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.<ref name=Adam2018/>

The first astronomer to record the Butterfly Cluster's existence was Giovanni Battista Hodierna in 1654.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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).<ref name=Burnham1705/> 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.<ref name=Adam2018/>

Estimates of the Butterfly Cluster's distance have varied over the years.<ref name=Kılıçoğlu2016/> Wu et al. (2009) found a distance estimate of Template:Convert,<ref name=Wu2009/> giving it a spatial dimension of some 12 light years.<ref name=trig/> Modern measurements show its total visual brightness to be magnitude 4.2. The cluster is estimated to be 94.2<ref name=Wu2009/> million years old. Cluster members show a slightly higher abundance of elements heavier than helium compared to the Sun;<ref name=Netopil2016/> what astronomers refer to as the metallicity.

120 stars, ranging down to visual magnitude 15.1, have been identified as most likely cluster members.<ref name=Kılıçoğlu2016/> 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,<ref name=Eggen1973/> 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.<ref name=Paunzen2006/><ref name=Kılıçoğlu2016/>

The cluster is located Template:Convert<ref name=Netopil2016/> 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.<ref name=Wu2009/>

See also

• List of open clusters
• List of Messier objects
• Nu Andromedae, a candidate escaped star system from this cluster

External links

• Messier 6, SEDS Messier pages
• Template:WikiSky