Andromeda is headed for us at 75 miles per second, right? Okay, 75 miles per second * 3600 sec/hr * 24 hrs/day * ~365 days/year*= 2.36 billion (2.36 times ten to the ninth power) miles per year.
Andromeda is 2.54 million light years away. What distance is a light year (approximately)?
D=RT, T = D/R
186,000 miles/sec = 186,000 * 3600 sec/hour * 24 hrs/day * 365 days/year = 5.87 * 1,000,000,000,000 (ten to the twelfth power) miles. At the rate that Andromeda is traveling, it will take roughly [(2.54 million) * (5.87 * ten to the twelfth power)] miles divided by (2.36 billion miles per year) = 6. 313 billion (6.313 times ten to the ninth power) years to get here.
I think we'd better start working on a Gayan-centric galaxy soon.
But wait...
Do you remember how you used to hate those word problems? (One train is headed east at this rate, the other headed west at that rate, when will they meet?)
What about our galaxy? Is it just sitting still waiting to be collided into by some big bully named Andromeda?
Relative to the centre of the Local Group, our Galaxy is moving at roughly 200 km/s the other way. Therefore, the Local Group of galaxies (to which we belong) is moving at 600 km/s relative to the Cosmological Microwave Background radiation. (from:
http://answers.yahoo.com/question/index?qid=20080903025801AAXuDc3 )