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M 6

Open Cluster M6 (NGC 6405), type 'e', in Scorpius

Butterfly Cluster

Right Ascension 17 : 40.1 (h:m)
Declination -32 : 13 (deg:m)
Distance 2 (kly)
Visual Brightness 5.3 (mag)
Apparent Dimension 25.0 (arc min)

Discovered by Hodierna before 1654.

A "charming group whose arrangement suggests the outline of a butterfly with open wings." (Burnham). Ake Wallenquist, in 1959, has identified about 80 cluster members in M6. The diameter of this star cluster is some 20 light years, with an estimated average density of 0.6 star per cubic parsec. Its age was estimated to 100 million years according to Burnham, and to 51 million years according to the Sky Catalog 2000. The brightest star in this cluster is a yellow or orange giant (spectral type K0-K3) of apparent magnitude 6.17 (the most left of the 4 bright stars forming the remarkable quadrangle of approximate parallelogram shape in our photo), while the hottest stars are blue main sequence stars of spectral type B4-B5. See the NOAO color photo of M6.

Trumpler has classified M6 as II,3,m, while the Sky Catalog 2000 gives its Trumpler type as III,2,p, Götz as II,3,r.

Of all Messier objects, M6 is situated at the closest angular distance from the Galactic Center, which is located in the constellation Sagittarius but very near to the 3-constellation edge of Sagittarius, Scorpius and Ophiuchus.

Burnham proposes that Ptolemy's mentioning of its apparent neighbor, M7, may include M6, but generally the discovery is credited to de Cheseaux, who definitely was the first to recognize it as "a very fine star cluster". According to Kenneth Glyn Jones, it was however seen previously by Hodierna who counted 18 stars, before 1654. Lacaille included it in his catalog of 1751-52 as Lac III.12, and Charles Messier eventually cataloged it on May 23, 1764.

  • Historical Observations and Descriptions of M6
  • More images of M6
  • Amateur images of M6
  • More images of M6 and M7

  • Jean-Claude Mermilliod's WEBDA cluster page for M6
  • SIMBAD Data of M6
  • NED Data of M6
  • Observing Reports for M6 (IAAC Netastrocatalog)

    Hartmut Frommert (spider@seds.org)
    Christine Kronberg (smil@lrz.uni-muenchen.de)

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    Last Modification: 17 May 2001, 00:38 MET