Messier 6 - Butterfly Cluster

Messier 6 / NGC 6405 / Butterfly Cluster is an open cluster located in the Scorpius constellation at approximately 1.600 light-years away from Earth. Although it was officially registered by Giovanni Battista Hodierna in 1654, there is proof that Ptolemy observed it with the naked eye in the 2nd century AD.
Having an apparent magnitude of 4,2, it can be seen with the naked eye if the sky is black enough. M6 has a diameter of 12 light-years and an estimated age of just 94 million years old.
The cluster has 120 stars, most of them B-type – however, the brightest star is BM Scorpii, an orange K-type giant star, which gives a beautiful contrast compared to the other bluish stars. BM Scorpii is a semiregular variable star with a range of apparent magnitude between 5,5 and 7.