What's the probability that two stars collide when galaxies collide? [with equations!]

There are 400 billion stars in the Milky Way and a trillion stars in Andromeda - so what's the probability that any of those stars will collide when the Milky Way and Andromeda merge over 2 billion years in the future?

With such numbers, you might think the probability is pretty high but there's also a lot more empty space than there are stars...


My main camera tried its best this week on face auto-focus but it didn't do so well. Please forgive its failings. And before you ask, I don't own a whiteboard.


Majewski et al (Sagittarius A stream paper):

Chemistry 101 collisions of particles in a box:

The Binomial Distribution:

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Dr. Becky Smethurst is a Junior Research Fellow at Christ Church at the University of Oxford.

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