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. The Abstract & Intro summarize the ideas, but go down to
page 9 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 is your man. Or, you can just mutter “The heck with all this heavy-duty head-scratching” and go outside with your binoculars.