How big is Pisces-Cetus Supercluster Complex?

The Pisces-Cetus Supercluster Complex is an enormous, flattened band of galaxy clusters that includes the Milky Way's own region of space. It is about 1 billion light-years long, measured along its longest dimension, where a light-year is the distance light travels in one year. The astronomer R. Brent Tully of the University of Hawaii described the complex in 1986 and 1987.

Size

Tully found the complex by mapping rich clusters, massive groupings each holding hundreds of galaxies. He linked together clusters that lay close to one another and found one network much larger than any other. In his 1987 paper, it contains about 60 rich clusters and about 10¹⁸ times the mass of the Sun, a 1 followed by 18 zeros.

Its longest dimension is about 300 megaparsecs, where a megaparsec is about 3.26 million light-years. That is about 980 million light-years. The complex is flattened like a thick pancake. Its main plane is about 40 megaparsecs (130 million light-years) thick, and its center lies about 650 million light-years away.

These distances depend on how fast the universe is expanding, a rate measured in kilometers per second per megaparsec. Tully assumed 75, while the Planck satellite measured about 67, which would make every distance about 11% larger. His 1986 paper, based on fewer clusters, gave a longest dimension of 360 megaparsecs, about 1.2 billion light-years. Either way, the complex is about a billion light-years long.

Size comparisons

About 10,000 disks of the Milky Way, each about 100,000 light-years across, would stretch the length of the complex.
If the Milky Way were shrunk to the size of a basketball, about 24 centimeters (9.4 inches) across, the complex would be about 2.4 kilometers (1.5 miles) long.
The Laniakea Supercluster, the Milky Way's home supercluster as mapped in 2014, is about 500 million light-years across, half the length of the complex.
The Hercules–Corona Borealis Great Wall, a much more distant structure that some astronomers have proposed, is estimated at 6.5–9.8 billion light-years across, 7–10 times the length of the complex.
The observable universe is about 93 billion light-years across, so about 93 complexes would fit across it end to end.

What the complex contains

Tully divided the complex into five parts. The most prominent is the Pisces-Cetus Supercluster, which shares the complex's name, and the complex is centered in the direction of the constellations Pisces and Cetus. Two long chains of clusters, the Perseus-Pegasus chain and the Pegasus-Pisces chain, form a filament first reported by other astronomers in 1985. The other parts are the Sculptor region and the Virgo-Hydra-Centaurus Supercluster.

Tully considered the Milky Way probably part of the Virgo-Hydra-Centaurus Supercluster. Much of that supercluster lies in the southern sky or behind the dust of the Milky Way, where clusters are hard to see. Tully linked it to the complex because of an alignment. Nearby galaxies lie in a plane, and that plane lines up with the main plane of the whole complex.

A newer way to map superclusters

In 2014, Tully and his colleagues mapped superclusters in a different way: by how galaxies move. Each galaxy is pulled by the mass around it. A supercluster can be defined as the region where galaxies flow inward toward the same place, like water running downhill into a valley.

By this measure, the Milky Way lies near the edge of the Laniakea Supercluster. Laniakea is about 500 million light-years across and contains about 100,000 galaxies, with the mass of about 10¹⁷ Suns. The name means "immense heaven" in Hawaiian.