Butterfly Cluster
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The Butterfly Cluster (cataloged as Messier 6 or M6, and as NGC 6405) is an open cluster of stars in the southern constellation of Scorpius. Its name derives from the resemblance of its shape to a butterfly.[1]
The first astronomer to record the Butterfly Cluster's existence was Giovanni Battista Hodierna in 1654.[2] However, Robert Burnham Jr. has proposed that the 2nd century astronomer Ptolemy may have seen it with the naked eye while observing its neighbor the Ptolemy Cluster (M7).[3] Credit for the discovery is usually given to Jean-Philippe Loys de Chéseaux in 1746. Charles Messier observed the cluster on May 23, 1764, and added it to his Messier Catalog.[1]
Estimates of the Butterfly Cluster's distance have varied over the years.[4] Wu et al. (2009) found a distance estimate of Template:Convert,[5] giving it a spatial dimension of some 12 light years.[6] Modern measurements show its total visual brightness to be magnitude 4.2. The cluster is estimated to be 94.2[5] million years old. Cluster members show a slightly higher abundance of elements heavier than helium compared to the Sun;[7] what astronomers refer to as the metallicity.
120 stars, ranging down to visual magnitude 15.1, have been identified as most likely cluster members.[4] Most of the bright stars in this cluster are hot, blue B-type stars but the brightest member is a K-type orange giant star, BM Scorpii,[8] which contrasts sharply with its blue neighbours in photographs. BM Scorpii, is classed as a semiregular variable star, its brightness varying from magnitude +5.5 to magnitude +7.0. There are also eight candidate chemically peculiar stars.[9][4]
The cluster is located Template:Convert[7] from the Galactic Center and is following an orbit through the Milky Way galaxy with a low eccentricity of 0.03 and an orbital period of 204.2 Myr. At present it is Template:Convert below the galactic plane, and it will cross the plane every 29.4 Myr.[5]
See also
- List of open clusters
- List of Messier objects
- Nu Andromedae, a candidate escaped star system from this cluster
References
External links
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