The Virgo Cluster: A History of Discovery and Exploration

The Virgo Cluster of Galaxies was discovered as an accumulation of nebulae by Charles Messier in 1781, as documented after the entry of M91 in his catalog. He noted that 16 and thus unusually many "nebulae" of his catalog are situated in this small region in the sky at the north-western edge of Virgo and overlapping to Coma Berenices. William and John Herschel, in their catalogs, also found many of their "nebulae" concentrating here, and Alexander von Humboldt commented that one-third of all nebulae known in the mid-19th century are situated in one-eighth of the sky around Virgo.

At that time, the concentration of "nebulae" in this region was not understood and related to the proximity to the North Galaxctic Pole.

The first study solely related to the Virgo "nebulae", more acurately to their positions, according to Tammann (ref. below), was undertaken by A. Schwassmann, probably influenced by Max Wolf (1902, Publ. Astrophys. Obs. Königstuhl-Heidelberg 1, 17), who observed 301 objects.

A. Hinks, in two papers published in the Monthly Notes (1911, M.N. 71, 588 and 1914, M.N. 74, 707), investigated the distribution of nebulae in an all-sky map, and suspected that the distribution of spiral nebulae might have own physical reasons and not only be related to the galactic poles.

It was Harlow Shapley and Adelaide Ames who in 1926 first used the term "cluster" ("of bright spiral nebulae") for this accumulation of galaxies, and determined data (brightness, color, diameter) for 103 of them. They estimated a common distance of 10 million light-years for these "nebulae", thus obviously thinking of a physical cluster. Ames extended this investigation in 1930 to 2278 objects brighter than magnitude 18 in this region, which she called Coma-Virgo group or cloud. This survey led to the identification of several background clusters and to the discovery of a Southern Extension of the Virgo Cluster.

In 1931, E.P. Hubble and M.L. Humason first used the term Virgo Cluster, and classified several hundred galaxies as members. S. Smith (1936, Ap.J. 83, ) was the first to apply the Virial Theorem to the Virgo Cluster for a mass estimate, which is still reasonably close to more sophisticated modern estimates, when rescaled to current values of the distance. The full extent of the Virgo Cluster was studied and visualized in charts by Fritz Zwicky in the late 1950s.

Using the work of Shapley and Ames, A. Reiz of Lund Observatory found in 1941 that there might be an extended halo of galaxies around the Virgo Cluster. Starting in the mid-1950s, Gerard de Vaucouleurs found that this halo extends well to our Local Group, and can explain an observed anisotropy in the distribution of nearby galaxies between galactic Northern and Southern hemispheres. The whole complex was first named Supergalaxy, a term first used by Shapley and Ames after having discovered the Southern Extension, but since the 1960s is often referred to as the Local or Virgo Supercluster. The term "supercluster" had been created by Fritz Zwicky in 1959 for "spherical systems of well defined individual clusters": The Local Supercluster is not really a supercluster in this sense as it is neither spherical nor are the "individual clusters" always well-defined - but this is common in the universe where most big clusters of galaxies seem to have such halos.

References

G.A. Tammann, Introduction to the ESO Workshop on the Virgo Cluster, Garching 4-7 September 1984. ESO Conference and Workshop Proceedings No. 20, 1985, p. 3-10.

Hartmut Frommert
Christine Kronberg
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Last Modification: April 28, 1998