<p>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.</p>

CC BY-SA 3.0
2017-12-07T17:17:36.777Z
Basically none.  See [Wikipedia](https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision#Stellar_collisions).  Of course many stars will be scattered (which in technical terms is a type of collision) but actually hits will be likely none.
StephenG - Help Ukraine
CC BY-SA 3.0
2017-12-07T17:27:22.217Z
This doesn't answer your question on calculation, but it's related.   https://astronomy.stackexchange.com/questions/10989/why-wont-the-milky-way-colliding-with-andromeda-affect-the-solar-system
userLTK
CC BY-SA 3.0
2017-12-07T17:54:52.820Z
@StephenG Yes I know the number will be negligible. But how to show it mathematically?
Arpan Das
CC BY-SA 3.0
2017-12-07T18:16:15.830Z
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.
userLTK
CC BY-SA 3.0
2017-12-07T23:01:38.810Z
See the answer to this question for one (crude) approach:
https://astronomy.stackexchange.com/questions/22545/what-are-the-chances-of-a-star-colliding-with-another-during-a-galactic-collisio
Peter Erwin
CC BY-SA 3.0
2017-12-08T14:20:19.787Z
In astronomy, a "collision" can mean numerous things. Two stars can "gravitational collide" without actually physically hitting each other. Are you referring specifically to physical collisions?
zephyr
<galaxy><cosmology><milky-way><dwarf-galaxy>
How to to compute the expected number of collisions between stars when Milkyway and Andromeda merged?
Arpan Das
# How to to compute the expected number of collisions between stars when Milkyway and Andromeda merged?

(Asked by: username_0 on 2017-12-07)

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.

**Tags:** galaxy, cosmology, milky-way, dwarf-galaxy

> **Comments:**
> **username_1**: Basically none.  See [Wikipedia](https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision#Stellar_collisions).  Of course many stars will be scattered (which in technical terms is a type of collision) but actually hits will be likely none.  
> **username_2**: This doesn't answer your question on calculation, but it's related.   https://astronomy.stackexchange.com/questions/10989/why-wont-the-milky-way-colliding-with-andromeda-affect-the-solar-system  
> **username_0**: @username_1 Yes I know the number will be negligible. But how to show it mathematically?  
> **username_2**: 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.  
> **username_3**: See the answer to this question for one (crude) approach: https://astronomy.stackexchange.com/questions/22545/what-are-the-chances-of-a-star-colliding-with-another-during-a-galactic-collisio  
> **username_4**: In astronomy, a "collision" can mean numerous things. Two stars can "gravitational collide" without actually physically hitting each other. Are you referring specifically to physical collisions?  

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