M6 and M7, Open clusters in Scorpius
I feel very blessed because from my current location I am able to see constellations from both hemispheres and therefore I can see Scorpius in its entire splendor. Scorpius is a majestic constellation that certainly looks like a scorpion and is formed by about a dozen stars that shine through a light polluted sky like the one I have in my city.
The first time I saw it, I was amazed because the color of Antares and by the way, the main stars shine so brightly in it. Since that night, I have never forgotten the shape of Scorpius.
In my humble opinion, Scorpius and Sagittarius are the two constellations with the most beautiful deep sky objects and why not they are toward the centre of the Milky Way. Of course, M7 and M6 located in Scorpius are no exception, both being of such beauty. Here is my first sketch of M6, Butterfly cluster. I had no moon in the early morning of the observation, but there was a fine layer of clouds which caused the light from the city to be worse, reflected in the southern sky. Finally at 3:00, the sky cleared and I took the chance to sketch this lovely cluster located about 5 arc degrees north-east from Cat’s Eyes: the stars, Shaula λ Sco and Lesath ν Sco. I had no doubt that M6 received its name because of its particular shape of a butterfly.
From my LPS I could not see the stars forming the butterfly antennas but the brightest stars formed a view like a pair of butterfly hindwings.
While most of the stars in the Butterfly cluster shine in a white-bluish color, there was a nice star just in the corner of the left wing shining in a very intense orange color: that star is BM Scorpii (HD 60371 or HIP 8652), an orange variable supergiant star located at 1600 LY away, of spectral type class K, more than 100 times bigger than our sun and more than 2000 times more luminous than it. I made some calculations about distances and according the size of BM Scorpii if it was in the place of our sun, it would occupy a size in the solar system just beyond the orbit of mercury. This star was so impressive that it truly stood out in M6.
Now, let’s talk about M7. It is also known as Ptolemy cluster, because it was described like a nebulous object by the Greco-Roman astronomer Claudius Ptolemy in his work Almagest. Based on my observations, I could say without doubt that thanks to its very bright stars, M7 is the second most luminous open cluster in the entire sky after the Pleiades. It is also the only one that in a very exceptionally clear night in my LPS (perhaps because my southern neighbors electricity went out) I could perceive with my naked eye as a very dim nebulous object (after a good eye adaptation).
M7 was really easy to spot with the binoculars as it was located at about 5 arc degrees to the east from the Cat’s Eyes. The star HIP 87261 (G Scorpii) in the lowest part in the sketch was located at just 2 arc degrees from M7. It provided a nice contrast between it and the brightest stars in M7 as it is an orange star class K while most of the stars in M7 are white-blue class A and B.
I decided to name this entry with the name the Bedouin of the Arabian Desert gave to these open clusters. I can just image all those very dark nights in the southern Arabian Desert where the sting of Scorpius was no higher than about 15 degrees from the horizon, the perfect darkness allowed them to perceive M6 and M7 as nebulous objects in the sky. I wish eventually to go into a dark place in my country to contemplate both of them with the naked eye and evoke those views from the Arabian Desert contemplating the venom of the Scorpion.
Please enjoy this marvelous creation of God.
Edited by Jennifer Steinberg (editor in chief)