Galaxies are huge.
So big, in fact, that they sometimes crowd together a bit, and when that happens they can pass closely, scrape each other, or even suffer a full-on collision.
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Galaxy collisions observed using Hubble Space Telescope. Clockwise from upper left: NGC 3256, NGC 1614, NGC 4194, NGC34, NGC6052, NGC 3690. Credit: NASA & ESA
That is a collage of six different galactic collisions involving a dozen galaxies, as seen by a Hubble survey called HiPEEC, for Hubble imaging Probe of Extreme Environments and Clusters. The idea is to look at interacting or colliding galaxies in the nearby Universe, find which ones are undergoing bursts of star formation, and then look at the clusters of stars in them to see what they look like. We think these kinds of star clusters formed early in the Universe as well, but it s hard to examine ones in galaxies billions of light years away. By looking at nearby ones we can get an idea of what they re like.