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It's fascinating to know that the Andromeda Galaxy, and our Milky Way galaxy will collide in 6 billion years' time, while the present time Andromeda Galaxy is visible to the naked eye (in good viewing conditions), and approaching us at enormous speed. I'm curious as to what the collision course is. I imagine, from the big bang theory, that everything is moving outwards from a central point, so cannot be a head-on collision. Is Andromeda "following/chasing" and catching up with the Milky Way? (or the other way around), or are both galaxies maybe travelling side by side, and gradually getting closer?, or is the collision course something different?

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The obvious answer is that while the Universe is expanding objects that are gravitationally bound together are not (the Milky Way is a rather obvious example). Our galaxy and Andromeda are bound together as part of the Local Group. M31 is moving towards us (relative to the Galactic centre) at about 120 km/s. Which of us is moving (or both) is hard to say as there is no absolute frame of reference other than to measure against distant objects. That method is used, for example, to say at what speed the galaxy is rotating.

There is a lovely merger movie by Dr. John Dubinski at: http://www.astro.soton.ac.uk/PH308/galaxies/mergers/MWmerge.mpg
which shows what happens when galaxies "collide". As galaxies are mostly empty space, with vast gaps between stars (except in crowded regions like the nucleus), the gravitational interaction is the key, rather than stars colliding (which will be very rare, unless the galaxy nuclei come together).

Paul O'Brien, University of Leicester