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I was looking for this post again, I am listening to podcasts "Astronomy 161" and went through Gravitation theory. Basically it is more an introduction and the talks are about rotating bodies planet around star, etc etc.
In responce to this thread I was wondering that solar systems are quite possibly rotating with other solar systems within a spiral arms of a galaxy and that gravitation theory would suggest that the multiple solar system could be rotating around each other creating their own stabilising orbit. This could hold the spiral arms together to a degree.
This post suggests that the spiral arms are flying apart but to what degree and what concentrations is unknown to me.
The question in my mind is has anyone tested the rotation of different solar systems or stars to see whether they are holding together within spiral arms or as we have only just touch the discovery of planetary discoveries there is not enough data to work with. Again this could reject the theory of dark matter which is the primary reason for holding of outer spiral arms.
Although this thread suggests proven flying apart the spiral arms, I always though they actually measured the spiral arms were being held together. So there is obvious discrepencies somewhere between this report and other reports too.
I apologise for the wording of this as I am writing this in response to my conversational mental thoughts. I am considering re-sitting year 12 english again.
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