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A Supercluster is a large group of smaller galaxy clusters or galaxy groups, which is among the largest-known structures of the cosmos. The Milky Way is part of the Local Group galaxy cluster (that contains more than 54 galaxies), which in turn is part of the Laniakea Supercluster. This supercluster spans over 500 million light-years, while the Local Group spans over 10 million light-years. The number of superclusters in the observable universe is estimated to be 10 million.
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Galaxies are grouped into clusters instead of being dispersed randomly. Clusters of galaxies, in turn, are grouped together to form superclusters. Typically, superclusters contain dozens of individual clusters throughout an area of space about 150 million light-years across. Unlike clusters, most superclusters are not bound together by gravity. The component clusters are generally shifting away from each other due to the Hubble flow.
The Milky Way galaxy falls within the Local Group, which is a poor and irregular cluster of galaxies. Poor clusters may contain only a few dozen galaxies, as compared to rich clusters with hundreds or even thousands. The Local Group is near the Local Supercluster (also known as the Virgo Supercluster), which has a diameter of 100 million light-years. The Local Supercluster contains a total of about 1015 times the mass of the Sun.
The biggest cluster in the observable universe is called the Great Attractor. Its gravity is so strong that the Local Supercluster, including the Milky Way, is moving in a direction towards it at a rate of several hundred kilometers per second. The biggest supercluster outside of the local universe is the Perseus-Pegasus Filament. It contains the Perseus supercluster and it spans about a billion light-years, making it one of the largest known structures in the universe.
Distribution: cosmic voids and sheets
Research has tried to understand the way superclusters are arranged in space. Maps are used to display the positions of 1.6 million galaxies. Three-dimensional maps are used to further understand the positions of these superclusters. To map them three-dimensionally, the position of the galaxy in the sky as well as the galaxy's redshift are used for calculation. The galaxy's redshift is used with Hubble's law to determine its position in three-dimensional space.
It was discovered from those maps that superclusters of galaxies are not spread uniformly across the universe but they seem to lie along filaments. Maps reveal huge voids where there are extremely few galaxies. Some dim galaxies or hydrogen clouds can be found in some voids, but most galaxies are found in sheets between the voids. The voids themselves are often spherical but the superclusters are not. They can range from being 100 million to 400 million light-years in diameter. The pattern of sheets and voids contains information about how galaxy clusters formed in the early universe.
There is a sponge analogy used often that compares a sponge to the pattern of clusters of galaxies in the universe – the holes are the voids and the other parts are the locations of the superclusters.
Existence
The existence of superclusters indicates that the galaxies in the Universe are not uniformly distributed; most of them are drawn together in groups and clusters, with groups containing up to some dozens of galaxies and clusters up to several thousand galaxies. Those groups and clusters and additional isolated galaxies in turn form even larger structures called superclusters.
Their existence was first postulated by George Abell in his 1958 Abell catalogue of galaxy clusters. He called them "second-order clusters", or clusters of clusters.
Superclusters form massive structures of galaxies, called "filaments", "supercluster complexes", "walls" or "sheets", that may span between several hundred million light-years to 10 billion light-years, covering more than 5% of the observable universe. These are the largest known structures to date. Observations of superclusters can give information about the initial condition of the universe, when these superclusters were created. The directions of the rotational axes of galaxies within superclusters may also give insight and information into the early formation process of galaxies in the history of the Universe.
Interspersed among superclusters are large voids of space where few galaxies exist. Superclusters are frequently subdivided into groups of clusters called galaxy groups and clusters.