M6 Constellation – Butterfly Cluster
The next two objects in the catalog of Messier are not globular clusters, but open clusters. Open star clusters are generally much younger, contain only a fraction of the number of stars of a globular cluster and are located in the Milky Way. Also they are much closer, making observations simple and you do not need a big telescope to see the stars. Unfortunately; this does not apply to the Benelux Butterfly Cluster. It is one of the southernmost objects in Messier’s catalog and comes to us only five degrees above the horizon. In southern latitudes it is easier to see and find. It is five degrees north of the star Lambda Scorpii.
Facts and figures about M-6 Messier 6
- Other designations: NGC 6405
- Position: right ascension 17h40m20s; declination -32d15m12s
- Type: Open cluster
- Slight concentration towards the center, large differences in brightness, more than 100 stars.
- Magnitude: 4.2
- Diameter: 25 light years
- Distance: 1600 light years
M6 was discovered in the mid-seventeenth century by the Italian priest, Giovanni Battista Hodierna (1597-1660). Although; there is evidence that it was seen by Ptolemy in the second century. Messier found it in 1764 in the records of the Chéseaux and Lacaille and added it to his catalog as number 6. M6 was described by him as, “…cluster of small stars between the bow of Sagittarius and the tail of the Scorpion. With simple sight, this cluster is a nebula without stars, but…examined with the smallest instrument then there is already a cluster of…faint stars.” The name, “Butterfly Cluster” was coined by American astronomer Robert Burnham Jr. (1931-1993) based on the fact that the cluster it forms the shape of a butterfly with open wings. M6 can be seen in the southern sky.
Young and bright cluster: M-6
With one exception, all bright stars are M6 types, B4V to B9V. These are hot blue to blue-white stars that result from the burning of hydrogen. These are stars that have not yet become giants and it shows that the M6 is a relatively young star cluster with an estimated age of 95 million years. With a medium-sized telescope it is easy to find about 80 stars that belong to the cluster. The total number is now around 120. These stars originated from a large gas cloud, are bound to each other by gravity and move in the same direction. This binding is not as high as that of stars in globular clusters. It is generally believed that most open clusters slowly disintegrate overtime. It is by no means M6. The blue stars are burnt and the cluster will no longer be as good as it is now.
The exception mentioned above is the brightest star of the cluster; BM Scorpii. This, like Betelgeuse , is a semi regular variable star of the type K2Ib (weaker, orange super giant). Its pulsation magnitude varies between 5.5 and 7.0 and falls from the tone of the rest of the bright stars. The mass of BM Scorpii is estimated to be 6 to 7 times that of the Sun. (Said to explode in the near future as a supernova.) Long before the falling apart of the M6 and the shedding of its outer layers, it spent tens of thousands of years as a beautiful planetary nebula. The core is a heavy white dwarf with oxygen and neon as the main ingredients. It should be noted that the properties of BM Scorpii are poorly studied.