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Hickson Compact Groups

January 31, 2021

Galaxies clump together in many ways. Our galaxy, the Milky Way, belongs to a group — a collection of a few dozen galaxies. Others belong to clusters, which can contain hundreds of galaxies. And the clusters can clump together to make superclusters of thousands of galaxies.

On the other end of the scale are the Hickson Compact Groups — handfuls of galaxies that are packed especially close together.

The groups are named for Paul Hickson, who cataloged a hundred of them almost 40 years ago. The galaxies in many of the groups are so close that they’re interacting with each other. Their gravity may pull out ribbons of stars that stretch across thousands of light-years. Or it may squeeze clouds of gas and dust, causing them to collapse and give birth to millions of new stars.

Because of their tightness, such groups shouldn’t last long. Their galaxies should merge quickly, forming a single super galaxy. The fact that we see so many of them suggests that there’s a lot of work to do to explain these tiny but interesting groups of galaxies.

A couple of fairly bright groups are in good view at this time of year. Group 16, which consists of half a dozen galaxies, is in the south-southwest at nightfall, in Cetus, the sea monster. And the most famous of them, Group 92, is in the west-northwest, in Pegasus. Under dark skies, each of them is visible through fairly strong binoculars, and an easy target for a small telescope.

Script by Damond Benningfield

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