The galaxy cluster Abell 959
Most galaxies lie in clusters containing from a few to thousands of objects. Our Milky Way, for example, belongs to the Local Group, a cluster of about fifty galaxies whose other large member is the Andromeda galaxy about 2.3 million light-years away. Clusters are the most massive gravitationally bound objects in the universe and form (according to current ideas) in a “bottoms-up” fashion with smaller structures developing first and larger groupings assembling later in cosmic history.
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