The Milky Way lives on the edge of a flattened galaxy supercluster
Our Local Group sits near the outskirts of the Virgo Supercluster, strung along a thin filament of galaxies running from the Fornax Cluster toward the Virgo Cluster at its centre. The whole structure spans about 147 million light-years and fills a volume some 100 billion times that of the Milky Way.
Clues appeared in 1863, when William and John Herschel's catalogue of nebulae showed an unusual crowding in Virgo. In the 1950s Gérard de Vaucouleurs argued this was a genuine large structure, calling it the Local Supergalaxy in 1953 and renaming it the Local Supercluster in 1958; Harlow Shapley preferred Metagalaxy. Through the 1960s and 1970s sceptics suspected a chance alignment, until big redshift surveys around 1980 revealed a flattened sheet of galaxies along what became known as the supergalactic plane.
In 1982 R. Brent Tully mapped its anatomy: a thin disk holding two-thirds of the bright galaxies, at least six times longer than it is thick, plus a roughly spherical halo with the rest. Survey data from 2003 marked it as a fairly small, poor supercluster, lacking a dense core apart from the one rich cluster at its heart. Galaxies thin out with the square of distance from Virgo, and almost all bright members gather in just 11 clouds, leaving most of the supergalactic plane empty.
At least 100 groups and clusters belong to it, one of roughly 10 million superclusters in the observable universe. Its main body, the Virgo Strand, forms part of a long filament stretching from the Centaurus Cluster to the Perseus-Pisces Supercluster; the Milky Way lies outside that strand, among the remaining third. Clusters and superclusters tend to sit where giant bubble-like voids, 20 to 60 megaparsecs wide, press against one another.
Since the late 1980s astronomers have known that everything within at least 50 megaparsecs, us included, is streaming at around 600 km per second toward the Norma Cluster. The cause, nicknamed the Great Attractor in 1988, is now seen as the centre of a far larger basin called Laniakea. A 2014 study concluded that Virgo is merely one component of it.
Source: Virgo Supercluster
