[troutman]

http://www.galaxydynamics.org/tflops.html

When will this collision occur? Plausible orbits and models of the Milky Way and Andromeda galaxies suggest that the big event could occur in as soon as 3 billion years. The Sun will still be burning brightly when this collision occurs and maybe life of some sort will still be around on Earth (or at least within the solar system) at that time. So what would people see in the night sky during this billion year galactic dance? As Andromeda approaches, it will grow in size and just before the collision the night sky will be filled by a giant spiral galaxy. When the two galaxies intersect, our familiar Milky Way arch over the sky will be joined by a second intersecting arch of stars but this will only last for 100 million years or so and will be a very confusing state of affairs for galactic astronomers. Finally, when the two galaxies merge our view will depend on which direction the Sun is thrown. There are two possible fates fort the Sun which depend closely on the details of where it is in its galactic orbit at the time of the collision. In the first case the Sun may take a ride on a tidal tail and be ejected into the darkness of intergalactic space. In this case, our star would be all alone with few stellar neighbours so the night sky would be very dark with few stars to see -- maybe like the disappointing view of the nightsky from an urban centre like downtown Toronto. In the second case, the Sun may be thrown right into the centre of the merging pair where a great starburst will be underway. The huge number of stars forming will result in supernovae going off at a rate of a few per year in the new merged galaxy. While these will likely not present a direct hazard to the Earth, they will truly light up the sky letting you read at night but probably frustrating the endeavours of backyard astronomers!