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Craig,
I agree that the motion of the stars will largely evolve chaotically. The result of this intermix as per theoretical modelling should match real observation.
I find the proposal given here fairly palatable. If stars did not rotate with the arms then there would be no arms! Inner stars migrating inwards, outer stars migrating outward. This is a lot like what happens in globular clusters but no arms there of course.
The stars move with the spiral pattern at the same speed. Meaning what? The speed of rotation of the arm? If this is the case, what causes the shearing that breaks up the spiral arms?
Regards, Rob
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