Circulation and Jewelry – The Galactic Spiral Structure (Part 5)

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 structure 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, it is a goal of this website, not just this series of postings to provide these explanations. Besides, this is meant to be an ongoing journey, rather than thesis. In any event here is what my goals 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 matter isn’t necessary to explain anything. Rather it is a snake oil equivalent – used to explain away mechanical and thermodynamic processes of real substances and maintaining a over-simplified description of what is going on.
• 5) What are emission nebula and what is behind the Halpha lines in our galactic images.

This, alone is a highly ambitious undertaking, so I will purposefully keep it at a very high level – with details to come in future postings, describing real images. Much of the galactic jewelry such as supernova remnants, planetary nebula, exotic bodies I will mainly leave alone here, but again will be the focus of future postings.

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. This is mainly true (I would say 99% true), but the 99.9% truth involves admitting that the spiral arms are continually forming via the simple reaction of “2H -> H₂” predominantly at one side of the spiral arms, and the backwards reaction of “H₂ -> 2H” predominant at the other side. In other words, the same physical hydrogen atom may spend part of its life with a partner as a molecule, subject to relatively high viscous force while happily living its existence within a spiral arm, when one day it breaks up with its partner to form a solitary atom, in the ISM. In other words, the two thermodynamic phase states (MC and ISM) created by fluid mechanical forces in a rotating disk, are not in equilibrium, but are continually forming and breaking apart molecules with the same recycled hydrogen atoms.

As an aside, the 100% truth is that both atomic and molecular (and even ionic) forms of hydrogen exist both within the spiral arms and within the gaps between them – just in different proportions. This distinction between the arms and the gaps is just in the relative abundance that favours H₂ within the higher pressure arms, and atomic H in the gaps. The two phase-states of hydrogen are indeed real, and there is a real interface between them – despite containing the same ingredients, only at different proportions and pressures. In parts of the galaxy the interface is invisible to our cameras, but in other parts – dust or hydrogen emissions show us that the interface is undeniably real. I make no apology for stating that the arms are material, because while they are always forming and breaking down I would consider a lake as material, despite water arriving and leaving the lake all of the time. For simplicity, however, I will just go ahead and pretend that the arms are H₂ while the ISM is atomic (or ionic) H just for simplicity of explanation.

If you think of water molecules in a lake, they are not locked in for life as lake molecules. It is likely that the lake is being supplied and drained by (different) rivers. Individual water molecules may stick around, forming the lake for a while before draining away and to live their lives as rivers, seas, sometimes via a phase transition as gas or ice, sometime positioned deep underground within an aquifer, sometimes high in the sky within clouds, or as rain. Some of these molecules will find themselves flowing back into the same lake some day. The lake itself remains as an entity – sometimes getting bigger/smaller, sometimes violent eddies, sometimes more placid. The movements of the lake itself are related to, but different from the movement of the individual water molecules that make it up. Similarly, individual atoms in the galaxy are not locked in as either “arm molecules” or as monatomic ISM. The dynamics of the hydrogen atoms is quite different and more complex from that of the arms.

To understand the movement of hydrogen I want to return to the graph we created showing the shape and periodicity of gravitational acceleration in both the radial and angular directions. The shape of the overall g-force is a spiral acting roughly perpendicular to the spiral arms. This gravitational field applies to the arms, as cohesive structure as well as to individual hydrogen atoms/molecules, both within the spiral arms (where the g gradient is largest) and within the ISM, where the gravitational pull is weaker. This tends to cause a continuous periodic imbalance between centrifugal and gravitational forces on any particle moving around the galaxy at a different velocity than the spiral arms themselves. To this periodic dance, we have to add a third viscous force, that essentially only exists within the galactic arms. This force acts acts at an angle to circular orbits and essentially causes the individual atoms/molecules to orbit apparent centre that does not align with the galactic centre.

For an example, a particle may find itself near the outside of a spiral arm, with a stronger gravitational pull toward the centre of the galaxy. This will cause gravity to be stronger than centrifugal force and pull the particle towards the centre to create a smaller radius of orbit. When the particle finds itself near the inside of a spiral arm gravity is locally less, and the particle will want to increase it’s radius of orbit to create a rebalance of inertial forces. Thus, for any particle moving in an orbit faster or slower than the spiral arms will find its orbital radius periodically getting shorter or longer in a sort of never ending dance to achieve inertial balance between gravity and angular momentum. As we shall see, hydrogen, as it circulates around the galaxy, is subject to this back and forth motion as it travels at a different speed than the arms themselves.

As you can imaging the dance that an individual hydrogen atom goes through as it orbits the galaxy is quite elaborate – getting alternatively pulled in to a shorter orbital radius by stronger gravity, and thrown outwards by weaker gravity and stronger centrifugal force, alternatively accelerated and decelerated by periodic gravitational strength in the angular direction too. Changing direction and periodic acceleration and decelerations is the norm in the life of a hydrogen atom, but at least it tends to do this in a somewhat regular pattern, at least at a large scale. I wish I could create my own simulation of this (I have not the skills, resources, or time), but I can “borrow” a simulation from the Density Wave Theory wikipage that almost gets it right.

Of course, on top of all this is the transformation of hydrogen alternatively between molecular and monatomic form is again, all just part of its dance. While the balance between gravity and centrifugal force is the same whether the hydrogen exists in it monatomic or diatomic (molecular) form inside or outside of the arm, it is only within the arms themselves where the higher pressure, molecular form brings significant viscous drag to bear on the particles motion. In terms of velocity – viscous drag keeps the retention time of any hydrogen particle longer within the molecular cloud and essentially makes the hydrogen move as part of the cloud – around a virtual centre at a virtual radius that differs from the true centre and true radius of the entire galaxy.

Near the galaxy’s true centre, the virtual radius is smaller than the true radius, and individual hydrogen particles will move slower around the true centre than the arms themselves, while further out, the virtual radius is larger than the true radius and the particles will orbit faster than the arms themselves. This effect can be seen in the simulation above – setting up a dynamic relationship between the arms and the very hydrogent atoms/molecules that make it up.

Just as the lake is continually being drained and resupplies, the spiral arms of the galaxy are continually being constructed with fresh (or better yet – recycled) hydrogen predominantly on one side, while also deconstructed predominantly on the other. So what does this all mean for a hydrogen atom moving through the galaxy with a period pull of gravity along different paths, always trying / transitioning to find an orbital radius that balances off gravity with centrifugal forces? Sometimes, it finds itself bound to another atom, subject to viscous effects, and at other times it is alone. Sometime it exists as part of a cohesive spiral arms, dragging and being dragged by its neighbours, and sometimes as an independent atom. The answer is indeed complex, so I won’t bore you with all the details, only to suggest the following rules..

• 1) Hydrogen tends to move faster in the angular direction (higher angular velocity) than the spiral arms towards the outside of the galaxy. This is because the arms are orbiting at a larger effective radius than the hydrogen. (recall our environmentalist/squirrel analogy).
• 2) The opposite occurs towards the centre of the galaxy, where the spiral arms are orbiting at a shorter radius than the molecules themselves.
• 3) The difference in velocity between the arms and the hydrogen is apparent from the continuous formation of molecular hydrogen on one side of the arms, and its erosion and destruction on the other. at the outer parts of the galaxy, erosion and deconstruction is dominant on the convex, leading side of the arms, while closer to the galactic centre, erosion and deconstruction is dominant on the concave, trailing side of the arms. Of course, both construction and deconstruction is occurring on both sides to some extent.
• 4) Due to viscous forces, the progress of hydrogen across the arms is slowed (and subject to turbulence). Within the arms, the hydrogen is subject to transverse gravitational pull within the arms, pulling towards the centre at the outside of the galaxy moving it radially inward. Once again, the opposite is true near the galactic centre.
• 5) Eventually, the hydrogen reaches the other side of the spiral arm where it disassociates with its molecular partner and associated strong(er) viscous forces. As it leave the spiral arms, it enters a region of local minima (or maxima) in the gravitational pull of the arms, where its gravitational pull is no longer appropriate for its velocity. Centrifugal forces will then move the particle on an outward (or inward) trajectory to restore balance with gravity.
• 6) Within the spiral arm the radial movement of hydrogen is counter, and at a higher velocity (but equal mass flux) to the movement of hydrogen within the arms, setting up the Bernoulli condition supporting the pressure within the arms
• 7) The different angular velocity of the ISM and the arms means that eventually hydrogen (now atomic) either runs into (or is hit by) another spiral arm on its concave (or convex) side, to join with another hydrogen atom to form a molecule and repeat its time as part of the spiral arm once again.

This whole concept of hydrogen moving in a different way than the MC spiral arms that it itself creates is part of a system that engineers describe as dynamic. One can’t create a static version of it because it is the movement and transformation of the galactic components that make it work and it only appears static in our images because of the enormous scale of the movement is occurring. In addition, the three forces (well, really two forces (gravity and viscous drag) plus angular momentum) are non-linear to one another. This sets up a special class of dynamic behavior known as chaotic dynamics, or turbulent dynamics. However, don’t let the word “chaotic” fool you, there is much order and beauty that can be found in chaotic systems and the creation of spiral arms, orbiting at a constant period and created by hydrogen orbiting at a different period, is just one of many examples. Chaotic dynamics is a field of physics and mathematics all on its own that is extremely important to understanding cosmology, but it is often pushed aside because it chaotic systems do not lend themselves well to numerical modelling and accurate simulation projections.

The back and forth dance would not be appreciated by astronomers and astrophotographers, however, if the galaxy were only hydrogen gas in its diatomic and monatomic forms. At prevailing galactic temperatures, both forms emit no visible light. Even in the infrared, only monoatomic hydrogen emits at a narrowband wavelength of 2.1 cm due to the quantum flip of its electron spin relative to that of the proton, but the picture we get at such a large wavelength ends up being pretty blurry. Diatomic hydrogen can’t even manage that, with its two electrons in quantum superposition. Now there are a few exceptions to this that we will get into later and in other posting, but the point I want to make is that primarily what we see in a galaxy ends up not being hydrogen in is gaseous forms at all, but what we actually see are hydrogen’s dance partners – either additional and still different forms of hydrogen (condensed stars or excited) or different elements altogether (dust and helium). Because of these dance partners have again different properties, they make themselves visible in different ways and dance with different moves, subject to both different magnitudes in inertial and drag forces as well as some different forces (electromagnetic, supersonic) altogether.

Macroscopically, I like to think of the galactic structure as a circulation system, driven by angular momentum rather than heart. As material is moving in and out radially, alternatively within the MC arms and the ISM gaps while rotating round and round, turbulence does its best at mixing things up and providing both the energy and material necessary at different places for the galaxy to due its thing. 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. Other components of this macroscopic circulation system include movement of material out of the galactic plane (z – direction), with hydrogen being ejected away from the galactic disk within the toroidal donut hole to eventually being brought back into the galactic plane due to the gravity acting along the disk itself.

We are now primed to introduce, what I call the jewelry into the story – in other words the components that aren’t so invisible as hydrogen (and helium) to our eyes and astrophotographic cameras. The first component is dust, that form the “dust lanes” in our galactic images. Dust, is a catch all term, used to describe any material that is no hydrogen, helium, photons, or electrons – essentially everything with an atomic weight greater than helium. It exists within the galaxies mainly as molecular or metallic chucks but can also exist individual atoms/ions. At the prevailing cool temperatures of the galaxy, the material is likely best thought of as condensed bits of material / particles held in suspension within the hydrogen. Although we conveniently think of it as a single substance – it is indeed a mixture of many things – both on the molecular and larger scale with varied chemical, electromagnetic, and astrophysical properties.

Dust, by virtue of being condensed material unlike hydrogen, can readily adsorb visible light, and it is via this ability that we can actually see dust in our images – at least in profile. Any photons emitted by stars or bright nebula that exist behind the dust tend to be adsorbed by the dust and dims the total light from behind it as we perceive it. In galactic images, such as below, we see the dust within the galaxy mainly as “dust lanes” indicating where the its concentrations are strongest, and some or most of the light from behind is blocked. Dust is mainly visible in our close-up Andromeda below, via its blocking of starlight that is coming from behind. I say, mainly, because dust can also reflect light, and concentrated light can also appear to be reflected off of it, and dust reflections are also the subject matter of many of the images we take.

Dust is, of course, largely subject to the same gravitational and centrifugal (inertial angular momentum) as the hydrogen and helium atoms and molecules. Recall that mass drops out of the equations used to balance gravity and centrifugal forces (same goes for stars, by the way). Thus, the flow of dust largely follows or traces the flow of the hydrogen circulation system. Thus, we tend to “see” dust as concentrated in the same place as hydrogen is concentrated, i.e. within the spiral arms. “Dust lanes” is somewhat synonymous with spiral arms although the former is used to explain our images, while the latter pertains more to the galactic structure.

The dust particles themselves, do move differently in several important ways than hydrogen, due to their very different properties and I will name but a few here. Most of the dust likely carries a dipole moment, if not bulk magnetism, by virtue of their chemical makeup. Molecular hydrogen will tend to form strong hydrogen-bonds, making the bulk fluid mechanical properties similar to hydrogen molecules themselves and be held in suspension within molecular clouds and the spiral arms. While suspended in molecular clouds the dust will behave as if it has viscous drag too! However, the apparent viscosity and inertial behavior will be quite different.

Compositional grading due to its higher molecular weight, coupled with a higher apparent viscosity will cause dust particles to bully their way to the centre of molecular clouds or the axis of the spiral arms while preferring not to join the ISM. In this manner, dust almost becomes a “tracer” for molecular hydrogen – allowing us to almost “see” what can’t be seen – the spiral arms themselves.

At an even smaller scale, even more forces are brought to bare that separates dust movement from that of hydrogen. Dust can be much more strongly influenced by electrical forces and inertial effects within turbulence (like a centrifuge). Definitely within nebula, dust will transport, accumulate, and disperse at different, more precise, locations and this is crucial for the creation of stars. In galactic images, however, the presence of dust generally indicates the presense of molecular clouds (although not necessarily visa versa.)

Dust is critical to the life of a galaxy, as it provides a cooling agent for the galaxy as a whole. All that viscous friction, although minimized by the structure itself, generates heat. It does this by re-radiating both heat energy and energy from absorbed visible/UV light primarily as infrared. For the most part and increasingly as IR wavelength increases, this infrared light can pass right through other dust and ultimately shed to the even colder (4K) regions outside of the galaxy itself. In this way, dust can also be thought of as the galaxies radiator.

At visible and UV frequencies, dust plays another role in our astrophotographs. While also blocking light from behind, dust particles tend to reflect light from their surfaces, while blocking light from behind them. In many of our visible light galactic images they are visible only by the absence of starlight and stars from behind or in other words, as a backlit profile. Often, the dust is so thick within the molecular clouds, it looks as if we spilled black ink across an image. Rather than acting as a heat radiator, at high temperatures where visible or UV light might be emitted by an object, the dust acts as an insulator – either adsorbing and back-emitting heat, or back reflecting it. This also make the dust a major actor in star formation. Diffuse fore-light from stars (closer to us) when cast upon the dust, will reflect a grey to reddish brown hue outlining the its presence. We call this “dark nebulosity”, or even “molecular clouds” due to the strong association of the dust with molecular hydrogen.

Aside from the early universe, the overwhelming majority of stars are created within galaxies, and our dust provides a critical role in this by both cooling hydrogen below its critical point, chemically by elevating condensation point, and gravitationally by helping the star material remain supercritical on its pressure and phase transition to becoming stellar liquid metal. This process is complex and tends to be oversimplified as “molecular cloud collapse” and that is fine on one level, but the deeper process is much more interesting and will be described in other postings at this site. Nonetheless, there are yet more phases of hydrogen to be considered, but the three ingredients we need to make stars all exist within the galaxy all exist within the molecular clouds – turbulence, dust, and molecular hydrogen.

We wouldn’t see much of the galaxy at all if it wasn’t for stars. Except for some more exotic galactic objects (which we indeed also like to photograph) almost a the light that gets to us is from stars. Even dark nebula are created by from starlight, either reflected or blocked by dust, such as the image below of the “Dark Shark Nebula”

Now we will turn to the literal and figurative stars of the show – the stars themselves. Stars are pretty easy, since we have all the ingredients we need to make them. I will make it even easier by avoiding the discussion about how they are made. Very simply, they are yet another form of hydrogen – not atomic or diatomic molecules, but hydrogen in a condensed, liquid metal form, with sometimes planets moving around them. While they are very small compared to our galactic dimensions, their range of influence around them can bely their actual size – especially when they form clusters. They are composed mainly of hydrogen, but also helium and dust which not only goes into their making, but also are generated via the fusion of hydrogen atoms (protons) into heavier elements within them. Fusion also causes them to get very hot, and the thermal photons (IR, visible, UV, and gamma) they give off either directly or indirectly light up just about everything we can see in our images. In addition, they emit solar winds composed of photons (hydrogen ions), electrons, and alpha particles that can clear out vast swaths of hydrogen and dust from their vicinity.

Stars are born mainly within the hydrogen and turbulent rich areas on the edges of the spiral arms, or in clouds turbulently expelled from the arms. From a transport point of view, they tend to have the angular momentum inherited from their ingredients (the hydrogen in the spiral arms) and are subject to the same spiral gravitational forms created by the spiral arms. As with hydrogen atoms, their orbit will be periodic in radius as they move from their original birthplace and travel around the galaxy and do their gravitational dance.

However there, is a couple of big differences between the stars in orbit and the hydrogen gas particles. Stars, are not subject to the viscous forces, nor the phase transitions, that the much smaller gas particles are. This means that they are not held captive within the spiral arms at all, and do not spend the residence time that hydrogen does within the arms because they are less trapped by turbulence. While proximity to spiral arms influence their gravity, they do not shift their orbital centre as the arms themselves and the hydrogen within them do. Overall, this means that, on average they move “faster” a shorter orbital period in the outer part of the galaxy (where we can best see them) than the spiral arms themselves and they move “slower” than the arms close to the galactic centre. In other words, they do part of the dance that the hydrogen does, but not with all the same moves.

At this point, we have to make two categories of stars in order for the story of the stars we see to make sense. There are big, bright, short lived giant stars that tend to be blue that tend to be see in the outer reaches of the galaxy, and the smaller red/yellow/white stars that tend to live closer in. (There are really way more categories of stars and they can actually evolve from one form to another, but these details can be addressed in future posts.) The main point I want to make is that the big blue bright stars, because they are brighter and hotter, burn through their hydrogen fuel more quickly and tend to be found in our images fairly close to their birthplace. A typical blue giant star might on shine for 10 million years (sounds like a long time), that is much shorter than the 225 million year period of the spiral arms of a galaxy like the Milky Way. This means that, when it is only moving marginally faster than the spiral arms, these stars don’t get too far from their birthplace before they burn out and go supernova. For this reason, we also use the position of these stars as another (but a blurry) proxy for the location of the arms themselves, but we should realize this difference when we examine astro-photos.

For modest stars (such as our sun, with a lifespan of 5 Billion years or so) or smaller – they have made many trips around the galaxy and their position is almost independent of the angular location of the spiral arms. Overall their position is more concentrated near the galactic centre, and a there are a number of possibilities why this is, but most often this concentration of light obscured their individuality and the details of the galactic structure here. Often it is deemed lucky if we can even make out the wisp of a dust lane close to the central black-hole attractor.

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. Fairly recently, the JWST space telescope – the ultimate specialist in IR imaging – has provided a picture of where the dust is and where it is emitting from through the galaxy.

If only gravitational and centrifugal forces were at play, dust would move and behave just like hydrogen, subject to the periodicity of gravitational acceleration in both the radial and angular directions. It’s back and forth dance will be similar to that of the hydrogen particles. However, viscous forces on stars are much, much weaker and stars will not be subject to the same retention times within the spiral arms as the hydrogen and the accelerations/decelerations that come with it.

Early in a star’s life, emitted UV light can strike surrounding H2, heating it up, breaking the molecules apart and ionizing the atoms. When these ions recapture their electrons and move to return to their previous atomic ground state, they emit photons in narrow bands of wavelengths, some of which are in the visible spectrum – creating light from hydrogen that we can see with our camera!. One of these narrow wavelength bands of emission is called Hydrogen-alpha (or Ha), that we can capture in our cameras – as part of a normal RGB galactic image, or enhanced through use of a filter that excludes all light but the specific narrowband wavelength characterised as Ha. Since Ha is in the red range of visible light, they appears as red blotches, loops, or strands in the galactic image, such as in our Andromeda galaxy close-up above.

Such red bands/loops/areas are so associated with star formation, that we can use them as beacons in the galaxy pointing to where stars are being born. They are indicators of where turbulence and star building blocks are coming together in the right proportions to make stars. Since the turbulence in the stars is mainly due to its circulation system and where clouds are next to highly turbulent ISM – we see them mainly on the edges of our spiral arms. In some galaxies that aren’t fully formed yet, or that are being yanked about from external tidal forces, a great deal of turbulence can be generated that can send some galaxies into overdrive in terms of star production – all of which make excellent astrophotography.

Unfortunately, these narrowband emissions is all we can actually see of the non-star hydrogen in our galaxy – which is why there has been much speculation on how the galaxy works and maintains its dynamic integrity. However, in its atomic form, hydrogen does give off a signature IR emission at 21cm due to flips in its electron spin. Of course, we can also see hydrogen in its stellar form quite readily. Astrophysicists have used this to estimate the quantitative mass that is contained in a galaxy. Corrections have also been made to account for the dust that can hide emissions. But in this process, I believe they have made an accounting error.

There are many similar graphs out there that examined how fast different elements orbit around the galaxy as a whole. The most common one you might find is the one below, which describes the orbital velocity of both stars and, where the stars are non-existent or sparse, monatomic hydrogen itself as a function of radius. – as if both the stars and hydrogen atoms were both independent acting particles subject only to their own combined mass. The entire mass of the galaxy was determined by adding up both the visible and IR visible (21 cm emitting hydrogen atoms) with adjustments for dust and other objects (central black hole) and assumed that this was material forms a disk shaped mass the size of the visible disk in the galactic plane. Particle velocity was determined by examining the red-shift of the starlight and atomic IR band.

As we have discussed, the orbital velocity around a disk or point mass or disk can be calculated as a function of radius such that the gravitational pull towards the centre of the galaxy is equal in magnitude to the apparent centrifugal force pulling away and this is shown in the dashed curve. (Note that the gravitational pull within a disk is a tricky calculation, but outside of a disk, it varies with the inverse of the square root of the radius cubed.) When the actual rotational velocities were plotted, an almost completely different curve was determined, although the two curves were closest to one another near the centre of the galaxy. After scratching their heads for at least a day or two, they concluded that there had to be some mass that they weren’t accounting for, and that this mass had to be invisible both to visible light and IR (at least at the prevailing temperatures of a galaxy). Fair enough – it was the 1970s and we already knew from 1933 that the apparent mass of galaxies seemed higher than what we added up. So astronomers made up a new substance, called fairy dust, or snake oil, or dark matter that simply made the numbers add up. (There is a parallel here with nebulium, the fake element that emits visible light that we now know is just ionized oxygen). Fair enough again – it was the 1970s and halucenogenic drugs were everywhere. What I don’t understand is why now that it is 2025, we still cling to this notion of dark matter.

In my opinion, we should not just make stuff up willy nilly to explain things that are difficult to explain. We should only make things up that are impossible to explain. What should have happened is a deep dive into the fundamental (and not so fundamental assumptions) that were made in creating this plot. The fundamental assumptions were that hydrogen is an ideal gas – subject to dispersion but not viscous drag – and that its phase behavior was simple – it existed in a monatomic state everywhere (at about a density of 100 to 300 in the decidedly non Avogadrian units of atoms/cm3). Pressure and temperature were unimportant.

What has been missed is that hydrogen is not an inviscid ideal gas, and exists as H₂ within the molecular clouds. H₂ 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 – not measurable anyways. 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 due to the warm temperature of these objects . Since we can’t see it either in IR or visible light it hasn’t really been accounted for in the assessment of mass. It sound much more plausible that 85% of the universe is made of H2, rather than something that no one has seen, yet alone weighed.

Astronomer have also though a while about where this mass could be, since there is none down here on earth. The conclusion is that it must exist as a halo around the outer part of the galaxy, since this is where the two velocity curves mismatch the most. The problem with this theory is that if a lot of this dark matter is placed as a halo around the galaxy, this will have outward pull of gravity on the visible disk within the halo. Then we would have to loop around and adjust this too. The problems with the anti-matter theory are just too long for this post.

Finally, I would point to the gravitational force plot at the top of this images, that was used to describe its periodicity both radially and angularly. I believe this periodicity is actually shown in the measurements made on M33 both in the periodicity of the graph and in the error bars. The periodicity arised from accounting for viscous drag as well as inertial forces in a spiral structure of H2 within the galaxy. As an added bonus, there is no need for a separate theory as to how the stars create the arms and the constant orbital period via traffic jams.

Consequently, I have concluded that something called dark matter is not needed at all in the galactic jewelry.

I hope you have found this series of postings on galactic spirals and structure. It has been quite a slog, setting up the website and writing this series and has taken away, somewhat, from my own astrophotography. Please, whether you found this helpful or not, please leave a comment to let me know what you thought. Future posting will not be anywhere near as long or involved, but will take a more gradual approach as the mysteries of the universe reveal themselves to me through images.