[TrevorW]

Target The Butterfly Cluster Messier 6 or M6, and as NGC 6405

Scope: GSO 8”(200mm) F9 CF RC
Mount : Losmandy G11-G2
Camera: QHY 8 OSC CCD
Exposures: 15 frames 9 minutes per frame 2hr 15m
Date: 19/5/2012
Time: 9:00 – 12:00pm
Seeing : Very Good
Conditions: Cool night no moon usual suburban LP
Guiding: Orion SSAG on 72mm WO Mergz using PHD
Stacking: DSS
Processing: PS CS 3, Max DL
The Butterfly Cluster 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 neighbour the Ptolemy Cluster (M7). 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 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.
Right Ascension

17 : 40.1 (h:m)

Declination

-32 : 13 (deg:m)

Distance

1.6 (kly)

Visual Brightness

4.2 (mag)

Apparent Dimension

25.0 (arc min)