APOD: Reflections of the Ghost Nebula (2020 Oct 26)

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APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by APOD Robot » Mon Oct 26, 2020 4:05 am

Image Reflections of the Ghost Nebula

Explanation: Do any shapes seem to jump out at you from this interstellar field of stars and dust? The jeweled expanse, filled with faint, starlight-reflecting clouds, drifts through the night in the royal constellation of Cepheus. Far from your own neighborhood on planet Earth, these ghostly apparitions lurk along the plane of the Milky Way at the edge of the Cepheus Flare molecular cloud complex some 1,200 light-years away. Over two light-years across and brighter than the other spooky chimeras, VdB 141 or Sh2-136 is also known as the Ghost Nebula, seen at toward the bottom of the featured image. Within the reflection nebula are the telltale signs of dense cores collapsing in the early stages of star formation.

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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by Ann » Mon Oct 26, 2020 4:46 am

I've recently talked about yellow reflection nebulas. Well, the Ghost Nebula is obviously another jaundiced cloud complex in the cosmos.

I'll leave the discussion of ghosts to other members of Starship Asterisk*. 👻 💀 🎃

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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by Chris Peterson » Mon Oct 26, 2020 4:55 am

Ann wrote:
Mon Oct 26, 2020 4:46 am
I've recently talked about yellow reflection nebulas. Well, the Ghost Nebula is obviously another jaundiced cloud complex in the cosmos.

I'll leave the discussion of ghosts to other members of Starship Asterisk*. 👻 💀 🎃
Just brown dust reflecting white light. What we generally see with dusty molecular clouds.
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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by Ann » Mon Oct 26, 2020 5:55 am

Chris Peterson wrote:
Mon Oct 26, 2020 4:55 am
Ann wrote:
Mon Oct 26, 2020 4:46 am
I've recently talked about yellow reflection nebulas. Well, the Ghost Nebula is obviously another jaundiced cloud complex in the cosmos.

I'll leave the discussion of ghosts to other members of Starship Asterisk*. 👻 💀 🎃
Just brown dust reflecting white light. What we generally see with dusty molecular clouds.
So... a yellow reflection nebula. :wink:

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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by Ann » Mon Oct 26, 2020 9:05 am

Chris Peterson wrote:
Mon Oct 26, 2020 4:55 am
Ann wrote:
Mon Oct 26, 2020 4:46 am
I've recently talked about yellow reflection nebulas. Well, the Ghost Nebula is obviously another jaundiced cloud complex in the cosmos.

I'll leave the discussion of ghosts to other members of Starship Asterisk*. 👻 💀 🎃
Just brown dust reflecting white light. What we generally see with dusty molecular clouds.


Brown dust reflecting white light usually creates a blue reflection nebula. Our Sun, which is white, can have its colors separated into blue and yellow hues by gas, dust and droplets of water in the Earth's atmosphere.


Nebulas in the Blue Horse.png
IC 4592: The Blue Horsehead Reflection Nebula.
Image Credit & Copyright: Mario Cogo











A very interesting patch of nebulas in the sky can be seen in and around blur reflection IC 4592. The blue color of this nebula is due to dust particles scattering light from the intrinsically very blue star Nu Scorpii, spectral type B2IV. I have marked this nebula with the number 1 in the image at left.

Yes, but there is also another, much smaller blue reflection nebula near the large reflection nebula surrounding Nu Scorpii. I have marked this other blue nebula with the number 3. It surrounds a small group of stars, of which two are visible. These stars are not at all as hot as Nu Scorpii, as they belong to spectral classes B9V and A0V. Therefore they don't produce nearly as much blue light as Nu Scorpii, but still enough for dust particles in the vicinity to scatter some of their light into a blue reflection nebula, called IC 4601.

Yes, but there is a third reflection nebula in this picture, a mildly yellow one. I have marked it with the number 2. This patch of light has a proper nebula designation, van den Bergh 101, so it is a reflection nebula all right. It surrounds a K-type star, which is yellower than the Sun, so the nebula is non-blue and in fact mildly yellow. It is the light from this K-type star that has been scattered by dust particles in the vicinity and produced the nebula.

Blue Horsehead Antares Rho Ophiuchi and the Milky Way.png
The Blue Horsehead (top right), Rho Ophiuchi and Antares (center right)
and the yellowish plane of the Milky Way (left). Photo: Mohammad Rahimi.
We may also note that some of the dust forming the shape of the Blue Horsehead nebula is indeed just brown, yet it appears to be slightly lit up. What is the light source for this faintly illuminated brown dust? Perhaps scattered light from the mostly yellowish disk of the Milky Way itself?

Ann
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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by VictorBorun » Mon Oct 26, 2020 9:32 am

I wonder what on Earth is making the cone of light in the bottom right corner
Image

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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by neufer » Mon Oct 26, 2020 1:45 pm


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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by Chris Peterson » Mon Oct 26, 2020 1:54 pm

Ann wrote:
Mon Oct 26, 2020 9:05 am
Chris Peterson wrote:
Mon Oct 26, 2020 4:55 am
Ann wrote:
Mon Oct 26, 2020 4:46 am
I've recently talked about yellow reflection nebulas. Well, the Ghost Nebula is obviously another jaundiced cloud complex in the cosmos.

I'll leave the discussion of ghosts to other members of Starship Asterisk*. 👻 💀 🎃
Just brown dust reflecting white light. What we generally see with dusty molecular clouds.
Brown dust reflecting white light usually creates a blue reflection nebula. Our Sun, which is white, can have its colors separated into blue and yellow hues by gas, dust and droplets of water in the Earth's atmosphere.
No, dust in molecular clouds rarely creates blue reflection nebulas. They are almost always brown, no matter what color the surrounding stars are. In such dust clouds, we may see patches of blue immediately around blue stars, but the larger nebula will be brown. Blue reflection nebulas are usually produced by very tenuous dust, not the much denser dust clouds found in molecular clouds, where the high density of hydrogen creates a strong gravitational gradient that concentrates dust. Optically, tenuous nebulas scatter light, while dense ones reflect it.

Observationally, tenuous, transparent nebulas will usually be blue (assuming blue stars are present), and dense, opaque nebulas will usually be brown.
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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by Ann » Mon Oct 26, 2020 3:47 pm

Chris Peterson wrote:
Mon Oct 26, 2020 1:54 pm

No, dust in molecular clouds rarely creates blue reflection nebulas. They are almost always brown, no matter what color the surrounding stars are. In such dust clouds, we may see patches of blue immediately around blue stars, but the larger nebula will be brown. Blue reflection nebulas are usually produced by very tenuous dust, not the much denser dust clouds found in molecular clouds, where the high density of hydrogen creates a strong gravitational gradient that concentrates dust. Optically, tenuous nebulas scatter light, while dense ones reflect it.

Observationally, tenuous, transparent nebulas will usually be blue (assuming blue stars are present), and dense, opaque nebulas will usually be brown.
Well, that agrees with what David Malin told me many years ago: Near hot stars, the particles of a nebula are "cooked" to a smaller size. So I can see your point there.

40968096974_dc107c6028_b[1].jpg
van den Bergh 101, 102 and 103 nebulas. Photo: Dan Crowson.
But what do you think about the yellowish nebula surrounding K-type star HD 146834? That nebula has a "nebula designation", vdB 101.

Isn't that a reflection nebula, in your opinion?

Ann
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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by johnnydeep » Mon Oct 26, 2020 4:47 pm

VictorBorun wrote:
Mon Oct 26, 2020 9:32 am
I wonder what on Earth is making the cone of light in the bottom right corner
Image
Nothing on earth, that's for sure!
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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by johnnydeep » Mon Oct 26, 2020 4:49 pm

So does the cloud in the top half of this image have a name? It's not shown in the linked pics of the Ghost Nebula proper.
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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by Ann » Mon Oct 26, 2020 7:32 pm

johnnydeep wrote:
Mon Oct 26, 2020 4:47 pm
VictorBorun wrote:
Mon Oct 26, 2020 9:32 am
I wonder what on Earth is making the cone of light in the bottom right corner
Image
Nothing on earth, that's for sure!
The APOD caption said:
Within the reflection nebula are the telltale signs of dense cores collapsing in the early stages of star formation.
So I guess it is at least possible that we are seeing the remains of a hole punched through the nebula by a Herbig-Haro jet emitted by a not yet fully formed star deep inside the nebula.

Either that, or a star that was just passing by plunged right through the nebula, leaving a hole behind.

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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by VictorBorun » Mon Oct 26, 2020 8:20 pm

Ann wrote:
Mon Oct 26, 2020 7:32 pm
So I guess it is at least possible that we are seeing the remains of a hole punched through the nebula by a Herbig-Haro jet emitted by a not yet fully formed star deep inside the nebula.

Either that, or a star that was just passing by plunged right through the nebula, leaving a hole behind.

Ann
wow
Went to read on Herbig-Haro objects and bow shocks.
If the jet is 1000 years old and is flowing at 300 km/s = 1/1000 c, then the jet should be 1 light years long, quite plausible here.

And the Dust Pillar of the Carina Nebula is a beauty and a model for two-eyed dusty head interpretation.

On the other hand, if the star at the vertex of the bow shock were the culprit, it should move away from us uncanny fast. And that movement would show as an uncanny red shift, wouldn't it?

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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by Ann » Tue Oct 27, 2020 5:17 am

VictorBorun wrote:
Mon Oct 26, 2020 8:20 pm

wow
Went to read on Herbig-Haro objects and bow shocks.
If the jet is 1000 years old and is flowing at 300 km/s = 1/1000 c, then the jet should be 1 light years long, quite plausible here.

And the Dust Pillar of the Carina Nebula is a beauty and a model for two-eyed dusty head interpretation.

On the other hand, if the star at the vertex of the bow shock were the culprit, it should move away from us uncanny fast. And that movement would show as an uncanny red shift, wouldn't it?



















Actually, I don't believe that the star seemingly at the vertex of the bow shock in the Ghost Nebula is anything else than a background object, and I don't believe that it has plunged through the nebula.

I think that the nebula would be very disturbed, full of shocks and eddies, if a star had plunged right through it. Look at the picture at right. Speeding runaway star AE Aurigae has run straight into a molecular cloud during its rush through space. As it entered the cloud, hot O9.5-type star AE Aurigae ionized hydrogen in the cloud, turning it into a red emission nebula. But AE Aurigae also lit up dusty regions in the cloud, making them reflect the blue light of AE Aurigae itself.

But note how disturbed the nebula surrounding AE Aurigae is, and how "calm" the Ghost Nebula is. I don't think the Ghost Nebula has had a star run right through it.



























But take a look at this nebula, NGC 1999, at left, one of the weirdest nebulas in the sky. Astronomers long thought that the pitch-black thing located right next to the illuminating star, multiple star V380 Orionis, was a so called Bok globule. A Bok globule is an evaporating dust cloud floating around in a nebula, usually inside a red emission nebula near very hot stars. In the picture at right, you can see several Bok globules floating around inside nebula IC 2944. This nebula is ionized by several very hot O-type stars with temperatures well over 30,000 K.

Yes, but the pitch-dark spot right next to the blue star illuminating blue reflection nebula NGC 1999 doesn't look like a dust cloud, does it? It looks like a hole that has been punched right through the nebula. And astronomers now agree that it is a hole.
Wikipedia wrote about nebula NGC 1999:

NGC 1999 is a dust-filled bright nebula with a vast hole of empty space represented by a black patch of sky, as can be seen in the photograph. It is a reflection nebula, and shines from the light of the variable star V380 Orionis.

It was previously believed that the black patch was a dense cloud of dust and gas which blocked light that would normally pass through, called a dark nebula. (...)

With support from ground-based observations done using the submillimeter bolometer cameras on the Atacama Pathfinder Experiment radio telescope (November 29, 2009) and the Mayall (Kitt Peak) and Magellan telescopes (December 4, 2009), it was determined that the patch looks black not because it is an extremely dense pocket of gas, but because it is truly empty.

The exact cause of this phenomenon is still being investigated, although it has been hypothesized that narrow jets of gas from some of the young stars in the region punctured the sheet of dust and gas (...)
So my best guess is that the strange "wake" and hole in the Ghost Nebula have also been caused by a jet.

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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by VictorBorun » Tue Oct 27, 2020 4:12 pm

I begin to see the picture.
A cloud is vulnerable… A Herbig-Haro jet punches right through it, a visiting star rumples it up.
A dense Bok globule will stay cool for a while but evaporate eventually if boiled by several large stars long enough.

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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by Ann » Thu Oct 29, 2020 7:50 am

Chris Peterson wrote:
Mon Oct 26, 2020 1:54 pm
Ann wrote:
Mon Oct 26, 2020 9:05 am
Chris Peterson wrote:
Mon Oct 26, 2020 4:55 am


Just brown dust reflecting white light. What we generally see with dusty molecular clouds.
Brown dust reflecting white light usually creates a blue reflection nebula. Our Sun, which is white, can have its colors separated into blue and yellow hues by gas, dust and droplets of water in the Earth's atmosphere.
No, dust in molecular clouds rarely creates blue reflection nebulas. They are almost always brown, no matter what color the surrounding stars are. In such dust clouds, we may see patches of blue immediately around blue stars, but the larger nebula will be brown. Blue reflection nebulas are usually produced by very tenuous dust, not the much denser dust clouds found in molecular clouds, where the high density of hydrogen creates a strong gravitational gradient that concentrates dust. Optically, tenuous nebulas scatter light, while dense ones reflect it.

Observationally, tenuous, transparent nebulas will usually be blue (assuming blue stars are present), and dense, opaque nebulas will usually be brown.
To all you Asteriskian*s out there: Chris knows almost everything about astronomy better than me. Except color. He doesn't, I swear.

So, long story short: Are there yellow reflection nebulas? Answer: Yes. There are.
Wikipedia wrote:

Reflection nebulae are usually blue because the scattering is more efficient for blue light than red (this is the same scattering process that gives us blue skies and red sunsets).
...
The giant star Antares, which is very red (spectral class M1), is surrounded by a large, red reflection nebula.
18_Rho-Antares-Complex-Wide_2015_Zeiss135_60D_1024B[1].jpg
The Antares-Rho Ophiuchi nebular complex. Antares is surrounded
by a yellow reflection nebula. Photo: Mario Cogo.
So Wikipedia states clearly that Antares is surrounded by a reflection nebula. Wikipedia calls the Antares nebula red, which is of course totally wrong. Emission nebulas are red, and there are two of them next to the Antares-Rho Ophiuchi nebular complex, one at bottom center, one at right. But the Antares nebula itself is very, very obviously yellow.



















Anyway, here's my point again: Yes, yellow reflection nebulas exist. They really do.

And by the way, did you know that the light from an old incandescent light bulb is as red as the light from Antares? No wonder rooms illuminated by such light bulbs look all orange. At least yellow-orange.

Ann
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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by Chris Peterson » Thu Oct 29, 2020 12:51 pm

Ann wrote:
Thu Oct 29, 2020 7:50 am
Chris Peterson wrote:
Mon Oct 26, 2020 1:54 pm
Ann wrote:
Mon Oct 26, 2020 9:05 am

Brown dust reflecting white light usually creates a blue reflection nebula. Our Sun, which is white, can have its colors separated into blue and yellow hues by gas, dust and droplets of water in the Earth's atmosphere.
No, dust in molecular clouds rarely creates blue reflection nebulas. They are almost always brown, no matter what color the surrounding stars are. In such dust clouds, we may see patches of blue immediately around blue stars, but the larger nebula will be brown. Blue reflection nebulas are usually produced by very tenuous dust, not the much denser dust clouds found in molecular clouds, where the high density of hydrogen creates a strong gravitational gradient that concentrates dust. Optically, tenuous nebulas scatter light, while dense ones reflect it.

Observationally, tenuous, transparent nebulas will usually be blue (assuming blue stars are present), and dense, opaque nebulas will usually be brown.
To all you Asteriskian*s out there: Chris knows almost everything about astronomy better than me. Except color. He doesn't, I swear.

So, long story short: Are there yellow reflection nebulas? Answer: Yes. There are.
I'm not sure where you got the idea I suggested there aren't. Brown and yellow are pretty much the same "color", just at different intensities.
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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by neufer » Thu Oct 29, 2020 1:32 pm

Chris Peterson wrote:
Thu Oct 29, 2020 12:51 pm

Brown and yellow are pretty much the same "color", just at different intensities.
Brown and orange are the same "color", just at different intensities.
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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by Chris Peterson » Thu Oct 29, 2020 1:41 pm

neufer wrote:
Thu Oct 29, 2020 1:32 pm
Chris Peterson wrote:
Thu Oct 29, 2020 12:51 pm

Brown and yellow are pretty much the same "color", just at different intensities.
Brown and orange are the same "color", just at different intensities.
Yes, certainly.

When we're talking emission nebulas, "color" (a physiological phenomenon) is closely related to simple wavelength (although it is still complex, because color is a function of intensity as well). But reflection nebula? Now we're talking about colored particles which are both reflecting and scattering colored continuum light sources. And all of this altered, usually significantly, by imaging, which captures something very different from what the eye sees. So when it comes to reflection nebulas, red/orange/yellow/brown... all are largely the same thing, depending on how the image was exposed, how it was processed, and the labels that people have learned to assign to the colors they perceive.
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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by VictorBorun » Sun Nov 01, 2020 3:09 pm

Still there is a difference between the cold, or blue and warm, or orange part of the low-sun sky on Earth. And the same separation mechanism gives unmistakable colors to any scattering nebula once we set the angles and the intensity of the scattering.
Do I get it right: if an illuminating star is
(i) in front of the scattering nebula, then the nebula is blue
(ii) behind the scattering nebula, then the nebula is orange
(iii) besides the scattering nebula, then the nebula is blue to silver to orange where the list starts from the part of the scattering nebula which is visually closer to the star i.e. farther away from us?

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Re: APOD: Reflections of the Ghost Nebula (2020 Oct 26)

Post by Chris Peterson » Sun Nov 01, 2020 10:49 pm

VictorBorun wrote:
Sun Nov 01, 2020 3:09 pm
Still there is a difference between the cold, or blue and warm, or orange part of the low-sun sky on Earth. And the same separation mechanism gives unmistakable colors to any scattering nebula once we set the angles and the intensity of the scattering.
Do I get it right: if an illuminating star is
(i) in front of the scattering nebula, then the nebula is blue
(ii) behind the scattering nebula, then the nebula is orange
(iii) besides the scattering nebula, then the nebula is blue to silver to orange where the list starts from the part of the scattering nebula which is visually closer to the star i.e. farther away from us?
Sometimes, but not always. The density of the nebula is a key factor. Molecular clouds, which are dense hydrogen clouds with a high dust content, are largely opaque, so don't have illuminating stars behind them. They are front lit and appear brown (brown/orange/yellow depending on the intensity and the image processing), because the dust is brown, and its reflected component is much brighter than any scatter. Optically thin nebulas will tend to redden light from behind them, and scatter blue from stars in front of them. And, of course, these colors are shifted somewhat depending on the color of the starlight itself.
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