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Distance to the Shapley Supercluster

The Shapley Supercluster is one of the most massive known concentrations of galaxies in the nearby universe. The distance from Earth to its core, around the galaxy cluster Abell 3558, is about 670 million light-years. A light-year is the distance light travels in one year, about 9.46 trillion kilometers.

Distance

At this distance, astronomers measure how far away galaxies are from their redshift. The universe is expanding, so distant galaxies move away from Earth, and the farther away a galaxy is, the faster it recedes. That motion stretches the galaxy's light to longer, redder wavelengths. Light from the core of the Shapley Supercluster is stretched by 4.8%, a redshift of 0.048.

A 2015 survey of the supercluster turned that redshift into a distance using an expansion rate of 70 kilometers per second for every megaparsec of distance. A megaparsec is a million parsecs, about 3.26 million light-years. The survey's figures put the core about 205 megaparsecs away, or about 670 million light-years.

The supercluster is deep as well as wide. It contains as many as 25 large galaxy clusters, with redshifts from 0.033 to 0.060. By the same method, its nearest and farthest clusters lie roughly 460 and 830 million light-years away.

Size comparisons

  • The Shapley Supercluster is about three times as far away as the center of the Great Attractor, about 230 million light-years away.
  • It is about 12 times as far away as the Virgo Cluster, the nearest large cluster of galaxies, which is about 54 million light-years away.
  • It is about 260 times the distance from the Milky Way to the Andromeda Galaxy.
  • If the Milky Way, about 100,000 light-years across, were shrunk to the size of a basketball, the core of the Shapley Supercluster would be about 1.6 kilometers (1 mile) away.

Discovery

In a 1930 paper titled "Note on a Remote Cloud of Galaxies in Centaurus", the astronomer Harlow Shapley described it from long-exposure photographs. He called it one of the most populous clouds of galaxies yet found. He estimated its distance at 45 megaparsecs, less than a quarter of the modern figure.

In 1989 two teams found it again, one as an unusual crowd of galaxy clusters and the other as an excess of galaxies, and the second named it the Shapley Concentration. A 2006 survey of more than 8,000 galaxies found that its main sheet of galaxies fills the whole area surveyed, 12 by 30 degrees of sky. At the supercluster's distance, that is about 140 by 350 million light-years.

Its pull on the Milky Way

The Milky Way and the rest of the Local Group are moving at about 630 kilometers per second compared with the cosmic microwave background, the faint glow left over from the early universe. The Shapley Supercluster lies in roughly the direction of that motion, so astronomers have long asked how much of it the supercluster causes.

Estimates differ widely. Studies summarized in 2006 range from a few percent of the Local Group's motion to about a quarter. A 2017 map of galaxy motions found that the flow around the Milky Way is dominated by a pull toward the Shapley Supercluster and a push away from a region with few galaxies in roughly the opposite direction.

Sources

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