Dark structure names

[Rodd]

Most of the dark structures have a Barnard catalogue designation.  I was wondering if dark structures contained within larger nebula complexes have designations.  A good example is this serpent like streamer that seems to weave through the ionized web that is IC-443 (The Jellyfish nebula).  This is a center crop of an image I captured using the TOA 130 and STT-8300.  This image is a reprocess of some older data that is markedly improved over my earlier attempts.  However, I still crossed the "over-processing" line by a few steps, which, in itself, is a marked improvement for me.  Normally I need binoculars to look back and see the line.  I feel this image is much closer--perhaps a good running jump, or dive to get back. 

Ha: 9 30min

OIII: 12 30min

SI: 10 30min

 

[vlaiv]

I guess it depends on "type" of the dark structure.

Not all dark nebulae are the same, in fact - dark nebula (also called absorption nebula) refers to actual nebulous matter blocking the light from behind and creating dark appearance. It is dust and dense gas that is not excited and therefore does not shine in narrow band, but instead it blocks the light from behind - both of other nebulae/galaxies but also stars (and in fact anything that is lying behind).

You can have appearance of dark structure without dark/absorption nebula - if there is simply gap in gas cloud and there is nothing but deep space behind - which is of course black .

You can distinguish the two by looking at the stars - dark nebulae tend to attenuate stars as well as other things, so they will be truly dark with almost no stars shining thru. Empty space will just look like a piece of background sky - so it will show stars regularly.

Here is nice example of dark nebula taken from wikipedia (lupus 4):

Note absence of stars in otherwise dense star field.

On the other hand look at jellyfish nebula in wider field (random screen shot from google image search):

Black space in nebula very much looks like that - place where nebula is not shining and not something that is actively blocking the light, stars don't appear less dense than in surrounding area.

[Rodd]

31 minutes ago, vlaiv said:

I guess it depends on "type" of the dark structure.

Not all dark nebulae are the same, in fact - dark nebula (also called absorption nebula) refers to actual nebulous matter blocking the light from behind and creating dark appearance. It is dust and dense gas that is not excited and therefore does not shine in narrow band, but instead it blocks the light from behind - both of other nebulae/galaxies but also stars (and in fact anything that is lying behind).

You can have appearance of dark structure without dark/absorption nebula - if there is simply gap in gas cloud and there is nothing but deep space behind - which is of course black .

You can distinguish the two by looking at the stars - dark nebulae tend to attenuate stars as well as other things, so they will be truly dark with almost no stars shining thru. Empty space will just look like a piece of background sky - so it will show stars regularly.

Here is nice example of dark nebula taken from wikipedia (lupus 4):

Note absence of stars in otherwise dense star field.

On the other hand look at jellyfish nebula in wider field (random screen shot from google image search):

Black space in nebula very much looks like that - place where nebula is not shining and not something that is actively blocking the light, stars don't appear less dense than in surrounding area.

I think I disagree with you if you are saying that the dark structures in the Jellyfish are not dark structures but the lack of Ha, OIII, or SII wavelengths.  These structures are present in broadband images as well.   To be clear--I use the tern dark nabula and dark structure interchangeably--perhaps that is my mistake.  But if you are trying to show that there are no dark structures in the Jellyfish.....you have not convinced me.

Rodd

[vlaiv]

29 minutes ago, Rodd said:

I think I disagree with you if you are saying that the dark structures in the Jellyfish are not dark structures but the lack of Ha, OIII, or SII wavelengths.  These structures are present in broadband images as well.   To be clear--I use the tern dark nabula and dark structure interchangeably--perhaps that is my mistake.  But if you are trying to show that there are no dark structures in the Jellyfish.....you have not convinced me.

Rodd

No, I'm not saying that there are not dark structures in jellyfish. I was trying to answer your question about names of "dark structures" or to be precise of dark / absorption nebulae.

Like I said, I believe that there are two types (maybe more) of dark structures - ones formed by something blocking the light from behind, and ones that just look like dark structure in the image because there is nothing there - empty space.

I'm rather convinced that empty space will not have catalog number / designation (why would it? after all, much of universe is empty space ). However, I do believe that dark / absorption nebulae, being actual things in space, although not emitting light but rather blocking it, will have designation as they are indeed objects of some sort.

[Rodd]

13 minutes ago, vlaiv said:

No, I'm not saying that there are not dark structures in jellyfish. I was trying to answer your question about names of "dark structures" or to be precise of dark / absorption nebulae.

Like I said, I believe that there are two types (maybe more) of dark structures - ones formed by something blocking the light from behind, and ones that just look like dark structure in the image because there is nothing there - empty space.

I'm rather convinced that empty space will not have catalog number / designation (why would it? after all, much of universe is empty space ). However, I do believe that dark / absorption nebulae, being actual things in space, although not emitting light but rather blocking it, will have designation as they are indeed objects of some sort.

So back to my question, as I believe that the dark structures in the Jellyfish are in fact "dark structures" and not "empty space".  I agree with you that empty space is typically not named (the key hole being one possible exception).  I will take it one step further and say that empty space can not by definition be a "dark structure" even if it looks like one, as there is no "structure" to empty space (forget quantum foam discussions and all that virtual stuff for a moment).  When you think about it--how could the dark "things" in the Jellyfish be empty space with nothing in them--if that were the case, one could see through them and they would not obscure the background structure.

Rodd

[vlaiv]

Not sure how you concluded that.

Above attached images outline main difference between empty space and absorption nebulae - empty space contains stars, or rather you can see thru it to see stars although there is no other nebulosity behind so it looks black. With absorption nebula, nothing gets thru in visible spectrum, although you can observe in infra red or radio and see what is behind.

Jellyfish nebula contains stars in dark region, so it stands to reason that it is in fact empty space.

Important bit is that Jellyfish is super nova remnant, so not quite ordinary molecular cloud nebula that can contain dust.

There is remote possibility that there is dust cloud sitting in front of Jellyfish nebula and covering it, but I fail to see what would give it it's shape. Stars seen there could be foreground stars instead of background stars. Most nebulous things out there are either driven by gravity or stellar wind or some other process shaping it. Nothing remarkable in that region that could twist it like so.

I was searching for minimally processed image, ideally scientific survey type to see what can be concluded, but most images that I was able to find were heavily processed. In most heavily processed images that sort of feature will look darker than it probably is and smoother than it probably is (local contrast enhancement + denoising).

 

[Rodd]

11 minutes ago, vlaiv said:

Not sure how you concluded that.

Above attached images outline main difference between empty space and absorption nebulae - empty space contains stars, or rather you can see thru it to see stars although there is no other nebulosity behind so it looks black. With absorption nebula, nothing gets thru in visible spectrum, although you can observe in infra red or radio and see what is behind.

Jellyfish nebula contains stars in dark region, so it stands to reason that it is in fact empty space.

Important bit is that Jellyfish is super nova remnant, so not quite ordinary molecular cloud nebula that can contain dust.

There is remote possibility that there is dust cloud sitting in front of Jellyfish nebula and covering it, but I fail to see what would give it it's shape. Stars seen there could be foreground stars instead of background stars. Most nebulous things out there are either driven by gravity or stellar wind or some other process shaping it. Nothing remarkable in that region that could twist it like so.

I was searching for minimally processed image, ideally scientific survey type to see what can be concluded, but most images that I was able to find were heavily processed. In most heavily processed images that sort of feature will look darker than it probably is and smoother than it probably is (local contrast enhancement + denoising).

 

Those stars could easily be in front of the dark structure.  Also, different parts of the explosion will cool at different rates--those dark structures are not empty space--they are cool gas and dust.  Answer me this....if the dark patch in front of the glowing gas  is empty space, why can't we see through it to the glowing gas behind?

[vlaiv]

1 minute ago, Rodd said:

Those stars could easily be in front of the dark structure.  Also, different parts of the explosion will cool at different rates--those dark structures are not empty space--they are cool gas and dust.  Answer me this....if the dark patch in front of the glowing gas  is empty space, why can't we see through it to the glowing gas behind?

Because glowing gas is not uniform ball. It has internal structure, and as such - it has filaments that are stretched out and areas of very low density. Don't capture enough signal - it just appears dark. There is probably faintly glowing gas that is transparent there as well, but it is not type of gas/dust that is blocking the light from behind (probably).

Let me find you another example of super nova remnant. That way you'll see what I mean. I could be wrong. This is not firm statement on my part - just trying to reason what it could actually be.

This is also SNR - Abell 85, we could similarly argue that dark regions in it are in fact dust clouds in front of it.

Here is better one as an example, look at this image (Simeis 147):

I've marked something that could well be dust cloud sitting in front of nebula - it looks about right.

Same feature in this other image does not look like cloud in front of nebula anymore:

Nor in this one:

 

[Rodd]

10 minutes ago, vlaiv said:

I've marked something that could well be dust cloud sitting in front of nebula - it looks about right.

Same feature in this other image does not look like cloud in front of nebula anymore:

Different gases will pass different frequencies of light.  I did not indicate that these were necessarily dust lanes--but colder gas could be considered a dark structure.  All explanations aside--in order to convince me that the dark structures (I will continue to call them that) visible in the Jellyfish nebula are actually empty space devoid of gas and/or dust, you will have to reference a source specific to this question and this nebula.  Its obvious that the dark things have structure because they pass in front of a ridge line that continues after them--Why is the light from the ridge line being blocked?  Certainly not because it is passing through empty space--or vacuum.  It can only be that the light from the glowing ridge line is obscured by something--be it cold, lightless gas, molecular dust--whatever.  But no way are these things empty space.  I refer to dark "things" on the outer shell--not the darkness in the interior.  In my mind they can be 1 of 2 things., Either nothing (space) or something (gas, dust, ice, etc, etc).  It matters not to me what they are--if they are anything other than vacuous space--they are dark structures.  Even if they are glowing at a brightness that can not be detected by eyes or sensors.

Rodd

[Martin Meredith]

The most extensive collection of dark cloud designations that I know about is the Dutra & Bica catalogue from 2002 which contains just over 5000 named objects. I checked and the Jellyfish does not contain any of these. They do note that the catalogue is not complete. It might be worth seeing who has cited this work to check if there are more recent catalogues.

Martin

[Rodd]

6 minutes ago, Martin Meredith said:

The most extensive collection of dark cloud designations that I know about is the Dutra & Bica catalogue from 2002 which contains just over 5000 named objects. I checked and the Jellyfish does not contain any of these. They do note that the catalogue is not complete. It might be worth seeing who has cited this work to check if there are more recent catalogues.

Martin

It does not surprise me that dark structures do not appear in the catalogue.  that could be for a number of reasons.   Take the Cygnus region--there are literally thousands of dark structures there--a few of which have names.  I think of catalogues like that similar to the Messier Catalogue--obviosuly there are many more DSOs to see than what is in the Messier catalogue.  Similarity, there are no doubt  more than 5,000 dark structures in the visible part of the galaxy.  But we digress from the real question that has materialized out of the vacuum of space--are these dark shapes something, or are they nothing.  In my original post that asked if they were named, I should have asked if they exist, or are illusion.  Vlad suggests that they are noting more than empty space. 

Rodd

[vlaiv]

1 hour ago, Rodd said:

Different gases will pass different frequencies of light.  I did not indicate that these were necessarily dust lanes--but colder gas could be considered a dark structure.  All explanations aside--in order to convince me that the dark structures (I will continue to call them that) visible in the Jellyfish nebula are actually empty space devoid of gas and/or dust, you will have to reference a source specific to this question and this nebula.  Its obvious that the dark things have structure because they pass in front of a ridge line that continues after them--Why is the light from the ridge line being blocked?  Certainly not because it is passing through empty space--or vacuum.  It can only be that the light from the glowing ridge line is obscured by something--be it cold, lightless gas, molecular dust--whatever.  But no way are these things empty space.  I refer to dark "things" on the outer shell--not the darkness in the interior.  In my mind they can be 1 of 2 things., Either nothing (space) or something (gas, dust, ice, etc, etc).  It matters not to me what they are--if they are anything other than vacuous space--they are dark structures.  Even if they are glowing at a brightness that can not be detected by eyes or sensors.

Rodd

You are right, it is indeed dust region. Not sure if it can be classified as a single dark/absorption nebula.

I was going to give you incentive to pursue this topic further by examining one of two possibilities:

1. either it was "cooled" part of super nova remnant - which was in my view highly unlikely (why would there be only few parts of it cooled enough?)

2. It is part of some molecular gas cloud/complex near by that is closer to us.

It turns out that option two is in fact true. Sh2-249 lies very close to IC443 (also known as Sh2-248). In Stellarium it is wrongly marked as reflection nebula and shares IC catalog number of 444, which is not true as it is Ha region. IC 444 is indeed reflection nebula a bit further away and not the same thing as Sh2-249. Central part of that region is filled with dark filaments. This APOD image shows complex and neighboring IC443 

If we find any data on distances to both objects, and Sh2-249 turns out to be closer to us, or they are roughly the same distance (meaning Jellyfish is remnant of the star that was embedded in molecular cloud), then it is very likely that obscuring dark absorption part of Sh2-249 is in fact what you see in the image.

Jellyfish is roughly 5000ly. Let's see if we can find info on Sh2-249.

According to Simbad, distance to Sh2-249 is about 6556ly +/- 489ly.

Ok, not sure what to make out of this.

[Rodd]

16 hours ago, vlaiv said:

Jellyfish is roughly 5000ly. Let's see if we can find info on Sh2-249.

According to Simbad, distance to Sh2-249 is about 6556ly +/- 489ly.

That's interesting.  I suppose its possible that the little dark "structures" visible in IC443 are in fact the same distance as the dark structures in Sh2-249.  The dark lanes may not be associated with either of these targets--so if they were present before the KaBOOM! they all could be the same distance in the foreground.  Now that I look at the entire complex--I can't see why that couldn't be true.  If that's the case I would suspect that these dark lanes and filaments were present prior to the super nova that created the Jellyfish--though I was reading an article (there's a bunch on-line you find when you Google it--can't remember which)--but there were apparently more than one super nova associated with this region--they found multiple shells, some of which are much older than the 10,000-30,000 year estimate for the most recent.  They also found the progenitor star--a neutron star that is located in a counter intuitive location.  One would think that the center would be a good place to look.  But the neutron star is actually at the edge of the Jellyfish--On the near side there is a ridge that runs form right to left--a big S shape.  It protrudes like the brow ridge of a Homo Erectus skull.  About midway there is a bright association of stars--it is somewhere in that vicinity.  I suppose the image they showed (a x-ray image, or some abstract looking frequency image) could have just been portraying the x-y location and the z axis location was not shown--so the star could be anywhere on the z axis, including in the center of the Jellyfish. 

One thing about naming the dark structures--it could be a naming protocol thing.  If a dark structure is associated with a named object, would they be less likely to give it its own designation?  Or a line drawing thing--the most comprehensive catalogue has 5,000 items--but if one names all the little dark features associated with this one target, they could come up with a pretty long list depending on where they draw the line....each filament could have a name, or a general region could have a name.   The NGC 7000 area would be even worse!

Rodd

Edited October 2, 2019 by Rodd

[vlaiv]

Yes indeed - it did occur to me that dark filaments are neither associated with Sh2-249 nor Sh2-248, but standing in foreground. I've read somewhere that such dark nebulae are usually part of molecular cloud complex, so I can't tell for certain if they can exist on their own (which would be the case here).

On the naming - it is possible that objects get catalog number if they are studied by people building the catalog, and small features don't make the list for practical reasons - too much time spent on studying other more interesting object in that class.