Editing Butterfly Cluster (section)

{{short description|Open cluster in Scorpius}}
{{distinguish|butterfly cluster compound}}
{{Infobox open cluster
 | name      = Butterfly Cluster
 | image     = [[File:Messier 6 - The Butterfly Cluster.jpg|300px]]
 | epoch     = J2000.0
 | class     = II,3,m
 | constellation  = [[Scorpius]]
 | ra        = {{RA|17|40.1}}<ref name=Wu2009/>
 | dec       = {{Dec|−32|13}}<ref name=Wu2009/>
 | dist_ly   = {{Convert|0.487|kpc|kly|order=flip|abbr=on|lk=on}}<ref name=Wu2009/>
 | appmag_v  = 4.2<ref name=Frommert2007/>
 | size_v    = {{Val|25|u=arcminute}}<ref name=Frommert2007/>
 | mass_msol = 
 | radius_ly = 6<ref name=trig/> light-years
 | age       = 94.2<ref name=Wu2009/>&nbsp;Myr
 | notes     = 
 | names     = Messier 6, [[New General Catalogue|NGC]] 6405, [[Collinder catalogue|Cr]] 341, [[Melotte catalogue|Mel]] 178, Lund 769, OCL 1030,<ref name=simbad/> ESO 455-SC030
}}

The '''Butterfly Cluster''' (cataloged as '''Messier 6''' or '''M6''', and as '''NGC 6405''') is an [[open cluster]] of [[star]]s 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>{{cite web |url=http://www.messier.seds.org/m/m006.html |title=Messier 6 |access-date=20 July 2024 |website=SEDS Messier Catalog}}</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 {{Convert|0.487|kpc|ly|order=flip|abbr=off|lk=on|disp=out}},<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/>&nbsp;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 star]]s but the brightest member is a [[K-type star|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 [[apparent magnitude|magnitude]] +5.5 to magnitude +7.0. There are also eight candidate [[chemically peculiar star]]s.<ref name=Paunzen2006/><ref name=Kılıçoğlu2016/>

The cluster is located {{Convert|7.54|±|0.04|kpc|kly|order=flip|abbr=on|lk=off}}<ref name=Netopil2016/> from the [[Galactic Center]] and is following an orbit through the [[Milky Way]] galaxy with a low [[orbital eccentricity|eccentricity]] of 0.03 and an [[orbital period]] of 204.2&nbsp;Myr. At present it is {{Convert|7|pc|ly|order=flip|lk=off|abbr=on}} below the [[galactic plane]], and it will cross the plane every 29.4&nbsp;Myr.<ref name=Wu2009/>