> So, as a starter, we can say that an average sized star would have to come closer than 2 light years from us to have any real effect. To figure out exactly how close would require more math than I'm willing to do (I had to do enough to convince myself that the previous paragraph was true, what with gravity following an inverse square law, and me being generally untrusting of my own instincts when math is concerned) so I'm going to quit now.

Okay. So, going with these figures, that gives us a volume of space approximately equal to 33.51 cubic light years (91.95 if the distance should be 2.8 light years instead of 2) in the shape of a sphere, centred around the Earth, through which a medium-sized star would have to pass in order to have any effect on us at all. That seems to be a tremendously large amount of space to me; is the average galaxy spread out thinly enough that it becomes unlikely for any star to pass through that volume of space?