NOAO / SCIENCE PHOTO LIBRARY NOAO / SCIENCE PHOTO LIBRARY
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Optical photo of the Stephan's Quartet cluster of galaxies & the galaxy NGC 7320 (lower left) which is not a member of the cluster but simply happens to lie in the same direction. Discovered in 1877, the group was originally thought to have four members, but later photographic plates revealed that one of the galaxies (centre) was in fact an interacting pair. The group was known as Stephan's Quintet until the 1960s, when the distances of the five galaxies were measured. NGC 7320 was found to lie much closer to the Earth than the others, & the quintet became quartet once more.
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