APOD: NGC 7129 and NGC 7142 (2019 Aug 30)

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APOD: NGC 7129 and NGC 7142 (2019 Aug 30)

Post by APOD Robot » Fri Aug 30, 2019 4:06 am

Image NGC 7129 and NGC 7142

Explanation: This wide-field telescopic image looks toward the constellation Cepheus and an intriguing visual pairing of dusty reflection nebula NGC 7129 (right) and open star cluster NGC 7142. The two appear separated by only half a degree on the sky, but they actually lie at quite different distances. In the foreground, dusty nebula NGC 7129 is about 3,000 light-years distant, while open cluster NGC 7142 is likely over 6,000 light-years away. In fact, pervasive and clumpy foreground dust clouds in this region redden the light from NGC 7142, complicating astronomical explorations of the cluster. Still, NGC 7142 is thought to be an older open star cluster, while the bright stars embedded in NGC 7129 are perhaps a few million years young. The telltale reddish crescent shapes around NGC 7129 are associated with energetic jets streaming away from newborn stars.

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Ann
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Re: APOD: NGC 7129 and NGC 7142 (2019 Aug 30)

Post by Ann » Fri Aug 30, 2019 6:56 am

Today's APOD is a very nice picture! :D And as always, it prompts my desire to find pictures of "similar" phenomena in order to shed more light on the properties of the objects that are highlighted in the APOD.

NGC 7129. Robert Gendler, Roberto Colombari.
Additional data: Roberto Colombari, Eric Recurt, Adam Block.
Subaru Telescope.























As for NGC 7129, it is a region of low-mass star formation. We can tell that the stars being born are not very massive, because the dominant color of the surrounding nebula is blue. That is because the stars being born are not massive and hot enough to ionize a big bright red hydrogen alpha nebula. On the other hand, outbursts of different kinds on the baby stars being born will result in small and often contorted little pieces of red nebulosity. The dominant blue color is reflection nebulosity, caused by the blue light of stars a little more massive than the Sun being scattered in the dusty natal cloud, which has not yet been dispersed by the stellar winds of the stars.

A site of massive star formation like M17, by contrast, will be surrounded by a huge bright cloud of red nebulosity, caused by the onslaught of ultraviolet photons from the hot massive stars tearing electrons from protons in the surrounding dust and gas, after which the electrons one by one emit a hydrogen alpha photon as the they settle one by one into their proper electron shell around a proton again.

NGC 7142. Donald P. Waid/Waid Observatory.


















As for NGC 7142, I agree that it looks a bit old. NGC 7142 is clearly a rich cluster, because is contains many stars. It is far richer than, say, the Hyades (the photo can be found here). Nevertheless, for such a relatively rich cluster, NGC 7142 appears to be a bit loose. Its stars have had some time to drift apart. Like the Hyades, the brightest stars of NGC 7142 appear to be F- and K-type stars.

Compare NGC 7142 with M11. M11 is extremely rich, and while it contains many evolved stars, it also home to at least one star of spectral class B8. The overall color of M11 is very much bluer than the rich stellar background of the Scutum star cloud, seen here in a photo by Chris Deforeit of Astrim. According to Wikipedia, M11 is thought to be 316±50 million years old, while the Hyades cluster is about twice that old.

Old globular cluster NGC 5053. Photo: Bob Franke.
Finally, take a look at an even older cluster, globular cluster NGC 5053. NGC 5053 is very loose indeed for a globular. Nevertheless, note that NGC 5053 contains three main types of stars: The bright red giants, the moderately bright blue horizontal branch stars, and a multitude of small main sequence stars. This is typical of globular clusters, even though not all globulars contain the blue horizontal branch stars. Check out the full resolution version of Bob Franke's portrait of NGC 5053.

There are really important things that you can conclude about the nature of a star cluster or a nebual just by looking at a good photograph of it.

Ann
Last edited by Ann on Sat Aug 31, 2019 4:28 am, edited 1 time in total.
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Alex_g444

Re: APOD: NGC 7129 and NGC 7142 (2019 Aug 30)

Post by Alex_g444 » Fri Aug 30, 2019 7:43 am

So the physics of emission nebulae is close to what we see on Earth when sun light photons diffuse in the atmosphere, with the exception that in emission nebulae the diffusive medium is dust. Does that mean that, if could travel to NGC 7129 with a space ship, we would see on arrival there an immense blue light like in our sky ?

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Re: APOD: NGC 7129 and NGC 7142 (2019 Aug 30)

Post by Boomer12k » Fri Aug 30, 2019 10:02 am

What amazing contrasts between so apparently close objects...
A good image...

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Re: APOD: NGC 7129 and NGC 7142 (2019 Aug 30)

Post by Ann » Fri Aug 30, 2019 11:01 am

Alex_g444 wrote:
Fri Aug 30, 2019 7:43 am
So the physics of emission nebulae is close to what we see on Earth when sun light photons diffuse in the atmosphere, with the exception that in emission nebulae the diffusive medium is dust. Does that mean that, if could travel to NGC 7129 with a space ship, we would see on arrival there an immense blue light like in our sky ?
A cosmic grain of dust.
Rayleigh scattering in the Earth's atmosphere.





















Basically the atmosphere of the Earth contains molecules of just the right size to preferentially scatter short-wave light photons over long-wave light photons, which is why the Earth's sky looks blue to us. This is called Rayleigh scattering.

So in the Earth's atmosphere, it is the gaseous molecules that scatter blue light and make the Earth's sky look blue. In space, it is dust grains of the right size that preferentially scatter blue light and create the typical blue reflection nebulas.

The Rho Ophiuchi cloud complex.
Photo: Takayuki Yoshida
Consider the colorful Rho Ophiuchi cloud complex. Rho Ophiuchi is the multiple star at upper right surrounded by a large blue reflection nebula. All the three stars that you can see clumped together are hot B-type stars, which produce lots and lots of blue-light photons. These are scattered by the dusty nebula that is particularly obvious in the upper part of the picture. In turn, this gives rise to a large and relatively bright blue reflection nebula. There is also another blue reflection nebula in the picture, at center left, surrounding another B-type star, 22 Scorpii.

But what about the huge yellowish patch of nebulosity in the middle of the picture?

There are several components that give this yellow nebula its color, among them dust reddening that makes the nebula extra orange-brown in its upper parts. Primarily, though, this yellow patch is a yellow reflection nebula, lit up by the light of red supergiant Antares (below center in the image). Antares is so cool that it hardly produces any blue light at all, which is why Antares can never give rise to a blue reflection nebula. But Antares is extremely bright, and some of its yellow light is scattered by the dust around it.


















So what about the patches of pink nebulosity at right and at lower left? They have nothing to do with dust, but are caused by the ionization of hydrogen.

Hydrogen, as you know, is the simplest and lightest element in nature. It consists of a proton and an electron that is in orbit around the proton. The electron is normally found in its lowest electron shell, the shell that is called 1n in the picture at right.

There are blue stars located right next to or inside the pink nebulas in the picture at left. Consider the blue star at middle right and the one at lower left. These are hot stars, the one at right is spectral class B1III+B1V, and the other one is spectral class B0V. They produce enough ultraviolet photons to knock electrons in hydrogen atoms from one electron shell to another. When an electron is hit by an ultraviolet photon and knocked from electron shell one to electron shell two, it will soon fall back to electron shell one again. And when it does so, it emits a photon of red light at 656 nm. Occasionally an electron is knocked into shell number three, and when it falls back to shell number one it emits a blue-green photon of light at 486 nm. Together the red light at 656 nm and the blue-green light of 486 nm creates the pink hydrogen emission nebulas that are relatively common in the sky.

What about Rho Ophiuchi, the multiple blue star that has given this cloud complex its name? The blue stars of Rho Ophiuchi are just a tad too cool to fail to ionize hydrogen. Their blue light is scattered by dust, but they fail to knock many electrons out of their electron shells. That's why the nebula at top of the picture is blue, not pink.

So if we could magically travel to Rho Ophiuchi, would we see the large blue reflection nebula surrounding Rho Ophiuchi itself, or the yellow nebula surrounding Antares? Would we be able to see the pink color of emission nebulas for ourselves?

I would say no. These nebulas are faint, and the clouds of gas and dust that scatter or emit their colors are extremely thin. Compared with the thickness of the Earth's atmosphere, most nebulas in space can be considered hard vacuums. So we would see nothing but the bright stars themselves if we were able to travel to the Rho Ophiuchi complex, or, at best, we would see the nebulas as a faint grey fog.

Ann
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Re: APOD: NGC 7129 and NGC 7142 (2019 Aug 30)

Post by sc02492 » Fri Aug 30, 2019 11:17 am

Thanks for the interesting discussion regarding my image. I would point out that the faint pink ribbon in the center of NGC 7129 doesn't seem to emit much in the way of pure Ha signal. In fact, I initially took about 4 hours worth of Ha in this field and didn't use it at all in this image (it's purely LRGB), since there was hardly any signal. In contrast, the broadband red filter channel showed this pink ribbon very well, and I believe that other studies have shown it to represent photoluminescence in a broader range of red light (i.e., not just Ha), emitted by dust grains that are excited by UV irradiation. This is similar to what might be seen in the center of the Iris Nebula, for instance. Steve

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Re: APOD: NGC 7129 and NGC 7142 (2019 Aug 30)

Post by orin stepanek » Fri Aug 30, 2019 12:05 pm

I really liked the reflection nebula NGC 7129; very colorful! Star clusters are always interesting because of their proximity to each other! 8-) Sol lives in the rural areas of space in comparison! :mrgreen:
Orin

Smile today; tomorrow's another day!

Alex_g444

Re: APOD: NGC 7129 and NGC 7142 (2019 Aug 30)

Post by Alex_g444 » Fri Aug 30, 2019 9:20 pm

Ann wrote:
Fri Aug 30, 2019 11:01 am
Alex_g444 wrote:
Fri Aug 30, 2019 7:43 am
So the physics of emission nebulae is close to what we see on Earth when sun light photons diffuse in the atmosphere, with the exception that in emission nebulae the diffusive medium is dust. Does that mean that, if could travel to NGC 7129 with a space ship, we would see on arrival there an immense blue light like in our sky ?
A cosmic grain of dust.
Rayleigh scattering in the Earth's atmosphere.





















Basically the atmosphere of the Earth contains molecules of just the right size to preferentially scatter short-wave light photons over long-wave light photons, which is why the Earth's sky looks blue to us. This is called Rayleigh scattering.

So in the Earth's atmosphere, it is the gaseous molecules that scatter blue light and make the Earth's sky look blue. In space, it is dust grains of the right size that preferentially scatter blue light and create the typical blue reflection nebulas.

The Rho Ophiuchi cloud complex.
Photo: Takayuki Yoshida
Consider the colorful Rho Ophiuchi cloud complex. Rho Ophiuchi is the multiple star at upper right surrounded by a large blue reflection nebula. All the three stars that you can see clumped together are hot B-type stars, which produce lots and lots of blue-light photons. These are scattered by the dusty nebula that is particularly obvious in the upper part of the picture. In turn, this gives rise to a large and relatively bright blue reflection nebula. There is also another blue reflection nebula in the picture, at center left, surrounding another B-type star, 22 Scorpii.

But what about the huge yellowish patch of nebulosity in the middle of the picture?

There are several components that give this yellow nebula its color, among them dust reddening that makes the nebula extra orange-brown in its upper parts. Primarily, though, this yellow patch is a yellow reflection nebula, lit up by the light of red supergiant Antares (below center in the image). Antares is so cool that it hardly produces any blue light at all, which is why Antares can never give rise to a blue reflection nebula. But Antares is extremely bright, and some of its yellow light is scattered by the dust around it.


















So what about the patches of pink nebulosity at right and at lower left? They have nothing to do with dust, but are caused by the ionization of hydrogen.

Hydrogen, as you know, is the simplest and lightest element in nature. It consists of a proton and an electron that is in orbit around the proton. The electron is normally found in its lowest electron shell, the shell that is called 1n in the picture at right.

There are blue stars located right next to or inside the pink nebulas in the picture at left. Consider the blue star at middle right and the one at lower left. These are hot stars, the one at right is spectral class B1III+B1V, and the other one is spectral class B0V. They produce enough ultraviolet photons to knock electrons in hydrogen atoms from one electron shell to another. When an electron is hit by an ultraviolet photon and knocked from electron shell one to electron shell two, it will soon fall back to electron shell one again. And when it does so, it emits a photon of red light at 656 nm. Occasionally an electron is knocked into shell number three, and when it falls back to shell number one it emits a blue-green photon of light at 486 nm. Together the red light at 656 nm and the blue-green light of 486 nm creates the pink hydrogen emission nebulas that are relatively common in the sky.

What about Rho Ophiuchi, the multiple blue star that has given this cloud complex its name? The blue stars of Rho Ophiuchi are just a tad too cool to fail to ionize hydrogen. Their blue light is scattered by dust, but they fail to knock many electrons out of their electron shells. That's why the nebula at top of the picture is blue, not pink.

So if we could magically travel to Rho Ophiuchi, would we see the large blue reflection nebula surrounding Rho Ophiuchi itself, or the yellow nebula surrounding Antares? Would we be able to see the pink color of emission nebulas for ourselves?

I would say no. These nebulas are faint, and the clouds of gas and dust that scatter or emit their colors are extremely thin. Compared with the thickness of the Earth's atmosphere, most nebulas in space can be considered hard vacuums. So we would see nothing but the bright stars themselves if we were able to travel to the Rho Ophiuchi complex, or, at best, we would see the nebulas as a faint grey fog.

Ann
Thank you very much Ann for this detailed explanation. I made a mistake/mispelling in my original question: of course I was mentionning reflection nebulae, not emission, you corrected it. So if I follow you well the perceived brightness of reflection nebulae at our distance comes from an integration along the path of sight in a rare diffusive medium. Fascinating !

Alex

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Re: APOD: NGC 7129 and NGC 7142 (2019 Aug 30)

Post by Ann » Sat Aug 31, 2019 5:19 am

Alex_g444 wrote:
Fri Aug 30, 2019 9:20 pm

Thank you very much Ann for this detailed explanation. I made a mistake/mispelling in my original question: of course I was mentionning reflection nebulae, not emission, you corrected it. So if I follow you well the perceived brightness of reflection nebulae at our distance comes from an integration along the path of sight in a rare diffusive medium. Fascinating !

Alex

Thanks, Alex. Your original question was, would we see what looked like an immense blue sky if we could magically travel to NGC 7129? I said no, because the nebula surrounding the stars would be too thin. Then again, the nebulosity surrounding NGC 7129 is probably thicker than the nebulosity surrounding Rho Ophiuchi, and a thicker reflection nebula might scatter more blue light our way. But it's tricky. If the dusty reflection nebula is too thick, it will start scatter light away from us, and then the blue light will be the first to go. Consider the appearance of our Milky Way galaxy as seen from our own vantage point. The dark dust band that we see bisecting the Milky Way is so thick that it completely blocks all optical light from the center of our galaxy, so we have to look at the center of our galaxy using infrared light.

The central dust band of the Milky Way.
Photo: Ivan Alvarado.
Infrared view of the dust band and center of the Milky Way.
ESO/VVV Survey/D. Minniti
Acknowledgement: Ignacio Toledo, Martin Kornmesser




















But back to your original question. Would it ever be theoretically possible for us to see a reflection nebula in space as blue with our own eyes?

Star formation in Corona Australis.
Franz Hofmann, Wolfgang Paech.
Overpoweringly bright Sun.
Hinode/JAXA/NASA



















Well, maybe maybe we just might see some blue light in a reflection nebula. Consider the picture of star formation in Corona Australis. There are two reflection nebulas in that picture, one at upper left, one at lower right. The one at lower right is far, far too thin for us to ever see blue light in it with our own eyes. But what about the reflection nebula at upper left? It is thick enough that the stars just barely peek out of it.

The problem is that in order to see blue light in that nebula, we would have to look straight at the stars inside! At least if we approached this nebula from outside. If we came close enough to see the nebula well, the stars inside would still be so bright that their light would probably blind us. Remember that when we look at the blue sky on the Earth, we look away from the Sun, not straight at it. And getting very close to the stars, so that we could watch the nebula while watching away from the stars inside, we would get a hefty dose of radiation from the hot stars inside. They are hotter and more massive than the Sun.

But maybe, maybe, maybe it would be at least theoretically possible to see blue light in a reflection nebula in space.

Ann
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Re: APOD: NGC 7129 and NGC 7142 (2019 Aug 30)

Post by Ann » Sat Aug 31, 2019 5:38 am

sc02492 wrote:
Fri Aug 30, 2019 11:17 am
Thanks for the interesting discussion regarding my image. I would point out that the faint pink ribbon in the center of NGC 7129 doesn't seem to emit much in the way of pure Ha signal. In fact, I initially took about 4 hours worth of Ha in this field and didn't use it at all in this image (it's purely LRGB), since there was hardly any signal. In contrast, the broadband red filter channel showed this pink ribbon very well, and I believe that other studies have shown it to represent photoluminescence in a broader range of red light (i.e., not just Ha), emitted by dust grains that are excited by UV irradiation. This is similar to what might be seen in the center of the Iris Nebula, for instance. Steve

NGC 7142 with a red ribbon across it. Photo: Tom Matheson.
Yes, I saw that red band of light in other pictures of NGC 7142. You say you didn't get any signal to speak of when you took 4 hours worth of Hα? Interesting. I think, like you do, that we are seeing a lower form of ionization than Hα. It could be that we are seeing, say, ionized sulphur, SII. Ionized sulphur is red too, just like but Hα, but it represents a lower form of ionization.

I would guess, too, that this red ribbon near NGC 7142 has nothing to do with the mature star cluster, but that it is associated with the energies produced by the young site of star formation, NGC 7129. Consider Barnard's Loop in Orion! Barnard's Loop is certainly associated with the hot young stars and the ongoing star formation in Orion.

Ann
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