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.
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
|Object Type||Open cluster|
|RA (J2000)||17h 40m 21s|
|DEC (J2000)||-32d 15m 15s|
|Apparent Size (arc mins)||25 x 25|
|Age (years)||95 Million|
|Number of Stars||80|
|Other Name||Collinder 341|