Our galaxy's most recent major collision

One of the characteristic features of modern cosmology is its description of how galaxies evolve: via a hierarchical process of colliding and merging with other systems. Nowhere in the universe do we have a clearer view of this buildup than in our own Milky Way. Currently one of our nearby neighbors, the Sagittarius dwarf galaxy, is being tidally disrupted (a dwarf galaxy has less than about 1% of the stellar mass of a normal spiral galaxy like the Milky Way, and often much less). Two other nearby dwarfs, the Large and Small Magellanic Clouds (with about 1% and 0.7% of the stellar mass of the Milky Way, respectively) are falling towards us. Meanwhile streams of globular clusters encircle the Galaxy, marking the effects of prior mergers. The record of even more ancient mergers can be extracted from the positions and motions of stars in the Milky Way's stellar halo, the roughly spherical distribution of stars (about one hundred thousand light-years in diameter) older than about 10-12 billion years. Meanwhile Andromeda, our nearest large neighboring galaxy, is about ten times farther away than these dwarfs; a merger with it is expected in another five billion years.

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