O.R.S.A.’S Astronomy Pages: M6


  1. The history
  2. To find M6
  3. To observe M6
  4. The stars of M6

The history

Until 1984, de Chéseaux was credited (1746) with the discovery of this splendid galactic cluster. But in 1985 G. Foderà Serio (Osservatorio Astronomico “G. S. Vaiana”, Palermo), P. Nastasi (Istituto di Matematica, Università di Palermo) e L. Indorato (Istituto di Geologia, Università di Palermo) irrefutably demonstrated the Hodierna’s priority. M6, in fact, is drawn and described in Hodierna’s “De Admirandis Coeli Characteribus” (pages 42 and 43), printed in Palermo in 1654. Giovan Battista Hodierna described M6 as “Quarta Nebulosa secunda, & exigua duarum est, quae candescunt iuxta Aculeum Scorpionis, haec vero ipsi Aculeo praeminet declinans à magna in Boream, & occasum.
According to Burnham, M6 was already observed by Ptolemy and Ulugh Begh. Messier found it in 1764, and described it as “a cluster of small stars between the bow of Sagittarius and the tail of Scorpius. To the naked eye it resembles a nebula without a star but even a small telescope reveals it as a cluster of small stars“.

To find M6 M6 lies about 5° N of the tail of Scorpius (stars l and n), and 3½° NW from the largest and brightest M7.

To observe M6

The shape of this galactic cluster suggest the outline of a butterfly with open wings, in fact it’s also called “Butterfly cluster“. The figure of the “butterfly” is about 9 l. y. in lenght, but the total diameter of M6 is more than 20 l. y.
M 6 is probably, among the Messier objects, the nearest to the center of our Milky Way. In his neighbourhood are several smaller clusters: 1.2° WSW, for exemple, lies the scattered cluster NGC6383.

The stars of M6

A. Wallenquist in 1959 identified, in in a field of 54′, almost 80 members of the cluster, with a central density of about 25 stars in a cubic parsec, and about 0.6 stars per cubic parsec for the over-all density of the whole cluster.
A study made at Harvard’s Boyden Observatory in South Africa in 1958 shows that great majority of the members are B-type main sequence stars, with the exception of the brightest, wich is a yellow giant.
Burnham suggest for the age of this cluster 100 million years. The Sky Catalog 2000 gives 50 millions. In any case, less than half the age of its neighbour M7.
The evaluetion of the distance range from about 1,300 l. y. (Hogg) to 1860 (Becvar).