Scientific American: Our Growing, Breathing Galaxy

Gradually, though, it has become clear that the Milky Way is not a finished work but rather a body that is still forming. Like the earlier discoveries, this realization has relied heavily on observing other galaxies and bringing the lessons back home. Most galaxies are now assumed to result from the merging of smaller precursors, and in the case of the Milky Way, we can observe the final stages of this process. Our galaxy is tearing apart small satellite galaxies and incorporating their stars. Meanwhile gas clouds are continually arriving from intergalactic space. No longer can researchers speak of galaxy formation in the past tense.