# What is the likely closest encounter for a typical star during the Milky Way Andromeda Merger?

This is closely related, but distinct question to What are the chances of a star colliding with another during a galactic collision?.

The accepted answer to that question places the odds of the Sun colliding with another star at about $$10^{-12}$$, what I want to ask is how near a near-miss is expected? In other words, what is the expected closest encounter between the Sun and another star during the collision? A more sophisticated answer would give the expected distribution of close encounters by distance.

A more general version of the question is -- what disruptive events would a typical star such as the Sun expect to experience during the merger?

• Given we do not even know how many stars are in each galaxy (including our own) except to an order or magnitude, it's pretty optimistic expecting someone to calculate a near-miss distance for encounters that's much better than a random guess. May 17 '19 at 7:55

Suppose the sun moves through a column of Andromeda stars $$L$$ parsecs long with density $$\rho$$ and random positions. The probability of encountering at least one star within $$r$$ of the sun along this column would be $$F(r)=1-e^{-\pi r^2 L \rho}$$ (basically, what is the probability of no "success" when doing a Poisson sample of the column volume times density).
Setting $$F(r_{1/2})=1/2$$ to find the distance where we should expect a 50% chance of a star more nearby we get $$r_{1/2}=\sqrt{\ln(2)/\pi L \rho}$$. If we plug in $$\rho=0.14$$ per cubic parsec and $$L=1000$$ parsec (assuming a flat disk-disk collision) I get $$r_{1/2}=0.02$$ parsec. That is 4125.296 AU, reassuringly far.
To get the mean distance we can use the not very well known but useful formula $$E[r]=\int_0^\infty (1-F(r)) dr =\int_0^\infty e^{-\pi r^2 L \rho} dr = 1/2\sqrt{L \rho}\approx 0.042$$ parsec.
Depending on your views of disk thickness and angle on merger $$L$$ and $$\rho$$ may vary a fair bit. A sideways collision might have $$L=67$$ kpc reducing the distances by a factor of 8.1. Direct collisions still remain very unlikely, but messed up Oort clouds seem very likely.