Sh2-155 - The Cave Nebula's Narrowband Emissions (HSO)
This is Part 2 of Cave Nebula Explanation - read this part after Part 1 - from Broadband Image
The start of the journey begins with a molecular cloud at an ultra-low pressure, consisting of what I call three components: hydrogen, volatile dust (such as CO, hydrocarbons, water?) and non-volatile dust such as metallic and minerals. These components exist as either molecules or micron sized particles that are all far too small and sparcely spaced for gravity to provide any significant cohesive force for hydrogen accumulation. In order to get the stage C, we have to create a large, dense mass for gravity to be significant – sort of a “critical mass” required to get the process going.
Stage A, consists of the cooling and compression of the molecular cloud that I have described elsewhere. During this stage, both volatile and non-volatile dust provides a conduit for heat to escape the molecular cloud and provide the necessarily, if not sufficient, for a thermodynamic phase change, from uncondensed gas to incompressible solid/liquid to occur under Stage B. It is only via such a phase change can a cohesive, sufficiently dense and massive protostar nucleus be created to accumulate hydrogen under Stage C.
On the phase diagrams, I have purposefully shown Stage B to be left of the critical point for hydrogen. The reason I have done this is to emphasize that one a nucleus of material has condensed, if it were to re-enter the “gas” phase it would self destruct and disperse. The progression of hydrogen needs to be condensation (phase change) and then transition to a super-critical fluid and bypassing the gas phase.
I note that a rocky mass, say the size of Jupiter, could accumulate hydrogen directly from a gaseous state to a supercritical state, but this would have to occur slowly to avoid dramatic temperature increase, and presupposes the existence of large dust “planets” which would then have to be explained somehow. Stars appear only to form in very cold molecular clouds, strongly suggesting that the cold condensation of hydrogen is a critical part of star formation. It is far more likely that the nucleus of protostars is a mixture of non volatile dust and condensed volatile dust and hydrogen more like what is believed to be at the core of Saturn, or comets – rather than the rocky core believed to be at the centre of Jupiter. As an aside, there is likely a great deal of analogous processes involved in both star and planet formation. Also, it is technically incorrect to call Jupiter, Saturn and other planets as “gas giants” when in reality they are supercritical fluid giants.
It would be just as great an oversimplification, to simply say the hydrogen condenses to form a protostar nucleus as to state that it is formed by the gravitational collapse of the molecular cloud even though both do occur (stage B versus C respectively). While indeed, should the molecular cloud temperature drop to single digits K, spontaneous condensation can occur even at the prevailing ultra low hydrogen partial pressures, the true process likely involves understanding the forces at play in compressible, multiphase flow equations and the roles played by the dog’s breakfast of phases and chemistry in participating in and catalyzing hydrogen condensation and condensed mass accumulation. The process is likely one of the ultimate cases of complexity and chaotic behavior that one can imagine and I have resigned to understanding it only at a high level. In future image description, I will indicate various pathways that hydrogen can take to become a condensed phase.
This image of the Cave Nebula is suggestive of some of the complexity involved in the nucleation of protostars. We see evidence of radiative displacement and compression and the presence of turbulent eddy flow in the dust, which can also greatly effect the pressure distribution within the molecular cloud. Finally, it takes little imagination to see that while overall dust density seems to correlate with gas density, at some level – based on emissions areas vs dark regions, the flow of dust versus hydrogen may only be weakly coupled and doesn’t necessarily flow together.
