M31 - A Strange Attraction - LRGBHO
A Strange Attraction - M31 in LRGBHO
Televue 127is; AP 1100GTO AE; QHY 600M, - Baader L,R,G,B & 6.5nm H,O Cmos Opt.
L: (97 x 120s 61 Bin 1, Gain 26R,G,B: (54,52,47 x 150s, Bin 1, Gain 26)H,O: (53,29 x 720s, Bin 1, Gain 26)
Total integration time = 26 hrs (Sep29,& Oct 1,2 , 2024) Maple Bay, BC
I decided to pull out my Televue 127is to do some moderately wide field imaging. I love this telescope, but the only thing that was frustrating me when I put it away was the wide field corrector, necessary for full frame imaging, yet never giving completely satisfactory stars. I haven’t had it out since getting RCAstro’s BlurrXterminator, so I wanted to see if it could high quality images by using it to correct the stars. I am happy to say this worked out extremely well. I was able to capture many of the tiny (but physically massive) stars in the outer regions of the Andromeda galaxy, together with Ha/OIII details of stellar nurseries. There is even a Planetary Nebula and many more tiny galaxies in the image.
I always like to provide an explanation to go with my images, particularly if it is slightly against common perception. For an image of Andromeda, what better topic than what makes a galaxy disk-shaped, with spiral arms. The short answer to this, is why, the same things that cause the spiral pattern of a hurricane, or simply watching water flow down a drain: a viscous fluid with angular momentum flowing toward a point attractor.
(If you haven’t before, please take a few moments to fill up your nearest sink, start the water moving around with your hand, remove the plug, and watch). The key word I want to focus on for the moment is “viscous”.
Viscosity is the quantitative description of how a fluid yields (strains) under shear (stress) and is due to the stickiness molecules have for one another when they are brought close to one another. (Technically, is the electromagnetic force due to molecules acting like dipoles (magnets) either permanent or induced on one another when in close proximity). When I have mentioned this, I am often sat down to receive an explanation that hydrogen gas is so rarefied that the molecules are generally so far apart, and hardly ever collide with one another. Therefore, we can treat the molecules like an “ideal” gas – or like very hard, but elastic billiard balls or ball bearings. As a bonus, an ideal gas assumptions makes the simulation of galaxy behavior simpler, and even possible, without having to deal with all that fluid mechanics and thermodynamics.
Without viscosity, a galaxy would consist of independently acting hydrogen molecules orbiting a central condensed mass (star, black hole) on various orbits (circular or elliptical). Only rarely would they collide and the two molecules would just end up finding new orbits. There is likely some bulk rotation of the cloud around some axis through the point attractor and this may result in a slightly higher density of particles perpendicular to this axis, but that is about all one can say for certain. Individual molecules would have no way of actually flowing to the attractor, because they would have no way of slowing down to shed their angular momentum as they approached it. They would simply be sling-shotted out to another orbit – like the planets, asteroids, and comets orbiting the sun. The system would not change much over time, only the loss of the odd molecule here and there as it randomly achieves escape velocity.
Instead, I would like you to consider viscous force that indeed is unimportant when molecules are far apart, but can be extremely important when molecules actually collide. We can think of “kinematic viscosity” as a property describing the effects of stickiness of molecule for another one, in our case hydrogen, but on a per collision or per molecule basis. In this way, the density of the gas can be completely ignored, and we only have to worry about it when molecules actually collide – which of course they do all the time – especially when you have a galaxy full of them.
When two “ideal gas” molecules collide their total (net) momentum in any and all directions are preserved, while their individual velocities are dependent on the angle at which they hit each other. In contrast, “real gas” molecules, when they collide they bond together either momentarily, or in some cases, for quite a few moments due to their “stickiness” or “magnetism”. Before, while, and after sticking together, the total net momentum remains the same, just as with ideal molecules, but in this case, while they are stuck together, their momentum will be equipartitioned between them – both travelling the same speed and in the same direction. If jiggling of the bond has too much energy, it will shortly cause the molecules to fly apart again. (if not the molecules will condense to form a liquid or solid). However, the molecules will have some retention of their momentum when they travelled together and their directions will now be somewhat alike. In other words, the molecules direction of travel will regress to the mean direction and energy, preserving the total or net momentum that they had going in.
If we now take our original ball of idealized hydrogen gas molecules travelling in chaotic orbits and add some stickiness to their collisions, their individual off-plane angular momentums will regress to form a disk along the plane with the angular momentum, with the same angular momentum around our point attractor source – all off axis momentum will simply cancel out via regression to the mean. In addition, elliptical orbits will regress to the mean of circular orbits, as viscosity works to give all the molecules at a given distance from the central attractor the same angular momentum. Without going into details on all of the other inertial forces at play, one can see this as at least a meta-stable configuration, with local perturbances dampened out through regression of identical mass hydrogen molecules to the mean velocity vectors at all points in space.
The stickiness of the molecules will try and hold them together while they orbit around the central attractor as if the disk were a solid – trying to maintain a constant angular velocity regardless of distance from the attractor. Gravitational forces, however, are pulling the molecules towards the attractor, causing the molecules to speed up their period of rotation to maintain angular momentum. As a result, these two forces are opposed to one another – with gravity speeding up the angular momentum, with viscous forces trying to slow it down. You might think of the molecules stuck together as two race cars moving around a race track with one car drafting off of another. The lead car, via wind resistance, ends up pulling the car behind it while the car behind is actually slowing the car in front. This is somewhat analogous to a pair of temporarily stuck molecules, the one closest to the attractor is trying to move faster, but is held back by the molecule behind it, that it is stuck to. This ends up creating a method for molecules to shed their angular momentum to the particles behind it as it gets closer to the attractor and in essence start to “spiral” into the attractor. This spiral pattern is essentially the same whether we are talking about air molecules attracted to the low pressure at the eye of a hurricane, water molecules attracted to the low pressure above a drain, or hydrogen molecules attracted to a central black hole.
This spiral disk pattern isn’t exactly what we see in the image/a hurricane/sink, - what we are seeing is a density wave of the molecules formed as they spiral to the centre. In bulk, the total magnitude of dynamic viscosity depends upon how many molecules colliding and sticking to each other along the surface plane of shear or in other words, local molecular density. The tug of war between gravity and viscosity is won by gravity and a lot of shear will occur in spirals of low density, while won by viscous forces in areas of high density. What our Andromeda image is actually showing is the areas of high density (noted by the “dust lanes” and the low density areas in between. This resulting density spiral wave rotates around the central attractor. To get a sense of the trajectory of the hydrogen gas, it is travelling in a trajectory that is tangent to the density spiral. Of course, the stars too have angular momentum and start out retaining the same angular momentum imparted to them by the hydrogen from which they were made. Stars are much less impacted by the viscosity of the media through which they travel and are free to take up independent orbits. If you see a spiral pattern in the stars, they are just vestiges of the density wave that created them.
I think that is enough for this explanation. I will be sure to write more in upcoming images – but I believe I have said enough for now to explain a lot of why spiral galaxies are the way they are.
