The Virgo Cluster is a galaxy cluster located about 60 million light-years from the Earth, consisting of over a thousand individual galaxies. It is at the astronomical centre of the Virgo Supercluster (a group of galaxy clusters, including our own), and features both spiral and elliptical galaxies, including the exceptionally bright Messier 87 galaxy.
- Galaxy Clusters -
Galaxy clusters are immense, relatively tightly bound groups of galaxies, linked together through their combined gravitational forces but not so close together that they are (necessarily) colliding to form new galaxies. They usually comprise at least several dozen individual galaxies, stretching over several million light-years. (The Milky Way Galaxy alone, by way of comparison, is just a hundred thousand light-years in diameter.)
Clusters, in turn, can be loosely gravitationally tied to one another in even larger, more diffuse structures known as supergroups or superclusters. Several million superclusters are known to exist, each consisting of dozens or hundreds of galaxy clusters.
- The Virgo Cluster -
The Virgo Cluster contains somewhere between one and two thousand galaxies, all locked together by their collective gravitational forces. The combined mass of these galaxies is equivalent to about one quadrillion stars the size of our own Sun. (Of course, the actual number of stars will be less than that, since large blue stars weigh several times as much as the Sun.) It stretches over a three-dimensional field roughly seven million light-years.
There is considerable variety in the galaxies which make up the Virgo Cluster, which occur both as spiral galaxies (like our own) and elliptical galaxies (ovoid forms). About three-quarters are spiral galaxies, although as some of these inevitably collide, they may be torn apart or reshaped into ellipses (as our Milky Way will it collides with the nearby Andromeda Galaxy in several billion years). Indeed, last year scientists discovered that the Messier 87 galaxy, although the largest in the cluster, was missing most of its customary halo of stars, presumably after having them already ripped away by the gravity of a neighbouring galaxy or galaxies.
We now know that sixteen of the Virgo galaxies were large enough to be detected at this immense distance even with the comparatively primitive telescopes of the 18th century, when they were included in the Messier star catalog. Virgo's massive size makes it the centre, both geographically but also gravitationally, of a much larger cluster of about a hundred galaxy clusters referred to as the Virgo Supercluster, or sometimes the Local Supercluster. The much smaller cluster which is home to our own Milky Way Galaxy, named (unimaginatively) the Local Group, lies on the periphery of the Virgo Supercluster.