What are collections of galaxies called? (Intermediate)

A collection of solar systems is called a galaxy. A collection of galaxies is called ------?

Galaxies are actually not grouped together in quite the same way as stars are grouped together in galaxies. Stars are generally confined to galaxies, with few (if any) existing in the space between galaxies. In general, it's not difficult to determine what galaxy a star belongs to.

With "collections of galaxies", it's a little more complicated. It's not always so easy to determine what group a galaxy belongs to, or even where one group ends and another begins.

Astronomers classify "collections of galaxies" in the following way:

Galaxy Groups - Groups are sets of just a few galaxies packed into a region that's a few million light-years across.

Galaxy Clusters - Clusters are sets of a few *hundred* galaxies packed into a region that's a few million light-years across (making them much denser than groups).

There are intermediate cases between groups and clusters, and no strict rule about where you draw the line. It's just like distinguishing between cities and villages. You have to make *some* distinction, but exactly how you define the cutoff is going to be somewhat arbitrary.

I should also add that there are even larger associations of galaxies called "superclusters", which are large associations of galaxies that can extend over distances of more than few 100 million light-years. Superclusters contain many groups and clusters in them. But that's as high up as the hierarchy goes. Astronomers don't have any reason to believe that there are larger structures than superclusters.

This page was last updated on June 27, 2015

About the Author

Christopher Springob

Christopher Springob

Chris studies the large scale structure of the universe using the peculiar velocities of galaxies.  He got his PhD from Cornell in 2005, and is now a Research Assistant Professor at the University of Western Australia.

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