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 foot 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 their proximity.
Three views are represent a broad spectrum RGB image combined with narrowband signal from atomic hydrogen Ha emissions. The first image represents an LRGB (broadband view) of the nebula, using filters that roughly match the same colour broadband light as our cones within our eyes do. The scene is one where a glowing backdrop of primarily red light is shining towards us, through a dark – dust containing molecular cloud. While the diatomic hydrogen and helium, that comprises most of this cloud are transparent, the dust (condensed higher molecular weight material) within it is not. The cloud is being swirled around in turbulent motion and the eddies this causes in the dust makes for interesting patterns. Stars are also visible in both the foreground and background of the cloud. The light from the foreground stars are also reflecting of the dust, revealing a dark reddish-brown colour.
In the second image, the normal brightness filter that is the equivalent to what our eyes experience has been replaced with a special filter, tuned to the red wavelength emitted by the background. This is a special form of light emitted by atomic hydrogen over only a narrow band of wavelengths when a molecular cloud is stimulated by strong UV emissions. The UV light breaks the molecules and frees some of the electrons from the nucleus of the resulting atoms. The red narrow band light we see, is a result of hydrogen atoms, having recaptured it protons, returning to their ground state – likely to reform with another atom to become a molecule again. We call this narrow band of emitted red light Ha (or hydrogen alpha) and it is one of the view emission of hydrogen that we can see in the visible range. When cold and not stimulated by UV, both forms (molecular and atom) hydrogen are invisible to our eyes and the camera.
To better see where the Ha light is coming from, we have exaggerated the brightness signal (roughly 10 fold). However, when we use the Ha signal filter, it dims the light from sources of other colours, and in our case, darkens any reflected light from the foreground dust. This allows for a much greater detail in the actual structure of the dust to be shown in our second image.
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. The stars themselves have been left their natural colour in all images. In the intense Ha (mouseover) 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 by the nebulosity 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 the nebula. In parts it is appears as smoke in other parts it is thick enough to black out even bright stars behind it. It almost has a sharp binary representation – black where there is condensed material (dust) and not where there isn’t.
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 shown 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 20 to 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 5K. between 20 and 100K, molecular 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. Light can readily enter deep into the diffuse nebula.
Meanwhile tidal forces and radiation may 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 to become dark and colder too. Shielded by the dust to cool towards the 7 to 15K temperature of space through emission of first infrared, turning to microwaves, and then turning to radio waves by the cooling dust. The longer the wavelength, the less effective the dust will be at blocking this outgoing radiation.
A reduction in temperature will cause the cloud to shrink in size and material to be more concentrated at the prevailing pressure. Due to electromagnetic forces between hydrogen molecules (van der Waal forces) hydrogen will start to condense into a liquid or solid 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 re-vaporize any solid/liquid. Tiny droplets or crystals will coalesce and become larger as they move about in a random walk process in the same process that water droplets in an atmospheric 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. In both cases, I am open to suggestions as to what these bright linear elements are.
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 and through phase change at low temperature seems the most digestible.