Stephan's Quintet is a visual grouping of five
galaxies of which four form the first
compact galaxy group ever discovered. The group, visible in the constellation
Pegasus, was discovered by
Édouard Stephan in 1877 at the
Marseille Observatory. The group is the most studied of all the compact galaxy groups. The brightest member of the visual grouping (and the only non-member of the true group) is
NGC 7320, which has extensive
H II regions, identified as red blobs, where active star formation is occurring.Four of the five galaxies in Stephan's Quintet form a physical association, a true
galaxy group,
Hickson Compact Group 92, and will likely merge with each other. Radio observations in the early 1970s revealed a mysterious filament of emission which lies in inter-galactic space between the galaxies in the group. This same region is also detected in the faint glow of ionized atoms seen in the visible part of the spectrum as a green arc.
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