Pelican / North America / Clamshell Nebulae in Cygnus

 Williams Redcat 51; iOptron HEM27; ASI6200MM, - Antlia Pro RGB & 3.5nm Ha Filters
H,O,S: (54,45,39 x 720s Bin 1, Gain 100); R,G,B: (25,25,24 x 120s, Bin 1, Gain 100)
Total integration time = 30.0 hrs (Aug 4-9, 2024)  Maple Bay, BC 

Three different views are presented from R,G,B broadband and S,H,O narrowband filters.  Care was taken in order provide enough narrowband data to really hone in on the structures within the Cygnus molecular cloud / stellar nursery area – encompassing the bright Pelican Nebula above the North America Nebula (2600 ly away) and the dimmer, but 400ly closer Clamshell Nebula.  In data acquisition and processing, I have been careful to bring out the dim emissions (and reflections) from the area and to not compromise, other than very locally, the relative brightness of the structures.  This is difficult to do given the huge dynamic brightness range offered up by the area.  There is a dense star field in the background (in addition to foreground stars), and the star's relative brightness have been turned down considerably relative to the narrowband emission.  Even with this reduced brightness, the stars can be a distraction to the nebular structure, so a starless version is also offered up.  Finally, the image is shown both in the hubble (SHO) and broadband (RGB) palettes although narrowband luminance incorporated in both version. 

The upper Pelican/NA stellar nursery first appears as two nebula, but scrutiny of the image reveals that there is only one nursery, with a dark nebula in the foreground dividing it into two – creating the Pelican’s beak and the NA eastern seaboard including the Gulf of Mexico.  If one imagines the nursery without the dark nebula it would be (very) roughly circular much as wide views of the Tadpoles/Elephant Trunk/Rosette etc nebulae.  The primary source of light and energy is thought to be an extremely hot, bright star now hidden behind the dark nebula.  Below the Pelican/NA stellar nursery there is a large scale version of mountain covered with streamers rolling across its surface.   These streamers likely originate from the ClamShell Nebula that is another stellar nursery that is "erupting" from the side of the mountain.

I want to use this image to debunk a number of misconceptions or oversimplifications that are often applied to the behavior of molecular clouds (or stellar nurseries) and how stars are formed within them.  I often hear, not just on the internet, but from reputable astrophysics, astronomer, and cosmological types such statements as: 
1)     The creation of stars is caused by the collapse of molecular cloud under gravity.
2)     The particles in a molecular cloud are too far apart to have any forces other than gravity play a role in their behavior.
3)     The contents of the molecular cloud are either gases (hydrogen and some helium) or dust particles.  The gases behave the ideal gas law while the other material is like non-volatile metal.  In other words, the gas molecules behave like mini planets, in orbit around each other, colliding perfectly elastically.
4)     Related to 3, gas molecules yield no resistance to shear forces other than dispersion.
5)     The stars are made of “gas” 

I hope to explain why these concepts are incorrect, and how it is the primarily the “stickiness” of real particles that is fundamental to the structure and behavior of the molecular cloud, the creation of stars, and even our ability to even image the molecular cloud in the first place. 

The collapse of the molecular cloud is due to cooling, with temperatures highest in the outer region of the cloud that has access to warming star radiation and can reach very high temperatures (in the thousands of K directly facing a close, hot, bright star (as at the Cygnus wall), to likely hundreds K on the backside as in this image:



which my represent what the Cygnus wall looks like from the back side.  Dust near the outside of the molecular cloud provides shelter from UV external by reflecting or adsorbing it and coverting to heat that is either distributing it to hydrogen molecules or re-emitting it.  At an intense front, like at the Cygnus wall the "wall of dust and relatively dense dust cloud is both being eroded and pushed back into the cloud:



The deeper one moves into the cloud, the more protection external radiation by dust there is, allowing less short wavelength light in and the cooler the cloud gets. 

At the same time, gas within cloud can cool, heat transfer from gas to the dust, which then re-emits the energy to deep space (at only 2.7K). This radiation is more longwave and these photons can more readily bypass dust to get to cold space.  The temperature gradient that is established within the cloud is cooling towards middleish of this in turn sets up an accompanying pressure gradient.  This pressure gradient creates a flow of the cloud itself towards its centre where the cloud density increases.  The flow, or flux from the outside makes it appear as if the cloud is contracting or collapsing, but is really the result of internal cooling.  The temperature and pressure gradients causing this inward flow of material is primarily regulated by the cooling ability of condensed material to convert hydrogens kinetic energy into photons and radiate into space.  The deeper one gets into the molecular cloud, the less light can reach the middle and warm it up, but the declining temperature means that the dust actually emits longer wavelengths allowing additional heat to be shed.

Here is an image of a cloud collapse.  Note the variability of both starlight and backlighting from nebulosity to make its way through the cloud.



Here is where molecular stickiness comes in., Technically termed Van der Waal forces, stickiness occurs when molecules collide with one another and influence the electron density function of the other colliding molecule.  This force is generally attractive causing the molecules to try and stick together in an inelastic way. Although these forces are not felt when molecules are indeed at a spacing equal to their “average density” the are certainly felt when they collide which they are readily doing all the time.  The strength of the stickiness is related to temperature.   At high temperature the stickiness is overwhelmed by the kinetic energy involved in the collision but at lower temperatures approach a substances “critical temperature”,  The net effect of this stickiness is a further reduction in pressure forcing more outside material inwards.  The strength of the stickiness also depend upon the composition of the molecules engaging in the collision.  Molecular clouds often contain volatile components other than hydrogen (hydrocarbons, inorganics) that will not only get sticky, but actually condense as the cloud cools.  For example, carbon monoxide – will actually condense due to molecular stickiness at about 150K.  This provides more radiant cooling agent.  Condensation will be nucleated on the edge of dust particles, so that the latent heat is radiated away.  Once condensed into several molecules, however, the condensate can radiate itself – adding to the cooling effect of dust.  At lower temperatures, helium may also condense, and at somewhere between 15 and 30K, hydrogen itself will condense into a liquid or solid.   The stickiness of hydrogen cooled to these temperatures is actually enhanced by causing the two atoms in the molecule to spin in the same direction, turning the molecule into a magnet for other hydrogen molecules.

The condensation temperature of hydrogen likely represents the coldest the molecular cloud can get and the rate of condensation is likely governed by the cooling ability of the cloud, coupled with the ability to the flowrate of supply hydrogen  In any event, this is not ideal gas behavior, despite overall rarefied cloud density, and the cloud is collapse due to a cooling at its core and not gravity, although gravity will later play a pivotal role in a star's creation.   

Molecular stickiness also plays a role in how the molecular cloud behaves.   Ideal gasses have no viscosity and do not resist shear.  (Although I did see a web site that purported to explain how it can, but it actually explained dispersion – not viscosity).   Viscosity describes how easily the molecules can move past one another.   Indeed it is determined by the degree of stickiness.  (Not only due fancy equations of state predict phase behavior and density, but also viscosity).  Total shear forces do indeed depend on the density, or total number of molecules moving past each other, but in a kinematic sense – that determines how the molecular cloud flows and behaves – the kinematic viscosity is not a function of pressure.  It is, however, strongly a function of temperature. The effect of viscosity is to give coherence to the molecular cloud – allowing for sharp fronts to exist on the edges of clouds and tendrils of material to maintain their integrity.   Here are streamers moving along the cloud face, demonstrating viscous integrity:



The most obvious effect of molecular cloud viscosity can be seen in “dust lanes” that spiral out from galaxies instead of the elliptical orbits taken by the viscosity-free stars.  The whole coherence of the molecular cloud as it is buffeted around by stellar radiation is due to its stickiness and viscosity.

Meanwhile, back at the coldest part of the collapsed cloud, condensed material will eventually flocculate/accumulate/coalesce into a growing mass, with an increasing hydrostatic pressure at its core.  I believe that this mass must become large enough so that when gravity can successfully take over in the accumulation of material.   The pressure at the core must become high enough so that the entire mass remains supercritical as it is heated via gravity by the compression of additional hydrogen molecules.  The pressure must rise first, followed by temperature because if not, the entire mass will simply vapourize back into the cloud.   Thus, the mass must be great enough to continue to accumulate via gravity, while remaining intact under the heating influence of this gas compression to a single point.

The end game in this gravity based accumulation phase is to cause the entire mass to change phase at high pressure and temperature to a liquid metal (not a gas!), where eventually fusion ignition take place.  At this point, a star will emerge from the cloud, several of which can be caught in the image of this molecular cloud complex.  Here are some young stellar objects "emerging" from the cloud, some complete with Herbig Haro jets (another discussion).