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What Is a Cluster of Galaxies?

Galaxy clusters, groups of galaxies bound to each other by gravity, hold great interest for astronomers. The stars, gases and dark matter contained within these clusters contain valuable information about how all galaxies are formed.

Definition
- A galaxy cluster is a group of galaxies connected by gravitational forces, according to NASA's Imagine the Universe Web site.
Types
- Scientists categorize galaxy clusters according to "richness" (number of galaxies), overall shape, and types of galaxies within the cluster, according to the University of California, San Diego (UCSD) Center for Astrophysics &Space Sciences Web site.
The Local Group
- According to the USCD Web site, the Earth is in a galaxy cluster known as the Local Group, which contains about 30 galaxies, including Andromeda, the Magellanic Clouds and our own Milky Way galaxy.
Composition
- NASA states that galaxy clusters can range from as few as two galaxies all the way up to thousands of galaxies, bound by gravity from dark matter and gases.
Study
- Scientists study galactic clusters with great interest, according to NASA, because these closed systems have retained massive amounts of gases left over from their creation. This preservation of conditions from their formative years can give us insights into how elements were created and how galaxies are formed.

Astronomy