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.

Last Modification: 28 Apr 1998, 21:30 MEST