View source for Messier Index/M6

{{ Cluster | <!-- put name or coordinates into the following URL -->
| name = [http://simbad.u-strasbg.fr/sim-id.pl?protocol=html&Ident=M6 Butterfly Cluster]
| image = [[Image:m6a.jpg|300px]]
| epoch = J2000.0
| class = II,3,m
| constellation  = [[w:Scorpius (constellation)|Scorpius]]
| ra = 17<sup>h</sup>&nbsp;40.1<sup>m</sup>
| dec = &minus;32° 13&prime;
| dist_ly = 1.6 [[w:light year|kly]]
| dist_pc = 491 Pc
| appmag_v = 4.2
| size_v = 25&prime;
| mass_kg = 
| mass_msol = 
| radius_ly = 6 light years
| v_hb = 
| age = 
| notes = 
| names = Messier 6, NGC 6405, Collinder 341, Melotte 178, Lund 769, OCL 1030, ESO 455-SC030}}

The '''Butterfly Cluster''' (cataloged as '''Messier 6''' or '''M6''', and as '''NGC 6405''') is an [[w:open cluster]] of [[w:star]]s in the constellation of [[w:Scorpius]]. Its name derives from the vague resemblance of its shape to a [[w:butterfly]].

The first astronomer to record the Butterfly Cluster's existence was [[w:Giovanni Battista Hodierna]] in 1654. However, [[w:Robert Burnham, Jr]] has proposed that the 1st century astronomer [[w:Ptolemy]] may have seen it with the naked eye while observing its neighbor the [[w:Ptolemy Cluster]] (M7). [[w:Charles Messier]] catalogued the cluster as M6 in 1764.  It was not till the 20th century that star counts, distance, and other properties were measured.

==Characteristics==
Most of the bright stars in this cluster are hot, blue [[w:spectral type|B type]] stars but the brightest member is a [[w:K type]] orange [[w:giant star]], BM Scorpii, which contrasts sharply with its blue neighbours in photographs. BM Scorpii, is classed as a [[w:semiregular variable|semiregular]] [[w:variable star]], its brightness varying from [[w:apparent magnitude|magnitude]] +5.5 to magnitude +7.0.  

Estimates of the Butterfly Cluster's distance have varied over the years, with a mean value of around 1,600 [[w:light year]]s, giving it a spatial dimension of some 12 light years.  Modern measurements show its total visual brightness to be magnitude 4.2.

==External links==
* [http://www.seds.org/messier/m/m006.html Messier 6, SEDS Messier pages]

{{Messier Index}}