M6, also known as the Butterfly Cluster, is a bright open cluster located in the southern constellation Scorpius. Its other designations are NGC 6405, Collinder 341, Lund 769, Melotte 178, OCL 1030, ESO 455-SC030, Lac. III.12. It lies in the direction of the galactic centre, at an approximate distance of 1,600 light-years from Earth at right ascension 17h 40.1m and declination -32°13′. Messier 6 was named the Butterfly Cluster by the American astronomer Robert Burnham, who described it as a “charming group whose arrangement suggests the outline of a butterfly with open wings.” The cluster has an apparent magnitude of 4.2. The Trumpler classification of II 3 r indicates it is rich in stars (r) with a high concentration (II) and a large range of brightness (3). Cluster members show a slightly higher abundance of elements heavier than helium compared to the Sun; what astronomers refer to as the metallicity.
The 80 stars that make up M6 are all moving through space together in an area spanning about 12 to 25 light-years across – and may have formed anywhere from 51 to 95 million years ago. The brightest of its stars is a variable known as BM Scorpii (HD 160371), a yellow or orange supergiant that changes its magnitude between 5.5 and 7 with a semi-regular period. However, most of the stars here are hot, blue main sequence stars of spectral type B4-B5. Members of this group were formed in the same giant molecular cloud and are still loosely bound to each other. Studies have been conducted on Messier 6’s upper main sequence stars for strong, highly structured magnetic fields – leading researchers further into understanding the origins and evolution of Ap stars in open clusters. The cluster contains more than 300 stars.
The cluster was discovered by the Italian astronomer Giovanni Battista Hodierna before 1654. It may have been seen by the Greek astronomer Claudius Ptolemy in the 2nd century while he was observing the nearby Messier 7 (Ptolemy Cluster), which lies only five degrees southeast of M6. The Ptolemy Cluster is a significantly brighter and larger open cluster, and it lies much closer to Earth, at 980 light-years. Swiss astronomer Philippe Loys de Chéseaux independently discovered M6 in 1745-46, describing it as “a very fine star cluster.” Abbé Nicolas Louis de Lacaille, the French astronomer famous for mapping many of the southern constellations, included M6 in his catalogue of 1752 as Lac III.12. Charles Messier entered the cluster in his catalogue on May 23, 1764. The distance, number of stars and other properties of M6 were not measured until the 20th century. In 1959, Swedish astronomer Ake Wallenquist identified around 80 stars in the cluster, all lying within a region of 54 arc minutes in diameter.
The cluster lies in eastern Scorpius, 5 degrees north and 1.5 degrees east of the multiple stars Shaula, or Lambda Scorpii. Shaula is the second brightest star in Scorpius, with a visual magnitude of 1.62, and it marks the scorpion’s tail. Shaula can be located by following the line of stars that curve from Antares in a southerly direction. Messier 6, Messier 7 and Shaula form a triangle that can be seen without binoculars to the right of the Teapot asterism in Sagittarius. The Ptolemy Cluster (M7) is the southernmost Messier object in the sky and it lies a little farther to the southeast of M6, closer to the horizon, while the Butterfly Cluster is a bit higher in the sky. The best time of year to observe the Butterfly Cluster is in the summer months when Scorpius appears high in the sky for observers in southern latitudes and is visible above the horizon to observers in the northern hemisphere. M6 is much easier to observe from locations south of the equator because Scorpius never rises high in the sky for northern observers. Messier 6 lies at a closer angular distance to the galactic centre than any other Messier object. The galactic centre, the rotational centre of the Milky Way, lies in the direction of Sagittarius constellation, near the borders with the constellations Scorpius and Ophiuchus. Credits: Messier Objects, Universe Today, Wikipedia.