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Old 26-07-2015, 05:58 AM
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Weltevreden SA (Dana)
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Paul Haese's M8 Lagoon, the untold story

Paul Haese recently posted an interesting take on the oft-imaged Lagoon Nebula. Paul's image is indeed aesthetically very pleasing, especially to anyone who looks at images from the science point of view but also appreciates their sheer artistry.

I've been studying M8 for awhile in the professional papers, and yet Paul's image give me more useable information than most of the observatory shots I see. For one, the wide field and exquisite detail illustrate features on large scale that more minute frames do not. Two details especially come to mind. First, the delicate, feathery outflows which nearly surround M8's interior structures are what is called champagne flow. When a large volume of gas has been bound by gravitational, magnetic field, and/or compressive turbulence, the strength of the binding process is reduced by the matter and energy consumed in making a new star cluster. Roughly 10% of the gas is consumed by star formation, and rather quickly (~5 million years) in the life cycle of a molecular cloud (~100 million years). The UV emission from the many hot O stars radiates enough energy to impel a net outward flow in gas which is not restrained by nearby dust clumps, magnetic fields, or other binding energies. When that gas reaches the interstellar medium, it can burst out quickly, just as champagne froths when you suddenly pop the cork. Paul's image captures this especially well in the entire E (left) quadrant, and also the quadrant running from 10° to 90° in the NE sector and the 170° to 220° sector in the S (bottom). Note how the champagne effect is constrained when there are masses of dark, dense gas around (some quite dense as you can see by the paucity of stars within). All this is a very pretty sight indeed. But is also vividly demonstrates how unused molecular cloud gas is released back into the galactic medium. Perhaps 100 million years or more may transpire before old Lagoon clumps here and there retained enough structural integrity to bind into dense clouds again. The cycle will commence again, endure 50 to 100 million years, and, like Sisyphus and his rock, go through it all again and again for an enduringly long time.

The second aspect of Paul's image is that the entire central Lagoon region and its surroundings are a gigantic bar magnet. Remember those things in elementary school? The teacher sprinkled iron filings around the bar while we watched the filings line themselves up in looping arcs streaming from the N pole around the bar to stream back in the S pole. Now look at the Lagoon again. The bar magnet stretches across the Hourglass region from the magnet's N pole at roughly 135° (i.e., SW sector of the nebula), through the Hourglass, to the magnet's S pole at roughly 275° near the middle of the image. It is a gas magnet, not an iron magnet, but the magnetic behaviour is much the same. How many times have we looked at that beautiful "river" of dark flowing from the E (left) side of the Hourglass region, to the S side, where it appears to dissipate away. That river is dense dark molecular cloud gas (perhaps 1000 atoms/cc) which obscures the distant field stars behind. In the physics of mass/energy transport, the languid river is a magnetic flux tube. These are columns of electrons flowing along magnetic field lines. The "flux" is a measurement term for the quantity of energy through a specific cross-section. The flowing electrons nudge along other charged atoms and dust with them, hence the opacity and the unitary flow. It’s a river with a fancy name, but a normal energy transport system all the same.

Look carefully: there is a similar flux tube on the opposite side of the Hourglass. It appears as a series of three striations on the W side of the Hourglass, though angled a but differently. If you carefully trace the thinner, weaker flow lines emanating out both ends of these flux regions, you'll see the same looping-back-in structure we saw in the iron filings, albeit distorted by the fact that it is a gas in a region with many competing forces in many small regions. But if this large structure is a magnet made of flowing gas, where does the gas come from? More important, where is it going?

In elementary school we were taught that field lines stream from the N to the S pole. In a metal bar the electrons barely budge, but in a hot nebula they speed sprightly along—velocities within the flux tubes range from 35 to 100 km/sec. (Do some sums and figure out how long that is from Sydney to Perth.) The Hourglass is illumined by Herschel 36, whose strong UV radiation strips electrons from the middle to outer electron shells of alpha-process atoms (N, C, O, S, etc). The large-scale magnetic "bar" which gives the stripped electrons a direction originated as a feeble micro-Gauss field streaming along the interarm regions between the spiral arms. Spiral galaxies literally carry magnetic rivers in their arms, very large but also very tenuous. The giant molecular cloud from which the entire M8 complex evolved carried traces of the galactic magnetic field with it. These were compressed as the cloud condensed into clumps and then cores, to finally arrive at a dense core (>10,000 atoms/cc) that evolved into the Hourglass. The dark blob around Rho Oph is another such core on its way to a cluster; so are two of the four dark blobs in Corona Austr, and one strung along the skinny thread of the Dark Doodad.

What happens to the magnetic field between initial dense core formation and what we see in the Lagoon today is a long, involved tale best suited for another day. Let's just pick up the story where the Lagoon is right now. How do those magnetic tubes feed large quantities of gas and dust into the furnace of the Hourglass? Here's a good presentation by the friendly folks at your own AAS observatory. Note that just to the right of the bright emission of the Hourglass is a small dense pocket towards which a stream of dark streaks arrives from the R lower quadrant (which is the SW quadrant of the Lagoon). It flows inward at what appears to be high velocity and ends abruptly in a dark messy clump. That stream is gas flowing into into the Lagoon’s S magnetic pole from the lazy rivers which all but surround the Lagoon region. By the time the gas arrives at the Hourglass it is highly energetic. When it arrives at the messy blob, the gas find that the puddly looking blob is in fact a solid wall of supersonic turbulence. The turbulence is a large shock zone made of an aggregate of many small supersonic shocks generated in the tumult of Herschel 36's UV radiation plus the twisted energetics of cluster formation that occurs throughout the Hourglass region. The shock front is hidden behind dust to our eyes, but not to IR imaging.

Magnetic flow is a powerful thing, but turbulence nibbles it like candy. The incoming flow is halted in its tracks by a wall of supersonic shocks moving at Mach 20 to Mach 90 over very short relative distances of a few dozen to several hundred astronomical units (AU, the distance from Earth to the Sun). At that, gravity can finally entangle the blob to initiate a star cluster's early collapse. The messy blob at the end of the inflowing streamers is a vital part of star cluster formation: turbulent shock and magnetic pressure weaken each other and enable gravity to contract it into many tiny pockets which will soon (100,000 years) become stars. The fact that the blob is so dark is a good indication how dense it is—we are now talking 100,000 to a million atoms per cc. Those densities are the levels gravity needs to shrink out several hundred small, pre-main sequence stars. Hence, lying in front of the Hourglass—which is a dense cluster about 1 million years old—we see yet another star cluster in the early childhood of less than 100,000 years. This looks to be occurring right next to the Hourglass, but is actually half a light year away from the Hourglass. At the pace of astronomy’s clock, these events are bang-bang-bang.

M8 is in fact a large cloud of gas and star clusters in various stages of development. The pretty NGC 6530 cluster to the left of the "river" was the first to form and is about 5 million years old; it lies 10 light years nearer to us than the Hourglass. With nearly a thousand members, N6530 is much larger than it looks (9 Sgr is one of its members, too). N6530 has six O stars, some in binaries. These will to go supernova in the next one to two million years. Already their UV radiation has cleaned out most gas from the cluster, although much remains which is we can visually detect as a diffuse glow using even a six-inch scope.

Directly S (bottom) of N6530 is a slender arc of gas wobbling off to the E (left). That is the Southern Wall, which looks wildly exotic in an APOD pic, but is actually nothing but a normal gas cloud which has had the hapless luck of lying near the white-hot O stars of N6530. It is not really a "wall"—we are viewing the surface of an irregular blob as seen edge-on. The Wall is a rather modest feature in the large Lagoon picture; it is more a curiosity than a powerful force in the basic mechanics of the place. The Wall looks ragged because it is yet another example of what happens to a cold cloud of gas when hit by a blaze of UV from a nearby young star cluster.

This isn't all. The Lagoon cluster-forming ménage is more complex than even Paul's image. Example: between the Southern Wall and the N6530 cluster itself is a baby cluster of over 1,000 small stars still in their infancy. They are protostars which emit x-rays from their poles as they eject excess birth gas. X-ray images of this area show at least a thousand specks. If we pay another call on the neighbourhood in around a million years we will see a pretty but smallish aggregate of sub solar-mass stars. It is what happens when the big fizz of nebular star formation is over and bubbly little fizzes consume the leftovers. Put a bit more formally, these are metals-enhanced third through fifth generation starform events. (I like the fizzy image better.) Alas for these little fizzes, their future as a cluster is not promising. They are low mass, have little mutual attraction (aka binding energy), and will dissipate into the Galactic spiral arm to join the many billions of others which got there the same way. We call them field stars, but they more liken to dust motes left trembling in the air after a great roaring storm has vanished into the distance.

Informative and beautiful as Paul's image is, it cannot reveal the most important story to one day emerge from that entire fulminous M8 star-forming region. In 100 million years we will see M8 as at least one Pleiades or Double Cluster with a few small clusters dotted about. There will be little nebulosity left. What we won’t see is the most interesting final consequence of all that immense tremor of energy we call the Lagoon. One—just one—of those tiny solar-mass x-ray babies might yield up an Earth. The invisible—indeed even nameless—x-ray cluster is the same kind of cluster our Sun originated within. All the Sun's family came from an origin like this, a smallish cluster of a few thousand stars. We see dozens of them in a low-power pan across 30° of the Milky Way. The Sun's cluster diffused away aeons ago into the the Milky Way disc. Astronomers think they may have found one Solar sibling far away in our Orion Spur, but that's a definite maybe. We here on earth are all that’s left of a once majestic Lagoon.

It's all right there in your image, Paul. Hoist a round to yourself over that one.

=Dana in S Africa
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Old 27-07-2015, 06:21 PM
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Most enjoyable Dana thanks for that wonderful account
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Old 28-07-2015, 05:05 AM
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ZeroID (Brent)
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Love these star 'stories'. Well written and quite enjoyable, I can imagine the protagonists as well as any good fiction. I will view M8 in a whole new light after this.
Thanks Dana for lovely descriptive and informative information.
Keep them coming.
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Old 28-07-2015, 06:07 AM
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Atmos (Colin)
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As a relatively recent astrophysics graduate it was an enjoyable read :-)
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Old 17-11-2018, 07:51 AM
stevous67 (Steve M)
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Hello Dana,

Thank you for sharing your study.

Regards

Steve
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