Re: @Sotorro -- Intriguing potential explanation for what a quasar really is
AFAIK there is an equal probability Andromeda passes through as as two flocks of birds can fly past.
It could be as little as 100 star collisions. Though a lot of edge cases can get flung out into deep space, the cores are travelling too fast to orbit and then collide.
A core may get "ejected" though should they get close together. Anyone fancy a long game of "musical chairs... um black holes".