M6 is a superb bright naked eye open cluster in the constellation of Scorpius that's also known as the Butterfly Cluster. The name was first coined by Robert Burnham who described it as a "charming group whose arrangement suggests the outline of a butterfly with open wings". At magnitude +4.2, it's one of the brightest open clusters in the Messier catalogue and a wonderful object for binocular and telescope owners. It covers 25 arc minutes of apparent sky and contains 80 stars. Located just a few degrees southeast of M6, in this wonderfully rich area of the Milky Way, is an even brighter and larger open cluster M7.

Despite being visible to the naked eye, it's commonly believed that the first person to record the position of M6 was Giovanni Battista Hodierna in 1654. However, Robert Burnham proposed that Ptolemy might also have seen M6 with the naked eye, while observing M7. Many years later, Charles Messier included both M6 and M7 in his catalogue on May 23, 1764.

M6 is located in eastern Scorpius. At the heart of the Scorpius is red supergiant star Antares (α Sco - mag. +1.0) the brightest star in the constellation and 16th brightest in the night time sky. Follow the stars from Antares, curving in a southerly direction, until arriving at lambda Sco (λ Sco - mag. +1.6). M6 is positioned 5 degrees north and 1.5 degrees east of this star.

M6 Butterfly Cluster (credit:- N.A.Sharp, Mark Hanna, REU program/NOAO/AURA/NSF)

Finder Chart for M6 (credit:- freestarcharts)

Finder Chart for M6 - pdf format (credit:- freestarcharts)

Finder Chart for M8 (also shown M6, M7, M18, M20->M24, M28, M54, M55, M69 and M70)  (credit:- freestarcharts)

Finder Chart for M8 (also shown M6, M7, M18, M20->M24, M28, M54, M55, M69 and M70) - pdf format (credit:- freestarcharts)

From a dark site, M6 is an easy naked eye target appearing as a hazy patch that hints on resolution. With 7x50 or 10x50 binoculars, it's a wonderful sight. The brightest six stars are of approximately the same brightness and line up to form the beautiful butterfly shape. Through a small 100mm (4-inch) telescope, the cluster is awash with stars of various colours. Most of the bright stars in M6 are hot, blue B type stars but the brightest member is K type orange giant variable star, BM Sco (mag. +5.5 -> +7.0 - period ~850 days). The contrast between this star and the surrounding hot white/blue stars is striking, a wonderful view.

M6 is 1,600 light-years distant and has a spatial diameter of 12 light-years. It's estimated to be 95 million years old and is best seen from the Southern Hemisphere during the months of June, July and August. From northern temperate latitudes, M6 never climbs particularly high above the southern horizon.

M6 Data Table

NameButterfly Cluster
Object TypeOpen cluster
Distance (light-years)1,600
Apparent Mag.+4.2
RA (J2000)17h 40m 21s
DEC (J2000)-32d 15m 15s
Apparent Size (arc mins)25 x 25
Radius (light-years)6
Age (years)95 Million
Number of Stars80
Other NameCollinder 341

Sky Highlights - September 2017

Neptune reaches opposition on September 5th

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for September

Northern Hemisphere
West:- Jupiter (mag. -1.7)
Southwest:- Saturn (mag. +0.5)
East:- Neptune (mag. +7.8)
South:- Neptune
East:- Uranus (mag. +5.7)
West:- Neptune
South:- Uranus
East:- Venus (mag. -3.9), Mars (mag. +1.8) (from second week), Mercury (mag. +0.5 to -1.3) (from second week)

Southern Hemisphere
West:- Jupiter
Northwest:- Saturn
East:- Neptune
West:- Saturn
North:- Neptune
Northeast:- Uranus
West:- Neptune
Northwest:- Uranus
Northeast:- Venus
East:- Mars (end of month)

Deep Sky

Small telescopes:-
Messier 13 - M13 - Great Hercules Globular Cluster
Messier 92 - M92 - Globular Cluster
Messier 11 - M11 - The Wild Duck Cluster (Open Cluster)
Messier 7 - M7 - The Ptolemy Cluster (Open Cluster)
Messier 6 - M6 - The Butterfly Cluster (Open Cluster)
Messier 4 - M4 - Globular Cluster
Messier 8 - M8 - Lagoon Nebula (Emission Nebula)
Messier 16 - M16 - Eagle Nebula (Emission Nebula with Open Cluster)
Messier 20 - M20 - Trifid Nebula (Emission and Reflection Nebula)

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