Butterfly Cluster

Butterfly Cluster

Amateur image of M6. Author: Ole Nielsen
Object type	Open cluster
Other designations	Messier 6, NGC 6405, Collinder 341, Melotte 178, Lund 769, OCL 1030, ESO 455-SC030
Observation data (Epoch J2000.0)
Constellation	Scorpius
Right ascension	17h 40.1m
Declination	−32° 13′
Distance	1.6 kly / 491 Pc
In visual light (V)
Apparent magnitude	4.2
Size	25′
Radius	6 light years
Related media on Wikimedia Commons

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

The first astronomer to record the Butterfly Cluster's existence was Giovanni Battista Hodierna in 1654. However, Robert Burnham, Jr has proposed that the 1st century astronomer Ptolemy may have seen it with the naked eye while observing its neighbor the Ptolemy Cluster (M7). Charles Messier catalogued the cluster as M5 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 B type stars but the brightest member is a K type orange giant star, BM Scorpii, 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.

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

External links

• M6 Butterfly Cluster Information and Images on UK Astro
• Messier 6, SEDS Messier pages
• The Butterfly Cluster on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images