According to Hickson: “Most compact groups contain a high fraction of galaxies having morphological or kinematical peculiarities, nuclear radio and infrared emission, and starburst or active galactic nuclei (AGN) activity. They contain large quantities of diffuse gas and are dynamically dominated by dark matter. They most likely form as subsystems within looser associations and evolve by gravitational processes. Strong galaxy interactions result and merging is expected to lead to the ultimate demise of the group. Compact groups are surprisingly numerous, and may play a significant role in galaxy evolution.”
Also called Stephan's Quintet, one galaxy of the sixsome, NGC 7320, is not part of the HCG, but a foreground galaxy in front of the five galaxies in the HCG. When discovered, two of the galaxies in the grouping were considered to be one galaxy, leading to the "quintet" for the 6 galaxies, but appropriate for the HCG.