NGC 7317, NGC 7318, NGC 7318A, NGC 7318B, NGC 7319, NGC 7320 - Stephan's Quintet

Stephan's Quintet, as its name implies, is a group of five faint galaxies (NGC7317, 7318A, 7318B, 7319 and 7320), all within an area of 4 arc minutes in the constellation Pegasus. Four of the five galaxies in Stephan's Quintet form a physical association, and are involved in a cosmic dance that most likely will end with a galactic merger.

The group was discovered by Edouard Stephan in 1877 at Marseilles Observatory, and is one of the most studied compact galaxy groups.

Visually, NGC 7320, on the southeastern edge of the group, is the most conspicuous group member. It is very faint with a small halo of even surface brightness. The other four galaxies are extremely faint and appear as ill-defined smudges.

NGC 7320 also shows a small redshift (790 km/ssec), while the other four exhibit large redshifts (near 6600 km/s). Since galactic redshift is proportional to distance, NGC 7320 is only a foreground object about 39 million light years from Earth, versus the 210-340 million light years of the other five. In fact, individual stars in the foreground galaxy can be seen in Hubble Space Telescope images, hinting that it is much closer than the others.

This unusual system has been used as proof that the redshift is not truly a distance indicator, which would completely overturn current cosmology. Opponents of conventional theory point to debris and tails around the low-redshift galaxy, suggesting that it is interacting with the high-redshift systems. This would require that all five galaxies be at nearly the same physical distance in space, despite their differing red shifts.