After dark

[Weltevreden SA]

Can anything new be said about the Coal Sack?

The good news is that it is a box of chocolates. Not much on the outside, but wow on inside. The bad news is light pollution eats it before you do. Half a magnitude of LP can drain the thrill out of all but the densest dark nebulae. In visual 6+ skies and a good pair of binocs, dark is more visually complex than bright. When a cold front gives me a half a mag deeper seeing, the Coal Sack acquires an intensity that surpasses the emission sky. In we Southies skies, the Sagg/Sco dark nebulae look like someone threw a black lace doily over a basket of diamonds. The Coal Sack is a hollow 3-D sphere in which you can see the surfaces from both the inside and the outside. It’s uncanny to see it floating there suspended between two Galactic arms. Its filamentary structures, clumps, and cores stand out more than the nearby Crux star clouds.

Vinyl record collectors say that their old pressings have a sound “presence” that CDs do not. The “presence” of dark nebulae is as textured visually as Brahms is to the ear. They can induce physical emotions, too. Pan across the Coal Sack at about 60x in a 6” scope and you go from glittery to dusty to icky to gloomy; it’s like being trapped in a spiderweb spun on quicksand in a dungeon. Move away into the stars again and it’s like taking a shower: all clean again. Whew.

Dark clouds like the Coal Sack are the most astrophysically complex structures in the Galaxy. Consider the Milky Way not as eyepiece star streams and dark blobs, but as matter density and energy density trying to come into balance. Their density is actually more important than their content. The popular perception that dark clouds are gravitationally free-falling into star clusters is true only in the last stage of a complex matter/energy exchange. Irrevocable collapse comes only after a bewildering list of other density interactions have worn themselves out. Supersonic shock waves are one. It seems weird to talk about Mach numbers in the vacuous realms between the stars, but the tenuous solar winds that buffet the upper atmosphere are breezing past Earth at local Mach 7. Sound is merely energy transfer detectable as compression/rarefaction waves (as is light.) We don’t hear the scream in space because it’s something like 50 octaves below Middle C. A rule of thumb is that at a density of 10,000 atoms per cc, Mach 1 is 200 meters/sec. That is also the density at which gravitational collapse becomes inevitable in a gas cloud three times the diameter of between us and Alpha Centauri (i.e., one cubic parsec).

Astronomers use an exotic vocabulary to describe the energy’s version of sound’s density dispersion. Magnetic flux tubes are simply vast hoards of electrons flowing like a river. The “lagoon” in M8 the Lagoon Nebula is a magnetic river we can see. Synchrotron radiation in the filaments of dark nebulae comes from the same spiraling electrons around magnetic field lines that give us the blue Cherenkov glow seen in nuclear reactors. High-density sound waves originating in supernovae or starburst clusters dissipate as ionic shock front turbulence. Shock waves are the most destructive processes at work in dark cloud evolution. The three closely layered curved bands next to the Orion Trapezium are supersonic shocks revealed by nearby bright stars; they are basically Mach 5 made visible. Our Galaxy seethes with fearsome energy densities, and we never see the show.

Only around 10 to 12 percent of a dark cloud condenses into stars. Throw in the stellar ruinations of hypersonic ejecta from supernovae and gas hurled supersonically from stars like our Sun, and energy density will recycle 99% of a dark cloud’s mass density back into space in one billion years. In the whole, huge Coal Sack, only one of its three dense cores will ever form a star cluster. The rest will be recycled into space. The Coal Sack is just a big softie. This unused gas can take half a billion years and more (four or more Galactic arm crossings) to weave its way into another dark cloud. Perhaps it be lucky enough to arrive at the burning-bright stage, but the chances are 9-to-1 it won’t.

The most digestible of the scads of papers on dust globule & molecular cloud evolution is Cambrésy 1999 (http://arxiv.org/pdf/astro-ph/9903149v1.pdf). The Abstract & Intro summarize the ideas, but go down to page 9 (http://arxiv.org/ps/astro-ph/9903149v1) to download the many GIF files used in the article. These hi-rez charts reveal the distribution of filaments, clumps, and cores which you can then trace across the sky with your unaided eyes. You can transfer these GIFs over to photocopies of your star charts so you can visualize both the whole cloud in the sky and the details of specific regions.

If you yearn for something more chewy, Bruce Elmegreen (http://arxiv.org/pdf/1410.1075v1.pdf) is your man. Or, you can just mutter “The heck with all this heavy-duty head-scratching” and go outside with your binoculars.

[madbadgalaxyman]

Hello Dana,

That is a very involving piece of scientific/artistic prose! I enjoyed your artistic and scientific experiment of imagining the weird sensations that come with being "trapped in the tentacles of a dark nebula".

In the same vein, I note that J.R.R. Tolkien refers to a giant spider monster as "weaving webs of shadow". Hmm, not a bad way to imagine the appearance of a dark nebula.

But, more seriously, most visual observers do not have access to your abnormally dark and transparent skies, so they must resort to using binoculars in order to increase the apparent contrast (as discerned by the "eyes + brain + mind" system) between the star-field and those inky "absences" where the line-of-sight density of the interstellar dust leads to >=2 magnitudes of dimming of the background light.

I have also found, from experience, that excellent light transmission of the optics can be an essential factor in achieving optimal viewing of dark nebulae, especially in average dark sky conditions;
so I strongly prefer using those binoculars that have the most effective anti-reflection coatings on each refracting air-glass surface and/or the highest-reflectivity mirrors

I used to use a pair of extremely-high-quality 4 inch refracting binoculars for dark nebula observations, but that was mainly due to lack of money for an even bigger pair of binoculars (!)..... ..it has always seemed to me that the dark nebula views through 5-8 inch binoculars were much better than those with 4 inch binos.

If ever did go back to regular deep sky obs, I would invest in a pair of binoculars in the aperture range 5-12 inches, as it gives you significantly increased contrast on deep sky objects, even if conditions are not absolutely optimal.

cheers,
Robert

I note that Herschel Space Observatory observed the cold dust of the Coalsack, in emission (!!), at far-infrared wavelengths of >=250 micrometers.
Yeah, she (the Coalsack) might look inky at visible wavelengths, but she lights up like a candle in FIR:
http://starformation-herschel.iap.fr/gouldbelt/
There is a most impressive extinction map of part of the coalsack on this website:
180119

I presented an extinction map of the Coalsack from a 1999 paper by Cambresy in this post:
http://www.iceinspace.com.au/forum/showthread.php?t=90824&highlight=coalsack

The fundamental question is why the Coalsack has not formed stars, yet other Very Similar molecular clouds have formed stars.
The coalsack continues to be an unpopular subject for research, as most investigators have instead focused on those molecular clouds that have formed stars.
One would think that the Coalsack is an important "no" case.

P.S
Dana, you may be interested in my recent posts in the Deep Sky imaging forum, relating to NGC 3621 and to the M81/M82 field.
N3621 is an excellent example of a very-extended disk of notably low optical surface brightness, in which star formation is occurring at large galactocentric radii (GALEX papers call objects like this "XUV" disks, and optical papers call them "Low Surface Brightness Disks).
The M81 field has inter-galactic blue knots.....how did the OB stars get out there, or did they form from the HI existing between the galaxies?

Did you know that a few elliptical galaxies also have disks with low-level star formation?!!?!

[Weltevreden SA]

Good to see your handle again on IIS, Robert. I’ve been out of the picture awhile and am delighted to be back.

For IIS readers of this thread, Robert’s paper on the Coal Sack is the best of the many I have read. Robert is blessed with clarity, a notion about which most of the professional papers are a bit vague on the concept. I wish varsities would teach writing before they teach calculus. The André and Saraceno paper that Robert cited is a good one, but it nearly extinguished my interest in extinction. IIS readers would be better off reading Robert’s paper to learn about the Coal Sack, then download & print the André and Saraceno maps of the regions you like most to observe. The paper’s web page with links to the maps is hard to navigate. The maps of the fields are presented in four different ways, only one of which we can actually go outside and look at. So I sorted through the unusable stuff and have provided links below to the maps useful to we dweeby folk who actually go outside and look up.

Most dark nebulae are so big they are easier to find naked-eye. After that, wide-field binocs provide those W-O-W views we all yearn for. So here’s the best of the lot. Prepare yourself for a shock: you will be humbled by what you are NOT seeing out there.

Coal Sack (http://starformation-herschel.iap.fr/gouldbelt/coalsack_ext.jpg) Invert the image to negative format, add stars, and that’s what you can see in binoculars. The bad news is: light pollution will ruin your tries. Take a spin out into the countryside, for once you don’t need a car full of gear. A six-pack will do just fine.

Dark Doodad in Musca, and Chamaeleon I & II (http://starformation-herschel.iap.fr/gouldbelt/gb_cham.jpg) I can see the Dark Doodad naked eye on the best nights. Come back in 10 million years and that lanky thread will be four glittery balls with bits of nebulosity. Over the next million years the first SN will come along and that will initiate a process that will eventually dissolve the cluster within another ten million years. Easy come, easy go.

Rho Ophiuchi extinction map 6.5° field (http://starformation-herschel.iap.fr/gouldbelt/ophiuchus_ext.jpg) Someday, all this will be another M4 and NGC 6144. Well, maybe.

Pipe Nebula region 8° field (http://starformation-herschel.iap.fr/gouldbelt/pipe_ext.jpg) This and the Sagg dark filaments are why Bulge globulars are so awful to spot.

Lupus extinction map 15° field (http://starformation-herschel.iap.fr/gouldbelt/lupus_ext.jpg) I never thought anything in astronomy could be boring until I arrived at Lupus.

Corona Australis map 4° x 8° field (http://starformation-herschel.iap.fr/gouldbelt/cra_ext.jpg) This is the region that makes me feel like I’m a pollywog in a mud puddle. The centre core of the three-cored clump on the right extinguishes 35 magnitudes of whatever’s in it and on the other side. The globular NGC 6723 is just rightward of the clump; it barely escapes being turned into a deeply reddened glowworm like GC 6526.

Serpens-Aquila Rift extinction 12° field (http://starformation-herschel.iap.fr/gouldbelt/gb_serpens.jpg) The dense core at RA 18.80 Dec –2.00 is why there are no globulars just north of the Scutum Star Cloud, in an area that should be dotted with them like the Ophiuchus group along the RA 18.00 latitude axis.

Orion A & B extinction map 16° x 12° (http://starformation-herschel.iap.fr/gouldbelt/orion_ext.jpg) M42 the Orion Neb region is in molecular cloud A and the Horsehead & M78 are in B. If the ambitious owner of a Tak 106FSQ ever gets around to overlaying a montage of this coordinate frame and its extinction map, the result will be The Last Word on the subject of Orion.

Taurus extinction map 18° field (http://starformation-herschel.iap.fr/gouldbelt/taurus_ext.jpg) Last January I traced the T Tauri complex naked-eye from my 31° South dark site. Even northerly molecular cloud zones can be seen if there’s no light pollution around.

If we lived in the middle of any one of those darker blobs, there would be utterly black and life would be brisk 2° K above absolute zero. The bottom of the Arctic Ocean is positively tropical by compare.

Another item of interest: Astronomers use the term “extinction” as a unit of measure. When they describe molecular cloud collapse, they cite the number of hydrogen atoms in a cubic centimeter from 1 to 10,000. At 10^4 H cc the gas density is high enough to extinguish a star or galaxy on the far side by one magnitude. At that point the astronomers start using Extinction as a calibration measure. I’ve seen Extinctions of 1,000 or more cited in papers devoted to the final collapse into a star—1,000 magnitudes of extinction is a galaxy’s version of a brick wall. Soon after, the collapse becomes a ball of hot rock.

Thanks again to Robert for nudging all this underway.

[madbadgalaxyman]

Note: This post has been substantially expanded in an edit.

Thanks, Dana, for the praise of my article about the Coalsack!

As I have not yet submitted it in IIS, I will start a new Science Thread which included my detailed article on the Coalsack.

Hey, Dana, were those extinction maps you presented from Herschel? Spitzer? 2MASS? other?? data.

I was not wrong when I said that the Coalsack remains an unpopular subject for research......searching the ADS (= the database of astronomical papers, for those of you who don't know) yields only 10 mentions of the Coalsack in the Paper Abstracts between 2011 and 2015, and most of these papers do not deal individually with this Dark Nebula.

The Coalsack was supposed to be included in the Herschel Space Observatory infrared survey of Gould's Belt, but I am not aware of any publication presenting the Herschel far-infared data on the Coalsack, at any time since the observations were made. I have a nasty suspicion that Herschel never got to survey the Milky Way properly because it ran out of coolant; one of the greatest tragedies for astronomy. It was an instrument easily as important as the HST, but apparently not able to be serviced.

I will try to extract some Far-infared observations of the Coalsack from the Herchel archive, if they are available. This is one of those very difficult "insiders only" data sources; that is nearly impenetrable to the ordinary mortal.
However, the Coalsack data seems to be missing from the data archive of the Herschel Gould's Belt survey; http://www.herschel.fr/cea/gouldbelt/en/Phocea/Vie_des_labos/Ast/ast_visu.php?id_ast=66
The NON star forming molecular cloud of the Coalsack is easily as important as the other similar clouds which HAVE formed stars, yet the other clouds have had much more study than the Coalsack.

[Weltevreden SA]

Robert, the André & Saraceno is a little unclear on where exactly its data comes from. The study’s home page (http://starformation-herschel.iap.fr/gouldbelt/) states, “We propose an extensive imaging survey of the densest portions of the Gould Belt with SPIRE at 250-500 and PACS at 110-170 microns”. The paper itself, in Sections 3 and 4 (http://starformation-herschel.iap.fr/gouldbelt/andre_saraceno_dusty04.pdf) discusses data from Herschel and Alma with the future-tense style of a proposal, not a paper. The paper is a good summary of what they hope to achieve, but is vague on where exactly the accompanying images derive. In this thread, I downloaded the image sets identified with the actual object’s real-sky location, e.g., Taurus, Coal Sack, Corona Aust., etc. There’s no ID on those charts, and they are a much higher-res set from an unidentified study subsequent to Cambrésy.

The other links in each “Target” box go to fields criss-crossed with green lines in rectangular form. Those must be the authors’ pointing instructions.

This is all kind of an ado about not much, because wherever the charts hail from, they are dadgum useful out in the dark skies. Agreed that’s a pity the way the Herschel was unable to be serviced.

Off-thread, Robert, I have read a couple of papers about footloose O stars out in the middle of where they’re not supposed to exist. I have logged two of them (one being Wd20A in Westerlund 2, the most massive binary in the Galaxy. A little due diligence reveals that there’s no way a binary of 160 Msols could be ejected out to where this thing lives; nothing is big enough to just toss it out there. Let me go ferret the sources out and let’s start a new thread. It’s relevant because these stars can be seen in a 7-inch.

=Dana

[madbadgalaxyman]

Hello there, all you Friends of the Coalsack.

Here are three images of the Coalsack......these three images were made in three different wavelength regimes. The three images are approximately in registration, and the imaging data has been displayed with the "Aladin Lite" tool found on the internet ( http://aladin.u-strasbg.fr/AladinLite/ ).

The most evident difference between the three images is the tendency for the Coalsack dark nebula to become ever more transparent to the background light coming from objects that are situated behind the dark nebula, with progressively increased (longer) wavelength of observation. This is as one would expect for sub-micron sized dust particles which are absorbing and scattering the light of background objects.
[[ for instance, if you look at the Coalsack at 22 micrometers (displayed as red in the WISE image), the dark nebula seems to be "not there".......because the light from background objects passes through it so easily at this wavelength ]]

However, the dust cloud itself is not emitting meaningful amounts of electromagnetic radiation in these three images;
the wavelength of the maximum emission of light coming from the cold interstellar dust particles at a temperature of 6 to 20 degrees Kelvin (that is, the peak of the Spectral Energy Distribution) is well longwards of 100 microns, so this thermal emission from the smoke-like dust particles that live in the cold "Molecular Hydrogen plus admixed Dust" clouds like the Coalsack was detectable with the now-deceased Herschel Space Observatory at far-infrared wavelengths.
For instance, Pagani et al. (2004, A&A, 417, 605) showed that 200 micron emission traces the outer regions of a cold dust cloud very well , though imaging at this wavelength does not pick up the coldest dust in the centre of a dust cloud.

[[[ in the following paragraph, I restate some information from Pagani et al. (2015, A&A, 574, L5) ( http://www.aanda.org/articles/aa/pdf/2015/02/aa25095-14.pdf
) ]]]

In principle, it is possible to use an infrared telescope to detect all of the dust in the Coalsack and in other similar Cold & Dark & potentially Star Forming clouds , emitting electromagnetic radiation at far-infrared wavelengths, if we observe at wavelengths longwards of 200 microns. For instance, according to this Pagani et al. paper, interstellar dust at 10 degrees Kelvin is brightest at 300 micrometers, while the coldest dust near to the cloud centres is at a temperature of about 6 degrees Kelvin and emits Electromagnetic Radiation most brightly at a wavelength of 500 micrometers.
(( The dust in dark nebulae is COLD, some of it not that much above absolute zero, which is why it emits only in the thermal far-infrared ! For the physicists among you, I note that the infrared spectrum of the dust in dark clouds is close to a Black Body curve, though with some discrepancies from a standard Black Body. ))

The longest wavelength of observation of the Spitzer Telescope, 160 microns, should also pick up at least some of the dust emission from the Coalsack.....but , would you believe, it, the researchers chose (and this is all too usual!) not to publish about far southern objects (like the Coalsack) in their first 6 papers (The Spitzer Survey of Interstellar Clouds in the Gould Belt). You would think that research astronomers actually have an aversion to the Coalsack.
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[[[ Dana, you presented some really good and useful maps of the space distribution of visual extinction in various dark nebulae. It would be a good idea to put a copy of them in the observational astronomy forum. (as a lot of people avoid the science forum..... as if it were the plague). Also, an explanation of how to use them would be helpful, for the benefit of the "lesser brethren". ]]]
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Anyhow, here are the three images........

Image 1: Visible light (Digitized Sky Survey 2, Red image)

Image 2 : Near infrared (image at 1 to 2.4 micrometers , from the 2MASS sky survey)

Image 3 : Longer wavelength Infrared image, from the WISE satellite:


Blue displays emission near 3.4 micrometers
Green displays emission near 4.6 micrometers
Red displays 22 micrometer emission

Note that extinction from dust in the Coalsack is so low at 22 microns that some of the red objects seen within the sky outline of this dark nebula are thousands of light years behind the Coalsack!
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(1)
180413

(2)
180414

(3)
180415


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