Sh2-135 – Eddies in a Cepheus Molecular Cloud
Askar 151phq; AP Mach2 GTO; ASI6200MM, - Chroma Broadband and 5nm Narrowband Filters
H,O,S: (49,35,45 x 720s, Bin 1, Gain 100); R,G,B: (28,21,27 x 180s, Bin 1, Gain 100)
Total integration time = 29.6 hrs (Aug 5-9, 2024) Maple Bay, BC, Canada

This narrowband image of Sh2-135 in was another attempt to show the turbulent eddies that exist within molecular clouds in the vicinity of new star creation. While not my best work, I think it demonstrates the large scale turbulent flow behavior quite well.

Of puzzlement to me was the question of how a protostar even nucleates within a molecular cloud. Any given hydrogen molecule, or even dust particle, has essentially no gravitational forces endearing it in any meaningful way to other molecules or particles – the masses are simply far to0 small for inter-molecular gravity to have any influence. In addition, any individual molecule / particle would find itself in sea of other molecules/particles in all direction, netting to nil even any small gravitational influence that may exist.   In order to attract hydrogen molecules to one another, there must be a large (relatively ginourmous) and dense object to collect hydrogen via gravitation, setting up a chicken vs egg type problem for protostar nucleation.   Somehow, a dense, large object of condensed material must appear in a molecular cloud to begin the gravity accumulation process, and this mass must appear without the help of the often invoked “inter-molecular gravity collapse”.

If a large enough chunk of integrated (once molten) rock such as believed to be at the core of Jupiter should wander into our molecular cloud, it is likely that it could exert enough gravitational force on gases to accumulate hydrogen to form a star. However, this begs the question of where could such a solid body come from, leaving such an explanation for star formation unsatisfying. In addition, the presence of such bodies, if common, would likely result in too many stars forming much qucker than observed.   In addition, once these bodies were spent, star formation would cease. Overall, this explanation of star formation simply kicks the can down the road – relying on a planet formation type explanation without the benefit of a star to help create the planet.

At the other end of the scale, and with the assistance of radio / microwave radiative dissipation of heat to deep space, the hydrogen gas may be cooled enough to spontaneously form a condensate. Sufficient condensate could aggregate to form a sufficient mass – under sufficient pressure head under its own weight – to gravitationally attract more hydrogen. However, there are a few barriers to this alternate mechanism.   Firstly, the average density/pressure of hydrogen gas is far too low to condense at temperatures in the double digits Kelvin. In fact, the local density of hydrogen must be between 10 and 12 orders of magnitude over the average in the molecular cloud to occur – even at 14K, representing the coldest temperature for which we have thermodynamic data.   Secondly, the density of dust, when averaged over the molecular cloud volume, is likely too low to adsorb and shed the enthalpy (heat energy) of both condensation and spin isomerization that ocurrs when hydrogen condenses.

To achieve higher densities of both dust and hydrogen in the molecular cloud, at least at specific locals where stars can form, requires work to be done to compress the clouds, at least in portions that we could initiate some form of condensation. In a previous images of the Andromeda Galaxy, I explained how viscous forces separate the interstellar medium into more dense viscous dust/molecular cloud lanes and much lower density gases comprising most (by volume) of the ISM. Viscous effects slow down material and reduce its angular momentum as it falls toward the galactic centre and transmit this energy back outwards through the galaxy via predominantly laminar flow. If these forces either become too much to transmit (Reynolds number too high), or something happens to interrupt the laminar condition (eg. Tidal forces), then the flow can become very turbulent and chaotic, in order to shed this excess mechanical energy. One of the results of this turbulence and its associated eddies is that densities of both dust and gases can vary tremendously over a wide variety of space scales to allow the condensation of hydrogen and the associated heat removal via long wave radiation by dust.

Visually, we are likely most familiar with waves on bodies of water.   Water, being an incompressible fluid, demonstrates its wave motion through height above the average water level.   A crest is noted where the water is highest and the opposite for a wave valley.   The movement of crests and valleys across the water represents the propagation of waves.   Compressible fluids (ie. gases) form waves of density, rather than volume or height, which normally can be seen (but can be heard!) unless the gases are emitting/reflecting light or actual hold visible particles in suspension. Molecular clouds of course are an example of where wave can be seen and densities somewhat correlated to the amount or absence of light that we detect from them.

My purpose in showing this image of Sh2-135, was to give a sense of the large scale turbulent eddies within the clouds associated with star formation areas. In this narrowband, the hydrogen in the clouds is emitting primarily Ha wavelength and the intensity is related to both hydrogen density and the energizing UV intensity. Dust tends to block Ha and other emissions from the cloud and its presence and density is mapped via the extinction of emissions. In dark nebula, it is the intensity of radiowave emissions from hydrogen and its extinction that is used to map hydrogen and dust densities and surmise its flow behavior.   Of course, this is difficult because the lack of emissions at any position could mean either or both reduced emissions or emission extinction.   Any reflections also muddy the waters. In this and many other narrowband emissions, it is clear that there are a lot of eddies swirling around creating both hydrogen and dust density hotspots due to multiphase, compressible fluid flow.

In 2016, astronomer/researchers Hopkins and Lee presented an article in a Royal Astronomy Society journal presented numerical simulation results of the effect of turbulent flow within molecular clouds on both dust and hydrogen gas densities. At the smallest end of the scale, simulation of small dust particles in and hydrogen with properties typical within MCs show very strong tendencies for both dust and hydrogen to aggregate in high concentrations under both subsonic, supersonic (dust relative to gas) turbulence and magneturbulent conditions. I have included one example simulation result here to illustrated the fractal similarity between the simulation results and the image of Sh2-135 presented here. In the simulation image colour is used to represent hydrogen density, while black represents dust.   The work also demonstrates that despite only covering some of the physics and mechanics involved (and none of the chemistry) material densities can vary over 10 orders of magnitude, and this is just at this smallest scale. It is worth noting that in these simulation, both high dust and gas concentrations/densities are correlated with each other, but not precisely – indicationing some independent flow characteristics.

Philip F. Hopkins, Hyunseok Lee, The fundamentally different dynamics of dust and gas in molecular clouds, Monthly Notices of the Royal Astronomical Society, Volume 456, Issue 4, 11 March 2016, Pages 4174–4190, https://doi.org/10.1093/mnras/stv2745As the authors/researchers describe in their paper, the result have strong implication on the formation of stars, in addition to the dust-based observations of molecular clouds. With the cooling and short-wave insulation of the molecular cloud, together with the density variability brought about by turbulent flow, the ingredients are coming together for the nucleation of stars via hydrogen condensation, despite the average low density of hydrogen in molecular clouds. Of course, there are few more twists and turns to this plot and there is a lot more going on in the molecular cloud to aid in the condensation and nucleation of protostars than I have described here. However, I am out of space and time, so I will continue this discussion associated with a future image – stay tuned.