Work in Progress…… Read at your own peril, please.
In four postings, I believe we have explained why a galaxy takes the spiral form it does using only two ingredients: a bunch of hydrogen and a black hole – all of which are invisible to our cameras in their native state. At the same time, they sure do make wonderful images, and I want to argue that what we are really imaging is really the icing on our underlying galactic system, or what I prefer to call more – jewelry on the galaxies invisible body. This jewelry is visible to our cameras, by virtue of either emitting light itself or reflecting light that is cast upon it. It consists of either hydrogen in some additional forms (stars, ionic or photon, stimulated atomic hydrogen), or additional substances higher up on the periodic table (dust). The stuff we can actually see actually moves differently around the galaxy in a somewhat different way too, being subject to some of the same forces (such as gravity), subject to other forces in a different way (viscous drag), additional forces that we haven’t considered yet (stellar winds, X-rays, supernovae explosions), and finally second and higher order effects that impact the galactic structure.
I believe a lot of the confusion of understanding the galactic structure stems from the fact that we are actually observing the movement of jewelry and not the movement of the galaxy itself. The puzzle is and has been all along, is how to we deduce the structure of the galaxy by only observing the jewelry (and also, without the invocation of magic). To do this, I will have to come clean on a bit of slight of hand that I used (in especially Part 4 of this posting series) to simplify the explanation by invoking both dynamic conditions and static forces at the same time. I will now come clean and suggest that the whole galaxy is dynamic with mass transformations, mass transfers, heat transfers, and relative movements going on as if a galaxy is a living thing that changes over time.
The explanations of how all of this jewelry is created, works, and moves around the galaxy is why it appears to us the way it does, is the goal of this website, not just this series of postings. Besides, this is meant to be an ongoing journey, rather than thesis. In any event here is what my goal is for this posting.
- 1) At a high level, the active, dynamic circulation of hydrogen through the galaxy and how this impacts what we can and what we cannot see
- 2) Dust, how we see it, how it circulates and moves and what role it plays in creating stars
- 3) Stars, where and why they form, and why they move differently from other hydrogen
- 4) Why dark matterisn’t necessary, or put differently – is dark matter really just gaseous molecular hydrogen?
- 5) What are emission nebula and what is behind the Halpha lines in our galactic images.
- 6) Speculation on galactic evolution
This, alone is a highly ambitious undertaking, so I will purposefully keep it at a very high level. Much of the galactic jewelry such as supernova remnants, planetary nebula, exotic bodies I will leave alone. You will have to read about these in my other postings and elsewhere.
First, I think it is time to come clean and admit that I was misleading at bit to describe the spiral arms as molecular clouds (MC) held separate from the ISM by a wall of turbulence in the ISM. The true story is that while this turbulence does create a regions of much higher pressure within molecular clouds, the interface is not so black and white. The eddies themselves are not neither high nor low pressure, but both at different points and they are very time dynamic. While dominant in the ISM, they do extend into the spiral arm MC to varying degrees – causing turbulence within them too. This turbulence spanning the ISM/MC intefrace can causing erosion of MC into the ISM space, where it may pressure diffuse back from its molecular form into its atomic one. The turbulence does not only cause erosion, but it is also necessary to trigger new star generation (or, more correctly, protstar nucleation) – both within the spiral arm or even eroded chunks of diatomic cloud.
The truth is that both within the spiral arms and the space between them, a semi-steady condition is reached between equilibrium atomic and molecular hydrogen forms. This high pressure / density condition does favour the condition of high H2 content as this is the predominant equilibrium state that the arms are trying to reach. The truth is that the arms are continually being supplied with a fresh supply of atomic hydrogen that is partnering up into H2 due to the high pressure existing in the ISM itself. Without star generation, the galaxy would reach a condition (pseudo-steady state) where new H2 generation in the spiral arms, supplied from the ISM, matches the rate of dispersion, diffusion, and erosion at other places in the spiral arms. The depletion of hydrogen through star formation creates a new dynamic altogether, which causes the whole galaxy to adjust, and evolve through time.
In Part 4, we used the cavitation of water into vapour and liquid phases in a centrifugal pump as an analogue to the MC and ISM phases that form in a galaxy. I will take this analogue a step further, by noting that even though the water pump becomes very inefficient at moving liquid water, it still circulates turbulent water vapour outward through its spiral impellers. Similarly, the spinning galaxy circulates ultra low density ISM from the donut hole of the galaxy to the outer regions. This provides the galaxy with a driver for its circulation system. While the ISM is very turbulent due to the Reynolds number associated with the galaxy, it also has a predominant direct, that is outward with radius, like channels through the galactic MC spiral impellers.
Of course the circulation system needs a return route, if the whole galaxy doesn’t deplete, and this return system is profided by the spiral arms themselves. In Part 4, we described the gravitational system where gravity spirals outward, almost perpendicular to the tangent of the spiral arms. This may or may not have alarmed you at the time, because it seems counter-intuitive that we can actually make gravity spiral. The fact is that what we were describing was only 1 of two components of the total gravity acting on the galaxy and just like force vectors adding together, we can separate force vectors into sub vector components. In this case, it is only one component of gravity acting in this manner – the component that shapes the spiral arms. The second component, only vaguely alluded to, is a component along the spiral arms that acts along its length, always tangent to the spiral arms. The predominant direction of this second gravity component is toward the centre of the galaxy and the flow it creates, still counter to centrifugal forces, along the almost inviscid spiral arms returns the hydrogen material back toward the centre.
The beauty of these gravitational components is that when you combine them, all angular components all cancel each other out, so that the areal mass weighted averaged gravitational field become straight again. Individually the components describe not only the shape of the galaxy and their wind-less motion, but also a critical part of the material circulation system. It is this circulation system the can adjust material in the parts of the spiral arms that are lost to either erosion by the ISM or due to star formation and this material compensation is mainly from the outer regions of the galaxy inward toward where it is needed to keep its integrity, although the galaxy will be slightly altered. Note there is a component of the flow that is countercurrent to the outward flow of to maintain a mass balance. Also there are other things occurring both at the donut hole itself, where polar and spherical coordinate meet and employ our out-of-plane dimension circulation – but that is for another posting.
Of course, all this circulation of material does cause the dissipation of energy of the galaxy. This dissipation is kept to a minimum as it travels through the ISM between the spirals under an ultra-low, even if turbulent, viscosity. The return trip, is made via the spiral arms themselves under nearly inviscid conditions. Nontheless, gradually the galaxy is using its angular momentum to power its circulation system and this results in the generation of some heat, but this only raises the temperature of the galaxy to about 20 to 50K or so (I would love opinions on this), which, in return, reduces viscosity further. The difficulty is, that for all practical purposes the hydrogen materials we have discussed thus far, are rather pathetic at getting rid of heat at these low temperatures. Atomic hydrogen can only emit at these low temperatures via a change to its electron spin, at spectral line at 2.1cm, that is actually used by astrophysicists to count hydrogen atoms, but pretty useless as re-emitter of heat.
Although I am not an astrophysicist, diatomic hydrogen can only emit by changing its molecular vibration or spin, and I am not sure if there is much of that to show for at these low temperatures. The state of electrons in diatomic hydrogen is one of quantum superposition, and while there are two proton spin isomers of hydrogen, any transition only happens when it the hydrogen is in a condensed form. Hydrogen lacks a permanent dipole and only in the last ten years have we begun to quantify the IR spectrum due to electric quadrapole and magnetic dipole transitions of molecular hydrogen, that likely occur at higher temperatures than our galactic model. To date H2 has only been detected in warm places in space, such as in the extended atmosphere and Herbig Haro jets associated with new stars. More on this later, but for now – if our galaxy were only made of hydrogen – it would be extremely poor at getting rid of heat.
So enter dust into the equation. As it turns out dust gives the galaxy its ability to cool itself via both broadband and potentially spectral emissions. Its makeup is considered to be everything heavier than helium – making it likely to exist as either heavier atoms or more likely crystalline or metallic chunks that, at galactic temperatures, are almost surely in a condensed state and therefore broadband emitters. It is also plays a major role in the thermal collapse of molecular clouds, and subsequent shedding of enthalpic energy during the nucleation of protostars. It likely also aids chemically and electromagnetically in creating local high pressures and cohesion necessary to nucleate protostars. I will go through star creation in other posts, but, in short, it is likely necessary for star creation on a big scale in the galaxy as a whole. Stars, in turn, when they grow old and either supernova or just plain nova, leave behind additional dust behind, where it is generally picked up by the galactic circulation system to be distributed along the spiral arms to help generate the next iteration of stars. Perhaps, in the early universe, galaxies were cool enough to create stars, without the need for dust radiation (or perhaps dust radiators is a better term). In our current model, however, it seems that to make stars, we need cold, and dust fits this role perfectly.
The properties, transport, and details of this process are again left for another day, but it is clear that overall its higher density and higher viscosity (if monatomic, likely is some sort of colloidal state) means it likely prefers to life within the spiral arms as far away from turbulence as it can. If only gravitational and centrifugal forces were at play, dust would move and behave just like hydrogen. However, viscous forces are not inertial forces and the incorporation of viscous forces into our galactic model enables the movement of dust in a different manner than hydrogen, while still subject to the same inertial forces. Furthermore, and perhaps even more importantly and especially in turbulent areas, are the creation of electromagnetic and Lorentz (with a “t” this time) forces that separates the movement of dust from that of hydrogen in either form.
While dust in a galaxy does provide infrared emissions that cools, it also both blocks other emissions from behind it – all but the longer IR wavelength, and will reflect visible light from in front (relative to an observer) of it. Although I haven’t established a visible light source, it is this reflectivity and darkness combined that we often describe as “dust lanes” in the galaxy. So, provided that there is dust there and there is light contrast, we can roughly see where the galactic arms reside. While similar to the properties of hydrogen molecules, its different density and flow properties means that it will move differently and inhabit different locations within the molecular clouds themselves. Closer looks at the molecular clouds will reveal beautiful patterns that we enjoy in our astrophotography. For astrophysicists who may be interested in counting hydrogen atoms, it makes life a little difficult due to the dust curtain that they hide behind.
Star formation
Star momentum and periodic gravity
Dark Matter and Diatomic Hydrogen
Emission Nebulosity
Galactic Evolution Speculation