"Topquark" wrote in
:

A program on the Science Channel last night talked about black holes
in the center of galaxies. It also mentioned that the galaxy in
Andromeda M31 is moving toward us and will therefore collide with our
galaxy some time in the future--They showed simulations. Is this
true? You can get the radial velocity from the Doppler shift, but how
can they know the angular velocity? Isn't it more likely that it's
just gravitationally bound?


That is most likely the case but both galaxies cover a fair bit of real
estate so their centres could be more than 200,000 ly apart at closest
approach and you would still get interaction.

LK