Superclusters

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Superclusters

Superclusters are currently the largest structures known in the universe. Their sizes range up to 100 to 150 megaparsecs (or, 325 to 490 million light-years, where one light-year is the distance that light travels in vacuum in one year). Stars and clusters of stars group together into galaxies so as to contain anywhere from a few million to a few trillion stars. Galaxies collect into groups known as clusters of galaxies. On a larger scale, superclusters are clusters of clusters of galaxies. As clusters of galaxies group into superclusters they leave empty spaces called voids between the superclusters. Superclusters and voids typically extend for hundreds of millions of light-years.

History

in 1924, American astronomer Edwin Hubble (1889−1953), after whom the Hubble Space Telescope was named, proved that there were indeed galaxies outside the Milky Way galaxy. The groupings of galaxies in the sky were so obvious that the existence of clusters of galaxies was accepted immediately. The existence of superclusters is less obvious. Finding the distances to galaxies, and their three dimensional distribution in space is difficult. Most astronomers accepted Hubble’s word that superclusters did not exist. American astronomer Clyde Tombaugh (1906−1997), who discovered dwarf planet Pluto, once showed Hubble his first map of a supercluster. Hubble refused to believe it so the idea was ignored.

French-American astronomer Gerard Henri de Vaucouleurs (1918−1995) was not so easily discouraged. In the early 1950s, he suggested that the Milky Way galaxy and its cluster, the Local Group, are at the edge of a much larger group. This larger group is now known as the Local Supercluster. De Vaucouleurs’ suggestion was initially “received with resounding silence” (his words) but after 25 years his view became widely accepted.

By the 1970s, improvements in instrumentation allowed astronomers to measure distances to a large number of galaxies and work out the three dimensional structure of the universe. Stephen Gregory, Laird Thompson, and William Tifft mapped out the first definite supercluster in the 1970s. They found the Coma Supercluster extending about 100 million light-years. Two clusters, the Coma cluster and A1367 (the 1367th cluster in a catalog by Abell), were bridged together by galaxies and small clusters. The trio also unintentionally found the first void in front of the Coma supercluster. Astronomers quibble about the details of superclusters but most accept the fact that the large scale structure of the universe includes super-clusters and relatively empty voids.

Large scale structures

Since the 1970s, large scale structure surveys have provided a picture of the superclusters and voids in the universe. All major known clusters of galaxies and at least 95% of all galaxies are found in superclusters. The voids between superclusters may contain faint galaxies but no bright galaxies. The voids tend to be spherical but superclusters are not. Superclusters have long filamentary or sheet-like structures that provide the boundaries for the voids.

The Earth’s galaxy (the Milky Way) is located at one end of the Local Group—a small cluster of galaxies with the Andromeda galaxy at the other end. The Local Group is near the outskirts of the Local Supercluster, which has a diameter of 100 million light-years. This somewhat flattened supercluster consists of two major hot dog-shaped groups of galaxies. It contains a total of 10¹⁵ times the mass of the Sun, most of which is concentrated into 5% of the volume of the supercluster.

The biggest supercluster is the Perseus-Pegasus Filament discovered by David Batuski and Jack Burns of New Mexico State University. This filament contains the Perseus supercluster and stretches for roughly one billion light-years. It is currently the largest known structure in the universe.

Margaret Geller and John Huchra have mapped a region that is 500 million light-years long but only 15 million light-years thick. This area, the Great Wall, has not been completely mapped. They may find it extends longer than 500 million light-years when the mapping is complete.

KEY TERMS

Cluster of galaxies —A group of galaxies that is gravitationally bound.

Galactic (open) cluster —A cluster of roughly a few hundred young stars in a loose distribution. This is not the same thing as a cluster of galaxies.

Galaxy —A large collection of stars and clusters of stars containing anywhere from a few million to a few trillion stars.

Light-year —The distance light travels in vacuum in one year, roughly 6 trillion mi (9.5 trillion km).

Supercluster —A connected group of clusters of galaxies that may extend for hundreds of millions of light-years.

Void —A region of space extending for hundreds of millions of light-years that contains few, if any, galaxies.

A number of other superclusters and voids are known but astronomers have only mapped the large scale structure of a small part of the universe. They need to do much more work before astronomers understand the structure of superclusters and voids. There may even be larger scale structures.

One of the pressing issues related to superclusters is how such structures formed in the early universe. This question remains unanswered but observations of the cosmic background radiation indicate that its beginnings developed even before galaxy formation.