Molecular Cloud "Collapse": Barnard 169 & 174

Askar 151phq; AP Mach2 GTO
ASI6200MM, – Chroma RGB & 5nm Narrowband Filters
Ha: (42 x 720s 61 Bin 1, Gain 100)
L☹73 x 120s, Bin 1, Gain 100)
R,G,B: (31,30,30 x 150s, Bin 1, Gain 100)
Total integration time = 14.6 hrs (July 14,15,16, 2024) Maple Bay, BC

The standard textbooks indicate that the start or conception of a new star formation is the collapse of a molecular cloud. But my background in thermodynamics, heat/mass transfer and fluid mechanics leaves this superficial explanation ungratifying (at least to me?) What would cause a molecular cloud or part of one to “collapse”. I have presented here, three variations of the same view of the Bernard 169 (the loopy one on the right), and Bernard 174 (shaped like a boot on the far left) – both molecular clouds in the process of “collapsing”, or as I would rather put it – condensing – towards star conception. Both B169 and 174 are dark nebulae that emit no light of their own, but rather block light from the background and reflect any starlight from stars in the foreground.

Molecular Cloud "Collapse": Barnard 169 & 174 in Balanced L+HaRGB
Molecular Cloud “Collapse”: Barnard 169 & 174 in Balanced L+HaRGB

The three views represent a broad spectrum RGB image, an image where the red Ha signal has been added in a balanced manner, and the reddest image, where the Ha has been exaggerated (roughly 10 fold) to be the dominant signal in order to remove broadband reflected light from the image. I am showing three images to illustrate that only condensed material (material in either a solid or liquid state) can block or reflect light. Gaseous material can neither block nor reflect broadband light.

The molecular clouds in this image are set against a backdrop of a rich starfield and weakly Ha emitting hydrogen gas stimulated by UV radiation from the background stars. This background hydrogen emission is dominantly red Ha, and is visible as a pinkish hue in the broadband (LRGB) image to bright red in the image where Ha signal has been exaggerated (HaRGB). The stars themselves have been left their natural colour. In the HaRGB image, the outline of the dark nebula is stark and sharp. The image (other than the stars) appear either a shade of red or grey/black where the background Ha and starlight is blocked Since this blocking can only be done by condensed material this image provides the best indicated of how dense/thick the blocking condensed material is. In parts it is appears as smoke in other parts it is thick enough to black out even brights stars behind it. It almost has a sharp binary representation – black where there is condensed material and not where there isn’t.

Molecular Cloud "Collapse": Barnard 169 & 174 in Balanced LHaRGB
Molecular Cloud “Collapse”: Barnard 169 & 174 in Balanced LHaRGB

In contrast, the LRGB image does include reflected broadband light from proximal/foreground stars. The pink/red background is still apparent, but now includes reflected browns, greens, yellows and blues. The thickest part of the molecular cloud still blocks light from behind it, but now the dark nebula appears somewhat like a cloud in the sky reflecting sunlight – an analogy I am about to draw upon again. The third, base image I show is a balanced view, that I believe is the most aesthetically appealing and is included for that reason. In is interesting to switch between views to see how the dark nebulosity effects the light we collect despite not emitting visible light itself.

So here is where the thermodynamics come in. In its diffuse form, a molecular cloud will exist at about 100K (–173C), very low density (2 to 5E-19 mol/l), and correspondingly low pressure. The temperature is pretty cold for us, but balmy compared most of space, where outside of the cloud it is likely only 15k. At 100K, hydrogen and helium are gaseous and only heavier elements and molecules are condensed – what we refer to as dust. Star radiation and light hitting the dust is partly adsorbed, keeping the diffuse cloud warm. Meanwhile tidal forces and radiation push the cloud around. Eddies in the cloud can form that can increase the density of the cloud here and there such that it increasing adsorbs and reflects the light from outside the cloud.

If the dust gets thick enough if will actually shield the molecular cloud behind it from warming photons by adsorbing it or reflecting it away. This will cause the molecular cloud shielded by the dust to cool towards the 7 to 15K temperature of space through emission of first infrared, turning to microwaves, turning to radio waves by the cooling dust being shielded. This reduction in temperature will cause the cloud to shrink in size and material to be more concentrated at the prevailing pressure.

Molecular Cloud "Collapse": Barnard 169 & 174 in HaRGB
Molecular Cloud “Collapse”: Barnard 169 & 174 in HaRGB

Due to electromagnet forces between hydrogen molecules (van der Waal forces) hydrogen will start to condense when the cold, light shielded portion of the cloud reduces to about 20K (-253C). Initial condensation will likely be nucleated by interior dust. An analogy would be dew or frost formation on blades of grass, only now it is hydrogen, rather than water condensing. Once tiny droplets/crystals are formed, these can further shield and reflect light, causing the portion of molecular cloud be extremely dark in our images. This process is what I believe is the cloud “collapse” in earnest.

This condensation will be diffuse across the interior of the cloud as its temperature drops, rather than at a single point, that a gravity driven process would imply. This is important, because diffuse condensation allows the heat (enthalpy) of condensation to be radiated away at radio wavelengths from distributed sources. If a single point condensation were to occur, the Joule Thomson heat would revaporize any solid/liquid.

Tiny droplets or crytals will coalesce and become larger as they move about in a random walk process in the same process that water droplets in a cloud will eventually form rain. At some point, an accumulation can become large enough (planet sized?) that it starts to pull in additional material through gravitational forces. However, this would have to be sizeable enough so that the gravity induced pressure within the condensed hydrogen accumulation causes it to morph into a super-critical fluid rather than a phase change to a gas. Getting rid of heat as pressure increases is a challenge as the same dust and condensed material will prevent it from dissipating. This heating will be tempered by the negative Joule-Thomson heating coefficient of helium, and the negative coefficient of hydrogen gas above 200K. Nonetheless, increasing pressure on hydrogen will eventually cause an additional phase change due to pressure increase of the hydrogen to a liquid or solid metal where the material will continue to grow, eventually ignite and emerge from the cold dark nebula as a newborn star.

Evidence that a new protostar is emerging from the base of the inverted L in B174, as this bright spot is a combination of broadband and narrowband light. A similar bright spot in B169 could be a Herbig Haro jet or just a hole in the dark nebulosity letting the background light through since this second spot is more NB dominated.

Now these are my own thoughts based on what I have read and heard on cloud collapse and new star formation. If there is evidence that confirms or refutes this view, I would love to hear it, but somehow we have to get from an ultra low density cold gas, to a liquid metallic/high density fusing star.

Molecular Cloud “Collapse”: Barnard 169 & 174