(Full Res in RASC Zenfolio, or full res mouseover in Astrobin)
(Full Res in RASC Zenfolio, or full res mouseover in Astrobin)
SH2-155 or Caldwell 9: The Cave Nebula in Cepheus
Askar 151phq; AP Mach2 GTO; ASI6200MM, – Chroma Broadband and 5nm Narrowband Filters
H,O,S: (56,48,50 x 720s, Bin 1, Gain 100) & R,G,B: (36,35,36 x 180s, Bin 1, Gain 100)
Total integration time = 36.5 hrs (Aug 28-31 & Sep 3, 2024) Maple Bay, BC
Here is presented both an RGB and HSO (using the Block/Cranfield narrowband colour technique) of the Cave Nebula star forming region. In my eye both make very nice pictures and the narrowband colourization technique provides a more consistent colourization method than created using the Hubble narrowband palette. However, when one it trying to delineate the structure and position of the molecular cloud it is helpful to have both images. This image was taken to further my understanding of star formation, and test if I could see if dust and gases always flow together in a strongly coupled manner in star forming regions.
Much of the journey that hydrogen molecules undertake in transforming from a member of a diffuse gaseous cloud with completely collective where gravity is almost completely negligible to a dense metallic spherical protostar under enormous gravitationally driven pressure is not well understood. It is almost as if nature doesn’t want us to know how it happens, at least not exactly, by shrouding the process in dust so we can’t see it, and making the conditions and scales under which a protostar forms beyond laboratory reproducibility. Numerical modelling is also of limited utility in understanding star formation and the process appears to be too chaotic and complex for computation, and in the end, models only echo what you want them to say anyways.
That doesn’t mean we cannot, at least, paint a fuzzy picture of what the prototstar formation should look like by considering a number of scientific principles/disciplines and hopefully combine them into a consistent story that makes sense. In this description, I choose to take a thermodynamic approach to the problem, because we know that the process must be consistent under the laws of thermodynamics in order to be viable. The thermodynamic description consists of mapping the state of hydrogen molecules as they move from a low temperature, ultra low pressure gas phase to a high temperature and pressure protostar through temperature and pressure space on Pressure-Temperature diagram. For a single substance, in this case hydrogen, its position on a PT diagram completely defines the microstate properties, such as density, phase (gas/liquid,etc.), viscosity, heat content, entropy, of the substance at equilibrium.
I am showing two phase diagrams (one linear, and one logarithmic) to sketch out what I believe are the three steps to protostar creation marked on the phase diagrams:
A) cooling and compression at very low pressure/density;
B) phase change to form a dense protostar nucleus; increase in pressure and density at low T;
C) hydrogen accumulation around the nucleus by gravitation forces increase in T and P
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. Ultimately, regions of gas and dust densities that are orders of magnitude higher than the average cloud density are required to condense the hydrogen, flocculate the dust, and nucleate a protostar. It is the eddies and shock fronts, occurring on multiple scales, that likely create such regions for star formation to progress along stage B.