Destiny: We'll Need A New Revised Hitchhiker's Guide To Galaxy
Interactions between galaxies can trigger an increase in star-formation activity as well an increased level of activity around the nuclear black hole. These general behaviors have been reproduced in simulations of merging galaxies, lending confidence to our understanding of the physical mechanisms at work. But not all interactions lead to such enhancements, and the reasons are not well understood; meanwhile the strength of the triggering mechanism(s) and many other details remain puzzling.
It even looks as though that in about three billion years, we'll need a new revised, Hitchhiker's Guide to Galaxy. According to recent research the Andromeda Galaxy may be destined to collide with the Milky Way. Andromeda, a spiral galaxy approximately 2.5 million light-years away in the constellation Andromeda is the nearest spiral galaxy to our own, the Milky Way. Andromeda and the Milky Way are approaching one another at a speed of 100 to 140 kilometers per second (62–87 miles/sec). However, this does not mean it will definitely collide with the Milky Way, since the galaxy's tangential velocity is unknown. If they do collide, the two galaxies will likely merge to form a monster elliptical galaxy.
Numerous studies have been conducted to observe the evolution of galaxies undergoing interactions. Merging galaxies have typically been identified either through their disturbed morphology (such as tidal tails and bridges), their infrared brightness (triggered star formation increases the infrared luminosity), or simply because they are close together. The first two methods presuppose the effects, and so tend to have selection biases especially since some distortions may be hard to see (for example, if the galaxies are far away or in the very early stages of merger) while infrared luminosity might in principle be caused by some other phenomena.