August 2016: M6/NGC 6405 – by Leslie Rose

The Butterfly Cluster

Image copyright: Leslie Rose

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.

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 distance 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.

Camera: Atki 383L+
Filters: Astrodon RGB filters
Exposures: 25 x 120 sec with each filter, 20 Darks, 20 Bias
Total Intergration Time: 2.5 hrs
OTA: GSO RC 8"
Mount: Celestron CGEM.