Can some one tell me how you can roughly estimate the number of collisions when Milkyway and Andromeda merged with each other. I know it is somehow related to the mean free path of the star. But couldn't figure out how to do it exactly.

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    $\begingroup$ Basically none. See Wikipedia. Of course many stars will be scattered (which in technical terms is a type of collision) but actually hits will be likely none. $\endgroup$ Dec 7, 2017 at 17:17
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    $\begingroup$ This doesn't answer your question on calculation, but it's related. astronomy.stackexchange.com/questions/10989/… $\endgroup$
    – userLTK
    Dec 7, 2017 at 17:27
  • $\begingroup$ @StephenG Yes I know the number will be negligible. But how to show it mathematically? $\endgroup$
    – Arpan Das
    Dec 7, 2017 at 17:54
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    $\begingroup$ A crude calculation, you need to look at 2 dimensional area when looking at collisions for objects passing through. Divide one of the two galaxies into slices, maybe 10 light years across, work out an average lateral distance between stars in each slice (5 light years I'd guess, less in the more dense center), work out what percentage is covered by stars per slice and how many slice deep. That's pretty rough, but that's where I'd begin. Too many shortcuts to be a real answer, but maybe a starting point. $\endgroup$
    – userLTK
    Dec 7, 2017 at 18:16
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    $\begingroup$ See the answer to this question for one (crude) approach: astronomy.stackexchange.com/questions/22545/… $\endgroup$ Dec 7, 2017 at 23:01


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