| cluster of galaxies | A group of galaxies held together by their mutual gravitational attraction.
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| explosion model | A model for the formation of clusters of galaxies in which the clusters form at the intersections of expanding shells of matter driven outward by gigantic explosions.
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| Great Attractor | A great concentration of mass toward which everything in our part of the universe apparently is being pulled.
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| hierarchical clustering model | A model for the formation of clusters of galaxies in which individual galaxies form and then begin to collect into clusters.
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| irregular cluster | A cluster of galaxies that lacks a symmetrical shapeand structure.
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| Local Group | The small cluster of galaxies of which the Milky Way is a member.
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| pancake model | A model for the formation of clusters of galaxies in which protoclusters form first and then fragment into individual galaxies.
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| regular cluster | A cluster of galaxies that has roughly spherical symmetry.
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| richness | A measure of the number of galaxies in a cluster. The more galaxies there are, the greater the richness.
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| voids | Immense volumes of space in which few galaxies and clusters of galaxies can be found.
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