The Universe is built up by various structures. Stars are collected together into galaxies, galaxies are collected into galaxy groups, and galaxy groups are collected together into galaxy clusters.
For example, our own Milky Way is part of the Local Group of galaxies (not exactly an original name…). The Local Group contains about 50 galaxies; mostly smaller dwarf galaxies. The Milky Way, Andromeda and the Triangulum Galaxy are the three large spiral galaxies in the Local Group. It contains about 1 trillion times the mass of the Sun.
At an even larger scale, the Local Group is part of a galaxy cluster called Virgo Cluster which contains up to 2000 galaxies. The center of the Virgo Cluster is located about 59 million light years away in the constellation of Virgo. It has a mass of about 1.5 quadrillion suns. One of the largest, brightest galaxies in the Virgo Cluster is the giant elliptical galaxy M87.
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And so, galaxy clusters are made up of galaxy groups. They’re large collections of thousands of galaxies held together by mutual gravitational attraction. A typical galaxy cluster contains 50 to 1000 galaxies, and has a total mass between 100 trillion and 1 quadrillion solar masses, stretching across a distance of 30 million light-years.
The galaxies and galaxy groups in a galaxy cluster are buzzing around a mutual center of gravity like bees around a hive. But here’s the thing. The individual galaxies are moving too quickly to be held by the mutual gravity of just the other members in their cluster. That’s because galaxy clusters are just a part of an even larger structure called a supercluster.
Superclusters really are the largest structures in the Universe, with millions of members and stretching across distances greater than 100 million light-years.
We have written many articles about galaxies for Universe Today. Here’s an article about a spectacular, and disturbed galaxy cluster. And here’s an article about a new galaxy cluster discovered by Galaxy Zoo.
If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.
We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.