Messier 6, NGC 6405 - Butterfly Cluster in Scorpius

Messier 6 is an open cluster of stars in Scorpius, known as the Butterfly Cluster, from the vague resemblance of its shape to that of a butterfly.

Historial Observation

Robert Burnham, Jr. speculated that had M 6 was seen with the naked eye by Ptolemy in the 2nd century A.D., and included by Ptolemy in his mention of its apparent neighbor M 7. However, the first certain observer of M 6 was Giovanni Batista Hodierna, sometime prior to 1654. Hodierna's records were not discovered before the 1980s; thus, Philippe Loys de Chéseaux independently rediscovered M 6 in 1745-46; Nicholas Louis de Lacaille included it in his catalog of 1751-52 as Lac III.12, and Charles Messier eventually included it in his catalog in 1764.

Visual Appearance

Burnham described M 6 as a "charming group whose arrangement suggests the outline of a butterfly with open wings." The Butterfly Cluster contains 80 stars spread over a 54 arc minute area, with its brighter members concentrated in a 30' x 15' rectangle. The irregular parallel lines of stars that form the long side of the rectangle are the butterfly's wings, and its antennae are a small but conspicuous V of 10th and 11th magnitude stars southeast of the cluster's center.

M 6 is seen against a darker background than M 7, so it appears somewhat brighter. The total visual magnitude of M 6 is about 4.2. Of all Messier objects, M 6 is situated at the closest angular distance to the Galactic Center.

Physical Properties

Estimates of the Butterfly Cluster's distance have varied over the years, with modern values around 1,600 light years. At this distance, the 25' apparent diameter of the cluster's main portion gives it spatial dimensions of some 12 light years. M 6 has an absolute magnitude of -5.0 (a luminosity of 8,300 suns), and an average density estimated at 0.6 stars per cubic parsec. The age of M 6 has been estimated at 50 to 100 million years.

The brightest star in this cluster is the semiregular variable star BM Scorpii, an orange supergiant of spectral type K0-K3 Ib whose apparent magnitude varies between 5.5 and 7. The cluster's hottest stars are blue main sequence stars of spectral type B4-B5. The contrast between the orange giant and the bright blue stars is obvious in color photos of the cluster.