What Is the Large-Scale Structure of the Cosmos? 141
Supercluster of Galaxies
A supercluster is an aggregation of several hundred groups or clusters of galaxies. A super-
cluster has a diameter of several hundred million light-years.
Astronomers formerly thought that since this kind of large-scale structure existed, the
galaxies would be distributed uniformly throughout the universe. However, in the 1980s,
regions were discovered in space in which no galaxies could be observed at all. The diam-
eter of each of these regions was more than 100 million light-years. When they were
further investigated, these voids were found to be lined up like bubbles, with groups of gal-
axies or clusters of galaxies distributed on their surfaces.
In 1989, Margaret Geller and John Huchra of the Harvard-Smithsonian Center for
Astrophysics named a grouping of galaxies the Great Wall, since it appears to be a long, con-
tinuous entity like the Great Wall of China. The Great Wall is a gigantic structure 500 million
light-years long and 200 million light-years wide. At the time of its discovery, the Great Wall
was the largest known structure in the universe.
However, on October 20, 2003, a new Great Wall was found. This structure is approxi-
mately 1 billion light-years from Earth and has a length of 1.4 billion light-years, which is
approximately three times the scale of the previous discovery. In other words, this structure
currently holds the record for being the largest thing in the known universe.
These structures are distinguished from each other by calling the one discovered in
1989 the CfA2 Great Wall and the one discovered in 2003 the Sloan Great Wall.
Galaxies form galactic walls.