Starship Asterisk*

by **Ann** » Sat Aug 28, 2021 7:02 pm

Chris Peterson wrote: ↑Sat Aug 28, 2021 1:24 pm
Ann wrote: ↑Sat Aug 28, 2021 5:08 am Filter channels should not normally be mapped as orange, violet or cyan, and absolutely not as brown. The reason for that is that our eyes have no receptors for orange, violet, cyan or brown. We have receptors for red, green and blue light, and all the colors that we see are a combination of stimulating our red-, green- and blue-sensitive receptors to various degrees.
Again, I feel it necessary to correct a lot of misunderstandings.

Color mapped images in general are of little scientific value. They might be simply aesthetic, or they might help us see interesting structure that warrants further examination in the individual data channels. The colors chosen are arbitrary (other than aesthetics). There is no reason they should correspond to the order of the wavelengths in the original data. The goal is to present information, and the careful choice of colors can make more structure apparent.

Our eyes do not have receptors for red, green, and blue light. They have what are called long, medium, and short wavelength receptors, which peak, respectively, in the yellow, cyan, and violet. We see color because of the large overlap in the sensitivity curves of these receptors. They do not match at all the red, green, and blue primaries commonly used for our display devices.

The only advantage to mapping three different data channels to red, green, and blue is that those channels can be separated again into their original data. If we map to any other color, it means we are mixing the data into two or more primary channels, and that's not a reversible operation. And, of course, if there are more than three channels of data, which is often the case, there's no choice but to mix them in a way that cannot be separated again.

On a display, orange and brown are not different hues. They are the same, with just a different intensity or saturation. In either case, you're just mapping your data into a mix of red and green.

: Click to view full size image

Wow. Thanks for the lecture, Chris. I feel as if I was getting bowled over by the power of your words. By the way, in the illustration I posted, our sexes are obviously reversed.

As for orange and brown being identical hues, just different degrees of saturation: Yes, I actually knew that, at least!

Ann

by **Chris Peterson** » Sat Aug 28, 2021 1:24 pm

Ann wrote: ↑Sat Aug 28, 2021 5:08 am Filter channels should not normally be mapped as orange, violet or cyan, and absolutely not as brown. The reason for that is that our eyes have no receptors for orange, violet, cyan or brown. We have receptors for red, green and blue light, and all the colors that we see are a combination of stimulating our red-, green- and blue-sensitive receptors to various degrees.

Again, I feel it necessary to correct a lot of misunderstandings.

Color mapped images in general are of little scientific value. They might be simply aesthetic, or they might help us see interesting structure that warrants further examination in the individual data channels. The colors chosen are arbitrary (other than aesthetics). There is no reason they should correspond to the order of the wavelengths in the original data. The goal is to present information, and the careful choice of colors can make more structure apparent.

Our eyes do not have receptors for red, green, and blue light. They have what are called long, medium, and short wavelength receptors, which peak, respectively, in the yellow, cyan, and violet. We see color because of the large overlap in the sensitivity curves of these receptors. They do not match at all the red, green, and blue primaries commonly used for our display devices.

The only advantage to mapping three different data channels to red, green, and blue is that those channels can be separated again into their original data. If we map to any other color, it means we are mixing the data into two or more primary channels, and that's not a reversible operation. And, of course, if there are more than three channels of data, which is often the case, there's no choice but to mix them in a way that cannot be separated again.

On a display, orange and brown are not different hues. They are the same, with just a different intensity or saturation. In either case, you're just mapping your data into a mix of red and green.

by **VictorBorun** » Sat Aug 28, 2021 11:12 am

Can't resist to apply the enhanced color rules to this particular set of narrow band channels:

673 nm ionized sulfur is a VIP channel
658 nm ionized nitrogen is a VIP channel
656 nm hydrogen alpha is a VIP channel
502 nm doubly ionized oxygen is a VIP channel
469 nm ionized helium is less important
373 nm ionized oxygen is less important

There are 5 source channels to spread evenly; 4 of them are VIPs and one is simply the shortest wavelength.
This five naturally maps to five RGB primaries:
RGB-red #ff0000 (630 nm, slightly orangish red)
RGB-yellow #ffff00 (570 nm, slightly orangish yellow)
RGB-green #00ff00 (532 nm, limish green)
RGB-cyan #00ffff (492 nm, slightly bluish cyan)
RGB-blue #0000ff (467 nm, violetish blue)

One channel then maps as to crowd with RGB-blue, not in the middle #0080ff, but to #0040ff.

Now there are brightness discriminations to apply:

673 nm ionized sulfur is a super VIP channel
656 nm hydrogen alpha is a sub VIP channel
469 nm ionized helium is not a VIP channel
373 nm ionized oxygen is not a VIP channel

So we have 4 levels of importance. Let's make them 4 none-zero levels of brightness:
ff A5 6E 37 00

These are total R+G+B brightnesses. So the result is:

super VIP 673 nm ionized sulfur to ███ RGB-red #ff0000 (630 nm, slightly orangish red)
just VIP 658 nm ionized nitrogen to ███ olive #525200 (570 nm, slightly orangish yellow)
sub VIP 656 nm hydrogen alpha to ███ dark RGB-green #006E00 (532 nm, limish green)
just VIP 502 nm doubly ionized oxygen to ███ teal #005252 (492 nm, slightly bluish cyan)
no VIP 469 nm ionized helium to ███ very dark cyanish blue #000B2C
no VIP 373 nm ionized oxygen to ███ very dark RGB-blue #000037 (467 nm, violetish blue)

by **Ann** » Sat Aug 28, 2021 7:54 am

Robert Eder wrote: ↑Sat Aug 28, 2021 7:12 am This is the Elephant's Trunk with stars: https://www.astrobin.com/6fy8mc/C/
It's a mix of RGB and HOO.

Robert Eder

Thanks a billion, Robert!

I hope you don't mind that I uploaded your "starry version of the Elephant's Trunk Nebula and Caravan" (don't know what the Caravan is), and posted it next to your starless version of the Elephant's Trunk and Caravan, just so people can compare the images.

I most certainly know which one I prefer!!!

Ann

by **tvhiggins2** » Sat Aug 28, 2021 7:53 am

But for those who prefer a starless Elephant's Trunk, I'll just switch off the star layer for them. It's just like turning off the lights...literally.

Like this: https://tvhiggins.com/wp-content/upload ... 664785.jpg

T.V Higgins

by **Robert Eder** » Sat Aug 28, 2021 7:12 am

This is the Elephant's Trunk with stars: https://www.astrobin.com/6fy8mc/C/
It's a mix of RGB and HOO.

Robert Eder

by **tvhiggins2** » Sat Aug 28, 2021 5:41 am

Chris Peterson wrote: ↑Sat Aug 28, 2021 4:21 am
VictorBorun wrote: ↑Sat Aug 28, 2021 1:37 am
Guest wrote: ↑Fri Aug 27, 2021 9:29 pm Amen, Ann. Starless images look like faces without eyebrows to me. They're just so unnatural looking, especially given that nebulae like Elephant's Trunk are all about star creation. Attached is the URL of a full-color image that I recently shot of Elephant's Trunk with the stars. It's done in the Hubble Pallet and is framed with a similar composition to today's APOD. I could have removed the stars--and in one preprocessing version of this photo I did just that--but I prefer to leave my eyebrows intact.

Tom Higgins

https://tvhiggins.com/wp-content/upload ... rked-1.jpg
Off topic.
I wonder why not use simple rules for enhanced color astrography:

1) never map your narrow-band images to hues outside the range of RGB-red to RGB-green to RGB-blue (that range for a modern UHDTV display is 630 nm to 532 nm to 467 nm). No mapping to pure red of ≥ 650 nm or pure violet of ≤ 440 nm! If you try to imitate pure red or pure violet in RGB image, you make your RGB color range narrower rather than wider. Your longest wavelength mapped hue should be exactly RGB-red and your shortest wavelength mapped hue should be exactly RGB-blue;

2) never map your narrow-band images of spectral lines λ₁ > λ₂ to RGB colors λ₁' < λ₂'. The wavelengths matter; longer waves make dust clouds more transparent. It's misleading to interchange the wavelengths. Therefore your narrow-band image of the longest wavelength should be mapped to RGB red and your narrow-band image of the shortest wavelength should be mapped to RGB blue;

3) feel free to slide all the other mappings within those ranges and with no breaking of wavelength order. It may pay well to spread all the mappings evenly, if you take into account what is evenly spread for a viewer: meanwaves cones are heavily used so colors with levels of the green subpixel are easily recognized, and such are red to yellow hues and cyan to blue hues. Yellow to green hues are poorly recognized because you have to register levels of the red subpixel with your longwave cones and green to cyan hues are recognized worst of all because you have to register levels of the blue subpixel with your rare shortwave cones.

4) as you may make brighter the mappings of your most informative narrow-band images, so you may make that same mappings more isolated in the range of the RGB hues. Let the mappings of less important narrow-band images be crowded. No need to merge them though; you won't save much contrast by denying those less important images a little space between their mapped hues
Scientific analysis of the images doesn't generally use any mapping at all. You seldom want to combine data channels into a single image. Mostly this is about aesthetics, or portraying some kind of structure as clearly as possible. In which case, there are no meaningful mapping rules other than what works.

The color-mapping scheme of the so-called Hubble Palette was the invention of astronomer Jeff Hester, who used it to create the HST image that came to be known as The Pillars of Creation. And the rest is history, as they say. If I had to guess why Dr. Hester chose this particular mapping scheme, I'd say it was because it provides good color separation for the emission lines of hydrogen and sulfur, which are both in the red and separated by only a few nanometers (656.2 & 672). Plus, images of star-forming regions look especially good in the Hubble Palette. So yeah, aesthetics, too. In any case, it has become a very popular narrowband mapping technique among astrophotographers, especially in light-polluted areas like the one I live in.

T.V. Higgins

by **Ann** » Sat Aug 28, 2021 5:08 am

VictorBorun wrote: ↑Fri Aug 27, 2021 9:50 pm
Guest wrote: ↑Fri Aug 27, 2021 9:29 pm Amen, Ann. Starless images look like faces without eyebrows to me. They're just so unnatural looking, especially given that nebulae like Elephant's Trunk are all about star creation. Attached is the URL of a full-color image that I recently shot of Elephant's Trunk with the stars. It's done in the Hubble Pallet and is framed with a similar composition to today's APOD. I could have removed the stars--and in one preprocessing version of this photo I did just that--but I prefer to leave my eyebrows intact.

Tom Higgins

https://tvhiggins.com/wp-content/upload ... rked-1.jpg
So the Hanging Stuff is connected to the Trunk in the 3d space, after all.
Moreover, it is now clear that there is a second Trunk, to the left of the Trunk, less favourably illuminated, but similarly bended, the whole composition like a hindu statue of a moving subject presenting two phases.

By the way, is the Hubble Pallet exactly this:

673nm ionized sulfur White
658nm ionized nitrogen Orange
656nm hydrogen alpha Brown
502nm doubly ionized oxygen Cyan
469nm ionized helium Blue
373nm ionized oxygen Violet

The Hubble palette is this:

673 nm, sulfur: Red.
658 nm, nitrogen: Red.
656 nm, hydrogen alpha: Green.
501 nm, doubly ionized oxygen: Blue.

The Hubble palette uses either ionized sulfur or ionized nitrogen, not both. Both wavelengths, 673 nm and 656 nm, look red to the human eye. You can say that they are mapped as red or shown as red.

656 nm looks very red to the human eye, but it is mapped as green in Hubble palette images.

501 nm looks green or greenish cyan to the human eye, but it is mapped as blue.

Black and white images taken through the same narrowband filters, 673 nm (SII), 656 nm (Hα) and 501 nm (OIII), can be mapped differently. I remember seeing an image where Hα was mapped as red (good, because it is red), OIII was mapped as green (pretty good, because it is greenish cyan) and SII was mapped as blue (as bad as it can be, because SII is very red). Mapping SII as blue made the image look absolutely wrong to me.

Filter channels should not normally be mapped as orange, violet or cyan, and absolutely not as brown. The reason for that is that our eyes have no receptors for orange, violet, cyan or brown. We have receptors for red, green and blue light, and all the colors that we see are a combination of stimulating our red-, green- and blue-sensitive receptors to various degrees.

Ann

by **Ann** » Sat Aug 28, 2021 4:40 am

Guest wrote: ↑Fri Aug 27, 2021 9:29 pm Amen, Ann. Starless images look like faces without eyebrows to me. They're just so unnatural looking, especially given that nebulae like Elephant's Trunk are all about star creation. Attached is the URL of a full-color image that I recently shot of Elephant's Trunk with the stars. It's done in the Hubble Pallet and is framed with a similar composition to today's APOD. I could have removed the stars--and in one preprocessing version of this photo I did just that--but I prefer to leave my eyebrows intact.

Tom Higgins

https://tvhiggins.com/wp-content/upload ... rked-1.jpg

Thanks a million, Tom!

I have to start by admitting that I am not the greatest fan of Hubble palette images, and I prefer RGB imagery for aesthetic reasons. But I do admit - yes, I do! - that the Hubble palette is far superior to RGB when it comes to bringing out details in an emission nebula.

I must say, too, that in my opinion your Hubble palette image (with stars intact) contains a lot more information about the Elephant's Trunk Nebula than Robert Eder's starless RGB APOD image does.

Ann

by **Ann** » Sat Aug 28, 2021 4:30 am

VictorBorun wrote: ↑Fri Aug 27, 2021 9:42 pm
Chris Peterson wrote: ↑Fri Aug 27, 2021 6:36 pm To the extent that there is some kind of "anti-gravity", it only applies across cosmological scales (as with the voids Art brought up). The scale of today's image, or indeed any image that contains optically visible bubbles, is nowhere near that. There is no anti-gravitational effect at these scales. Bubbles are simply the consequence of gas and dust blown outward by well understood forces.
Today's image is tan(2°)*3000 ly = 100 ly wide. An ordinary star's stellar wind bubble can not be as large as this but a Supernova's can.
Now are you saying that the Trunk is a cometary nebula, a tail of wind shadow cast by a dense globule?
And the Stuff 'hanging' from the Trunk further away from the blotted star — is it cometary too?

Click to view full size image

Thanks for combining the two images I posted, Victor! Very nice!

However, I'd say that yes, the "wind bubble" (or ionizing sphere) of a (pair of) hot O-type stars can certainly be tens of light-years wide.

Ann

by **Chris Peterson** » Sat Aug 28, 2021 4:21 am

VictorBorun wrote: ↑Sat Aug 28, 2021 1:37 am
Guest wrote: ↑Fri Aug 27, 2021 9:29 pm Amen, Ann. Starless images look like faces without eyebrows to me. They're just so unnatural looking, especially given that nebulae like Elephant's Trunk are all about star creation. Attached is the URL of a full-color image that I recently shot of Elephant's Trunk with the stars. It's done in the Hubble Pallet and is framed with a similar composition to today's APOD. I could have removed the stars--and in one preprocessing version of this photo I did just that--but I prefer to leave my eyebrows intact.

Tom Higgins

https://tvhiggins.com/wp-content/upload ... rked-1.jpg
Off topic.
I wonder why not use simple rules for enhanced color astrography:

1) never map your narrow-band images to hues outside the range of RGB-red to RGB-green to RGB-blue (that range for a modern UHDTV display is 630 nm to 532 nm to 467 nm). No mapping to pure red of ≥ 650 nm or pure violet of ≤ 440 nm! If you try to imitate pure red or pure violet in RGB image, you make your RGB color range narrower rather than wider. Your longest wavelength mapped hue should be exactly RGB-red and your shortest wavelength mapped hue should be exactly RGB-blue;

2) never map your narrow-band images of spectral lines λ₁ > λ₂ to RGB colors λ₁' < λ₂'. The wavelengths matter; longer waves make dust clouds more transparent. It's misleading to interchange the wavelengths. Therefore your narrow-band image of the longest wavelength should be mapped to RGB red and your narrow-band image of the shortest wavelength should be mapped to RGB blue;

3) feel free to slide all the other mappings within those ranges and with no breaking of wavelength order. It may pay well to spread all the mappings evenly, if you take into account what is evenly spread for a viewer: meanwaves cones are heavily used so colors with levels of the green subpixel are easily recognized, and such are red to yellow hues and cyan to blue hues. Yellow to green hues are poorly recognized because you have to register levels of the red subpixel with your longwave cones and green to cyan hues are recognized worst of all because you have to register levels of the blue subpixel with your rare shortwave cones.

4) as you may make brighter the mappings of your most informative narrow-band images, so you may make that same mappings more isolated in the range of the RGB hues. Let the mappings of less important narrow-band images be crowded. No need to merge them though; you won't save much contrast by denying those less important images a little space between their mapped hues

Scientific analysis of the images doesn't generally use any mapping at all. You seldom want to combine data channels into a single image. Mostly this is about aesthetics, or portraying some kind of structure as clearly as possible. In which case, there are no meaningful mapping rules other than what works.

by **VictorBorun** » Sat Aug 28, 2021 1:37 am

Guest wrote: ↑Fri Aug 27, 2021 9:29 pm Amen, Ann. Starless images look like faces without eyebrows to me. They're just so unnatural looking, especially given that nebulae like Elephant's Trunk are all about star creation. Attached is the URL of a full-color image that I recently shot of Elephant's Trunk with the stars. It's done in the Hubble Pallet and is framed with a similar composition to today's APOD. I could have removed the stars--and in one preprocessing version of this photo I did just that--but I prefer to leave my eyebrows intact.

Tom Higgins

https://tvhiggins.com/wp-content/upload ... rked-1.jpg

Off topic.
I wonder why not use simple rules for enhanced color astrography:

1) never map your narrow-band images to hues outside the range of RGB-red to RGB-green to RGB-blue (that range for a modern UHDTV display is 630 nm to 532 nm to 467 nm). No mapping to pure red of ≥ 650 nm or pure violet of ≤ 440 nm! If you try to imitate pure red or pure violet in RGB image, you make your RGB color range narrower rather than wider. Your longest wavelength mapped hue should be exactly RGB-red and your shortest wavelength mapped hue should be exactly RGB-blue;

2) never map your narrow-band images of spectral lines λ₁ > λ₂ to RGB colors λ₁' < λ₂'. The wavelengths matter; longer waves make dust clouds more transparent. It's misleading to interchange the wavelengths. Therefore your narrow-band image of the longest wavelength should be mapped to RGB red and your narrow-band image of the shortest wavelength should be mapped to RGB blue;

3) feel free to slide all the other mappings within those ranges and with no breaking of wavelength order. It may pay well to spread all the mappings evenly, if you take into account what is evenly spread for a viewer: meanwaves cones are heavily used so colors with levels of the green subpixel are easily recognized, and such are red to yellow hues and cyan to blue hues. Yellow to green hues are poorly recognized because you have to register levels of the red subpixel with your longwave cones and green to cyan hues are recognized worst of all because you have to register levels of the blue subpixel with your rare shortwave cones.

4) as you may make brighter the mappings of your most informative narrow-band images, so you may make that same mappings more isolated in the range of the RGB hues. Let the mappings of less important narrow-band images be crowded. No need to merge them though; you won't save much contrast by denying those less important images a little space between their mapped hues

by **Chris Peterson** » Fri Aug 27, 2021 9:56 pm

VictorBorun wrote: ↑Fri Aug 27, 2021 9:42 pm
Chris Peterson wrote: ↑Fri Aug 27, 2021 6:36 pm To the extent that there is some kind of "anti-gravity", it only applies across cosmological scales (as with the voids Art brought up). The scale of today's image, or indeed any image that contains optically visible bubbles, is nowhere near that. There is no anti-gravitational effect at these scales. Bubbles are simply the consequence of gas and dust blown outward by well understood forces.
Today's image is tan(2°)*3000 ly = 100 ly wide. An ordinary star's stellar wind bubble can not be as large as this but a Supernova's can.
Now are you saying that the Trunk is a cometary nebula, a tail of wind shadow cast by a dense globule?
And the Stuff 'hanging' from the Trunk further away from the blotted star — is it cometary too?

ElephantTrunkCaravan..png

All I'm saying is that over this scale there are no "anti-gravitational" forces at play. The only such force we are aware of (and have a poor understanding of) is dark energy, and the effects of dark energy are not apparent over distances less than billions of light years.

by **VictorBorun** » Fri Aug 27, 2021 9:50 pm

Guest wrote: ↑Fri Aug 27, 2021 9:29 pm Amen, Ann. Starless images look like faces without eyebrows to me. They're just so unnatural looking, especially given that nebulae like Elephant's Trunk are all about star creation. Attached is the URL of a full-color image that I recently shot of Elephant's Trunk with the stars. It's done in the Hubble Pallet and is framed with a similar composition to today's APOD. I could have removed the stars--and in one preprocessing version of this photo I did just that--but I prefer to leave my eyebrows intact.

Tom Higgins

https://tvhiggins.com/wp-content/upload ... rked-1.jpg

So the Hanging Stuff is connected to the Trunk in the 3d space, after all.
Moreover, it is now clear that there is a second Trunk, to the left of the Trunk, less favourably illuminated, but similarly bended, the whole composition like a hindu statue of a moving subject presenting two phases.

By the way, is the Hubble Pallet exactly this:

673nm ionized sulfur White
658nm ionized nitrogen Orange
656nm hydrogen alpha Brown
502nm doubly ionized oxygen Cyan
469nm ionized helium Blue
373nm ionized oxygen Violet

by **VictorBorun** » Fri Aug 27, 2021 9:42 pm

Chris Peterson wrote: ↑Fri Aug 27, 2021 6:36 pm To the extent that there is some kind of "anti-gravity", it only applies across cosmological scales (as with the voids Art brought up). The scale of today's image, or indeed any image that contains optically visible bubbles, is nowhere near that. There is no anti-gravitational effect at these scales. Bubbles are simply the consequence of gas and dust blown outward by well understood forces.

Today's image is tan(2°)*3000 ly = 100 ly wide. An ordinary star's stellar wind bubble can not be as large as this but a Supernova's can.
Now are you saying that the Trunk is a cometary nebula, a tail of wind shadow cast by a dense globule?
And the Stuff 'hanging' from the Trunk further away from the blotted star — is it cometary too?

by **tvhiggins2** » Fri Aug 27, 2021 9:29 pm

Amen, Ann. Starless images look like faces without eyebrows to me. They're just so unnatural looking, especially given that nebulae like Elephant's Trunk are all about star creation. Attached is the URL of a full-color image that I recently shot of Elephant's Trunk with the stars. It's done in the Hubble Pallet and is framed with a similar composition to today's APOD. I could have removed the stars--and in one preprocessing version of this photo I did just that--but I prefer to leave my eyebrows intact.

Tom Higgins

https://tvhiggins.com/wp-content/upload ... rked-1.jpg

by **Chris Peterson** » Fri Aug 27, 2021 6:36 pm

VictorBorun wrote: ↑Fri Aug 27, 2021 6:23 pm
Chris Peterson wrote: ↑Fri Aug 27, 2021 3:10 pm
De58te wrote: ↑Fri Aug 27, 2021 2:58 pm

Anti-gravitational bubbles? I have never read anything about them.
Because there's no such thing. I presume he simply means a region where material is moving outwards due to some existing or previous force, and not moving inwards due to gravitational attraction.
Why, speaking of cosmologically large part of the space containing galaxy clusters or a part of a disk galaxy containing stellar clusters we have to consider that every cluster was once a clump just a little thicker than the average density of the media and then gravitationally collapsed and that every void was once a bubble just a little thinner than the average density of the media and then anti-gravitationally blown up.
And every place with no bias in density, neighter positive nor negative, just remained as it was, without any gravitation or anti-gravitation.

Or you can put it this way: the gravitation force is always attractive and the mass density is always positive, but a region filled with matter gets gravitationally pulled in all directions, and the total effect is to collapse a clump, to blow up a bubble and to leave a zero biased place alone.

There are other phenomena beside gravitationally amplified clumps and bubbles if you please. A bright star can blow up a 10 ly bubble and an active galactic nucleus can blow up a 1 Mly bubble with energetic particles. An arm of a disk galaxy is subject to non-inertial forces and to density waves. So astrophysic filaments can be other things than a crack between three touching voids

To the extent that there is some kind of "anti-gravity", it only applies across cosmological scales (as with the voids Art brought up). The scale of today's image, or indeed any image that contains optically visible bubbles, is nowhere near that. There is no anti-gravitational effect at these scales. Bubbles are simply the consequence of gas and dust blown outward by well understood forces.

by **VictorBorun** » Fri Aug 27, 2021 6:23 pm

Chris Peterson wrote: ↑Fri Aug 27, 2021 3:10 pm
De58te wrote: ↑Fri Aug 27, 2021 2:58 pm
VictorBorun wrote: ↑Fri Aug 27, 2021 8:54 am
3) a gravitationally collapsed crack between 3 touching bubbles of anti-gravitationally blown up voids
Anti-gravitational bubbles? I have never read anything about them.
Because there's no such thing. I presume he simply means a region where material is moving outwards due to some existing or previous force, and not moving inwards due to gravitational attraction.

Why, speaking of cosmologically large part of the space containing galaxy clusters or a part of a disk galaxy containing stellar clusters we have to consider that every cluster was once a clump just a little thicker than the average density of the media and then gravitationally collapsed and that every void was once a bubble just a little thinner than the average density of the media and then anti-gravitationally blown up.
And every place with no bias in density, neighter positive nor negative, just remained as it was, without any gravitation or anti-gravitation.

Or you can put it this way: the gravitation force is always attractive and the mass density is always positive, but a region filled with matter gets gravitationally pulled in all directions, and the total effect is to collapse a clump, to blow up a bubble and to leave a zero biased place alone.

There are other phenomena beside gravitationally amplified clumps and bubbles if you please. A bright star can blow up a 10 ly bubble and an active galactic nucleus can blow up a 1 Mly bubble with energetic particles. An arm of a disk galaxy is subject to non-inertial forces and to density waves. So astrophysic filaments can be other things than a crack between three touching voids

by **neufer** » Fri Aug 27, 2021 4:06 pm

Chris Peterson wrote: ↑Fri Aug 27, 2021 3:10 pm
De58te wrote: ↑Fri Aug 27, 2021 2:58 pm
VictorBorun wrote: ↑Fri Aug 27, 2021 8:54 am
3) a gravitationally collapsed crack between 3 touching bubbles of anti-gravitationally blown up voids
Anti-gravitational bubbles? I have never read anything about them.
Because there's no such thing. I presume he simply means a region where material is moving outwards due to some existing or previous force, and not moving inwards due to gravitational attraction.

https://en.wikipedia.org/wiki/Void_(astronomy)#Dark_energy wrote:
Click to play embedded YouTube video.
Click to play embedded YouTube video.
<<Cosmic voids are vast spaces between filaments which contain very few or no galaxies. Voids typically have a diameter of 10 to 100 megaparsecs; particularly large voids, defined by the absence of rich superclusters, are sometimes called supervoids. They have less than one tenth of the average density of matter abundance that is considered typical for the observable universe.

Voids are believed to have been formed by baryon acoustic oscillations in the Big Bang, collapses of mass followed by implosions of the compressed baryonic matter. Starting from initially small anisotropies from quantum fluctuations in the early universe, the anisotropies grew larger in scale over time. Regions of higher density collapsed more rapidly under gravity, eventually resulting in the large-scale, foam-like structure or "cosmic web" of voids and galaxy filaments seen today. Voids located in high-density environments are smaller than voids situated in low-density spaces of the universe.

Voids appear to correlate with the observed temperature of the cosmic microwave background (CMB) because of the Sachs–Wolfe effect. Colder regions correlate with voids and hotter regions correlate with filaments because of gravitational redshifting. As the Sachs–Wolfe effect is only significant if the universe is dominated by radiation or dark energy, the existence of voids is significant in providing physical evidence for dark energy.

The simultaneous existence of the largest-known voids and galaxy clusters requires about 70% dark energy in the universe today, consistent with the latest data from the cosmic microwave background. Voids act as bubbles in the universe that are sensitive to background cosmological changes. This means that the evolution of a void's shape is in part the result of the expansion of the universe. Since this acceleration is believed to be caused by dark energy, studying the changes of a void's shape over a period of time can be used to constrain the standard ΛCDM model, or further refine the Quintessence + Cold Dark Matter (QCDM) model and provide a more accurate dark energy equation of state. Additionally the abundance of voids is a promising way to constrain the dark energy equation of state.>>

by **Chris Peterson** » Fri Aug 27, 2021 3:10 pm

De58te wrote: ↑Fri Aug 27, 2021 2:58 pm
VictorBorun wrote: ↑Fri Aug 27, 2021 8:54 am
3) a gravitationally collapsed crack between 3 touching bubbles of anti-gravitationally blown up voids
Anti-gravitational bubbles? I have never read anything about them.

Because there's no such thing. I presume he simply means a region where material is moving outwards due to some existing or previous force, and not moving inwards due to gravitational attraction.

by **De58te** » Fri Aug 27, 2021 2:58 pm

VictorBorun wrote: ↑Fri Aug 27, 2021 8:54 am
3) a gravitationally collapsed crack between 3 touching bubbles of anti-gravitationally blown up voids

Anti-gravitational bubbles? I have never read anything about them. Would they be the opposite of a black hole? Whereas if an astronaut flying in a spaceship would cross the event horizon of a black hole, there is no known force in the universe which could make him turn around and fly back out - if an astronaut in a spaceship would fly up to the event horizon of an anti-gravitational bubble then the ship would immediately bounce back the way he had come like a rubber ball hitting an unmovable object?

by **bystander** » Fri Aug 27, 2021 2:37 pm

orin stepanek wrote: ↑Fri Aug 27, 2021 12:26 pm
Interesting APOD; I couldn't open APOD's interstellar URL

It's not just you, http://www-ssg.sr.unh.edu seems to be down.

by **neufer** » Fri Aug 27, 2021 1:23 pm

Click to play embedded YouTube video.

by **orin stepanek** » Fri Aug 27, 2021 12:26 pm

Interesting APOD; I couldn't open

APOD's interstellar URL

I'll go back again and try later!

There are a lot of stars and
I do like the photo with them removed!

by **VictorBorun** » Fri Aug 27, 2021 8:54 am

I wonder if the two filament parts are continous in 3d: the proper elephant's trunk above and the winding flow below (looking like water from an elephant's trunk).

All in all there seem to be 3 kinds of astrophysical filaments:
1) a pair of magnetically contained jets from a tight accretion disk with an orbital electric current around a protostar or a neutron star/black hole when devouring a red giant companion
2) cometary shadow tails casted by a dense clump in a stellar wind if the source is small and far compared to the size of the clump
3) a gravitationally collapsed crack between 3 touching bubbles of anti-gravitationally blown up voids

Starship Asterisk*

APOD: Elephant's Trunk and Caravan (2021 Aug 27)

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