Society for Popular Astronomy

dynamics of galaxies.
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Author:  Deimos [ Mon Mar 21, 2011 12:40 pm ]
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maurice wrote:
Hi Ian. I realised you favour the dark matter idea. What I am asking is how does it work. Is it just like ordinary matter except not visible? If so, how does it cause the galaxies to break the Newtonion and Keplarian rules of orbital dynamics?
All the best Maurice.

Happy to explain my understanding but to be honest I would be repeating the thoughts of others and almost certainly introducing errors at the same time (bit like "Chinese Whispers"). Better read the explanations available closer to the "source" (e.g. on the internet).

I agree it is a fascinating subject and, hoping I'm not breaking forum rules and as it seems to interest you, I can thoroughly recommend the short course on Galaxies run by Liverpool JMU ( It covers all this sort of stuff (and more). Distance learning (still subsidised for UK/EU residents !!)


Author:  maurice [ Mon Mar 21, 2011 2:47 pm ]
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I’m sorry Ian but I thought with your commitment to the Dark Matter idea you would have had a simple explanation of what is going on. Such as the simple explanation for Newtonian orbital dynamics which goes. If you fire a cannon ball horizontally across the ground with enough force, the curvature of the Earth will match the balls rate of fall, so the ball will carry on falling continuously. It will in fact be in orbit. What would a ball of dark matter do?
All the best. Maurice.

Author:  Deimos [ Mon Mar 21, 2011 10:13 pm ]
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maurice wrote:
I’m sorry Ian but I thought with your commitment to the Dark Matter idea you would have had a simple explanation of what is going on.

I'm not committed to an idea. Just that from what I have seen I favour it as an idea. One thing I am completely sure about is that whatever our thoughts, ideas and theories are today, time will modify them as we get a better understanding.

As to giving you my explanation - well there are many published explanations available on the internet from far more knowledgable and capable people than myself.


Author:  maurice [ Tue Mar 22, 2011 3:12 pm ]
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When I first started this thread I was planning to dive straight in with my nuts and bolts version of what is really happening with galaxies. Thanks to the intervention you, Cliff, Ian and Stella, I have discovered two facts which I feel re-enforces my Idea.
Previously I thought Vera Rubin’s work was just to confirm that galaxies rotate as a wheel. The fact that this is not the case, and the rotation curves are flat, show that galaxies are neither rotating as a wheel nor as objects in free fall orbits, but in a third way with most outer parts travelling at the same speed. This is a real gift and clears up a little problem I had with my original idea!
The second thing I found out is that Vera Rubin doesn’t like the dark matter solution, so to my mind the field is still wide open for crack pots like me to have a go, very encouraging!
Going to see a man tonight, about the simple task of moving drawings about in my lap-top!
All the best Maurice.

Author:  Cliff [ Fri Mar 25, 2011 8:30 pm ]
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Dear Maurice
When I was a youngster I recall doing a self imposed projectat assessing the behaviour of asteroids using basic equations :-
v = u +at
S = ut + 1\2at(squared) a
v(sq) - u(sq) = 2as
I think ? I'm getting a bit rusty that was 60 years ago!
Whatever, I recall it seemed to work well enough to satisfy my curiosity.
From what I gather dealing with 2 bodies with regards gravitation is fairly simple but once it comes to three bodies it gets tricky and with a lot a nightmare.
From what I gather even modern computer models for galxies are simplified not using the real quotas of stars, although my understanding of that may be well out of date and models are now getting nearer to reality. Hoever, I recently read a biography about James Clerk-Maxwell who is creditted with "proving" that Saturn's Ring system is not a solid disc. Maxwell did his proof in the mid-nineteenth century and was the only person who submitted a credible answer when a competition was announced.
The biography said that Maxwell did not provide a completely definitive answer. Apparently he worked out that the Ring System had to be made up of numerous particles, but another alternative that seemed possible was that the system would probably work if it were a solid disk with a significant part of the total mass concentrated at a single point on the outside edge of the ring.
I suppose there might have been other weird possibilities Maxwell did'nt have time to consider. Bare in mind of course Maxwell didn't have the benefit of computers in those days. However,the author of the biography reckonned that since Maxwell's day no one has still been able to come up with a better proof even with the aid of computers.
How true that is I don't know and there is no way I could prove it myself.
However, I mention the above because I think theat understanding the movement of materisl (eg stars, dust and gas) in galaxies is hellishly difficult, and it is not something that ordinary mortals are likely to be able to answer.
Best wishes from Cliff

Author:  maurice [ Sat Mar 26, 2011 12:00 am ]
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Hi Cliff. Thanks for that. I didn’t know Maxwell had looked at the problem of Saturn’s rings. I will look it up on Google!
I’m surprised they were trying to prove the rings are not solid. Perhaps they meant not moving as a solid body, or in other words, moving like a wheel.
From the evidence of the rotation curves, the outer parts of galaxies are travelling too fast so should be moving away from the centre. This would be very difficult to detect, and if cosmologist have already decided it is not happening, so haven’t taken the trouble to check, they could be missing a vital clue that would alleviate the need for dark matter.
I feel sure the scientist who work out how to put satellites like soho into neutral gravity orbits could soon explain how galaxies rotate using normal orbital dynamics.
I’ve made some progress with copying and pasting diagrams but can’t get them onto this forum yet. It would save a heck of a lot of explaining if I can manage it, but if not, it will have to be the long winded route. All the best Maurice.

Author:  Cliff [ Sat Mar 26, 2011 7:37 pm ]
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Dear Maurice
My understanding is that galaxy dynamics are quite complicated.
I think you may be looking at the matter too simplistically.
Even properlt understanding Saturn's Rings isn't easy - although I was surprised to read in the Maxwell-Clerk biography that modern astro-physicists haven't been able to improve on Maxwell- Clerk's prize winning answer to the Rings problem wat back mid-19th century.
I cann't remeber the biographers name and do not know how mathematically minded that writer was himself - unfortunately I don't have that book now.
If I recall correctly it was pretty well conclusively show that Sturns are not a solid ring by Keeler in the 1880s (?) by observation spectroscopy.
Best wishes from Cliff

Author:  maurice [ Sun Mar 27, 2011 4:17 pm ]
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Hi Cliff. I took a look at the latest information on Saturn’s rings, and I have to admit, it looks pretty convincing. I lived in hope that Maxwell might mention the possibility of static electric charge being involved but quite the reverse. He proves the rings do consist of particles in gravitational orbits as confirmed by Keeler. There is obviously a lot my simplistic approach can’t hope to explain when confronted by evidence like this. Even so, I would love to see a practical explanation of how the rings behave the way they do, and if every particle has its correct orbital speed, or are Newtonian orbits modified in the rings? One question someone might know the answer to, are the moons that are embedded in the rings, travelling at normal free fall orbital speed? therefore different from the rest of the rings. One last straw to clutch at, gasses are being produced from the rings by the action of ultraviolet light and Ions ejected from the moon Enceladus. Wouldn’t this leave the particles in the rings electrically charged?
All the best. Maurice

Author:  David Frydman [ Sun Mar 27, 2011 5:20 pm ]
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I would think thatany sizable moon withinthe ring system would move at the normal orbital speed. There cannot be enough mass in small ring particles nearby to affect it too much. This is just a guess.
Some pairs of moons within the ring system I recall swap positions pereiodically as they pass each other.
I think these moons are kilometres in diameter.

Regards, David.

Author:  maurice [ Tue Mar 29, 2011 3:57 pm ]
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Thanks for that David. It may seem far removed from the problem of galaxies but I think understanding one will help in finding a solution to the other. I am talking about my solution of course! Though the published solutions are from impeccable sources, the lack of details and simple explanation available, prompt me to press on with my own ideas.
For instance the exact speed of a moon embedded in the rings is an important clue to what is happening. The moon looks as if it has cleared a path in the rings. If the moon and the rings travel at the same speed this wouldn’t happen. If the rings are moving faster than orbital speed, the moon will be increasing its speed and moving out to a wider orbit. The converse would result in the moon moving to a smaller orbit. In both instances clearing a path indicates that the rings are not moving at free fall speed.
Other related, unresolved, questions concern the thickness of the rings, galaxies, asteroid belts and satellites orbiting in formation. They are breaking the rules. The outer parts should fall behind while the inner should move ahead and the sides should exchange places on each orbit. If two moons are of similar size, and in almost the same orbit, they will change places as noted at the edge of Saturn’s ring system. They are following the rules but it appears the rest, galaxies etc aren’t. Unless we, just as amateurs, never get the facts. TV CGI leaves a lot to be desired I’m sorry to say.
All the best Maurice.

Author:  David Frydman [ Tue Mar 29, 2011 4:29 pm ]
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I think the thickness of Saturn's rings is fairly well known nowadays although there could be clumpiness..
The pairs of mooons that change position obvioyusly clear a path as they change position.


Author:  Cliff [ Tue Mar 29, 2011 4:54 pm ]
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Dear Maurice
You suggest "They're breaking the rules".
Which rules ?
I'll suggest they are behaving the rules (or trying to ?) but the way the rings formed might be such that they are continually changing in trying to behave the rules, but perhaps in the main the changes are generally occurring slowly in human life terms
As far as I know the actual time Saturn's Rings have already existed or will continue to exist in much their current form is not fully understood.
I don't think the origins of Saturn's Rings is properly understood. Maybe their very nature is such that the Rings are unstable (let's say "not obeying the rules" and might gradually dissipate or even disappear catastrophically, but that might not mean the rings disappear overnight.
Assessing the movements of a simple two body system is said to be relatively simple but assessing a so-called three system is much more difficult. The problems you are talking about are probably very very difficult and not adequately discussed in ordinary simple descriptive terms.
Undertanding galaxies might be a different ball game but there might be some aspects of galaxies' rotational movements that have some similarities to Saturn's Rings eg they aren't individual solid bodies - they're made of countless bits.
Best wishes from Cliff
Best wishes from Cliff

Author:  maurice [ Wed Mar 30, 2011 3:57 pm ]
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Hi. More developments. I’ve found the moons in Saturn’s rings do gain angular momentum from being, I presume, bombarded by the faster moving particles. And will eventually move out from the ring system. This is very strong evidence that the rings are not in any kind of gravitational orbit.
The two moons that swap places is more evidence to support my electro static idea. Have you ever tried to clear up “crumbs” of polystyrene and place them in a plastic bag. The bag and the polystyrene often carry the same electric charge making it impossible to get the crumbs to stay together or stay in the bag. The reason the moons swap places is because they are obeying the rules of gravitational orbits. The moon nearest Saturn will gradually catch up with the outer moon. As they get close, their own gravity will draw them together. This will cause the inner moon to speed up, so it moves out, to a “slower” orbit, and the outer moon to slow down, so it falls to a “faster” orbit, so the process, continually repeats itself. Though the moons are attracted to each other by gravity, they still carry a repulsive electric charge. The effect of this can be seen in the “braiding” effect, as charged particles, in the ring, move away from the increase in charge, as the two moons come close together. I think that’s enough for now. All the best. Maurice.

Author:  Cliff [ Wed Mar 30, 2011 8:47 pm ]
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Dear Maurice
Funnily enough I didn't want to mention it because I think doing so might only confuse things even more than they are already.
However, I do have a vague hypothesis about electrostatic forces being a possible important influence with respect to Saturn's Rings, but if I grasp your ideas correctly my own hypothesis is very different to your ideas.
My thoughts are that the electrostatic effects may have some effects with regards to the smaller ring particles, but NOT on the satellites which have more mass and are therefore much more influenced by gravity.
With regards Dark Matter I think I can understand in my own simplistic way that by introducing Dark Matter as I understand it to be the excessive sppeeds of the stars in the outer regions of spiral galaxies can be satisfactorily accounted for keeing the galaxy rotation stable.
I can accept that as a potentially correct answer to the problem.
However, for many years I have pondered over the possibility that the laws of physics (and gravity in particular) might not be constant throughout time or the whole universe. That is largely conjecture of course and I cann't claim to be the originator of such ideas.
From being a youngster I was happy with the Big Bang (or at least my simplistic understanding of it), but as time went on, and I felt I was learning a bit more about the subject (though arguably not enough!) I gradually got somewhat dis-illusioned by the Big Bang as it got more complicated. The first big thing to upset me was the need to introduce "Inflation" - which admittedly I don't pretend to properly understand - but seems a bit of a bodge to me. Then comes Dark Matter and Dark Energy to make things worse.
So my current attitude is to sit on the fence - half accepting current mainsteam cosmology ie Big Bang, Inflation, Dark Matter and Dark Energy. However, my other half likes the potential simpicity of MOND.
However, I am inclined to think that whilst there are almost certainly some similarities about Saturn and its satellites and ring system as compared to say spiral galaxies (ie both are influenced by gravity) but even so there are possibly considerable differences in the two ypes of system (ie Saturn's rings and spiral galaxies).
There is no way I personally would dream of solving the nature of spiral galaxies myself.
As I said I do have a pet hypothesis of my own about Saturn's rings but it is very different to yours and not relavent to the current discusion.
One thing I will ask about your ideas about the nature of Saturn's Rings is, are you suggesting that the Spokes observed from time to time, act like "real" spokes in a wheel (like a cart wheel) ?.
Best wishes from Cliff

Author:  maurice [ Wed Mar 30, 2011 10:52 pm ]
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Hi Cliff. Would you please read through my last post again. The moons are in gravitational orbits, and the gravitational affects on each other, is far stronger than the repulsive force of their static charge, which does not affect them. But this does not stop the static charge they carry from affecting the nearby ring particles.
The speed of objects in gravitational orbits is determined by their distance from the body they are orbiting. Regardless of mass or density, at the same distance, they all orbit at the same speed. The farther away they are, the slower they orbit, because this is the speed they fall, at that distance. And all objects fall at the same speed.
For simplicity we can say the embedded moons act as a speed indicator and the nearby particles should be travelling at a similar speed. The fact that the moons have cleared a path and are moving to wider orbits indicates that the particles are moving faster than the moons so are not free falling in gravitational orbits.
The spokes do act like the spokes of a wheel. The outer travelling faster than the inner, which is contrary to the findings of Keeler. I would have accepted that they are standing waves on the disc was it not for the evidence of the moons. All the best Maurice.

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