ROBERT GENDLER / SCIENCE PHOTO LIBRARY ROBERT GENDLER / SCIENCE PHOTO LIBRARY
Whirlpool galaxy (M51), optical image. The Whirlpool galaxy is a spiral galaxy, and one of the brightest galaxies in the sky. The smaller galaxy to the right, NGC 5195, is a companion galaxy that is thought to have collided with the Whirlpool galaxy around 500 million years ago, and is now bound by its gravitational pull. This interaction is probably responsible for the larger galaxy's classic spiral structure. The Whirlpool galaxy lies 23 million light years from our own, in the constellation Canes Venatici.
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