dust. The large blue nebula is the Rho Ophiuchi nebula, surrounding multiple star Rho Ophiuchi star of spectral class B2. This star is borderline hot enough to produce a (relativelhy small and faint) emission nebula, but we don't see one. Don't ask me why.
I believe that Rho Ophiuchi is the largest blue reflection nebula in the sky. One reason might be that so much of the blue light of Rho Ophiuci and its two companions has been scattered away from the stars themselves, making the stars themselves appear yellowish, according to one observer. I don't believe that. But if we compare Rho Ophiuchi with 22 Scorpii , we can see that 22 Scorpii has "retained" a lot more of its blue light (i.e., seems to emit much bluer light our way) than Rho Ophiuchi. Perhaps the conditions for scattering blue light in all directions are particularly favorable in the vicinity of Rho Ophiuchi, but not so much near 22 Scorpii.
Finally, we can see a lot of very dark dust in the Rho Ophiuchi region. Much of this dark dust is impenetrable to optical light. Stars behind it can't be seen at optical wavelengths.
To conclude:
My objections have been that the caption of today's APOD described the yellow Flame Nebula as an emission nebula, but a red emission nebula can't become yellow because of dust. I have never seen a single instance of this in all the thousands of astronomical images I have looked at.
Emission nebulosity may [i]contribute[/i] to the color of the Flame Nebula. But the Flame Nebula can't be a red emission nebula whose color has become bluer because of dust.
I have tried to explain what I think may be the reason for the highly unusual color of the Flame Nebula. I believe light from Alnitak has been filtered though the Flame Nebula, allowing only some yellow and red light to make it through, and I think that the yellow color has been enhanced by embedded star formation in the Flame Nebula.
Ann