M6 - The Butterfly Cluster
Credits: NASA Wikisky
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). 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.
120 stars, ranging down to visual magnitude 15.1, have been identified as most likely cluster members. 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.
Facts about M6 by Keith Turnecliff
The Butterfly Cluster (also known as NGC6405) is an open cluster of astars in the southern constellation of Scorpius.
M6 is best seen in binoculars. Its apparent size is roughly the same as that of the full Moon. The cluster contains more than 300 stars. Binoculars reveal only a few dozen and a small telescope will show about 80 stars brighter than 11th magnitude.