Open Clusters in Scorpius
The most recognizable constellation in the sky (after the Southern Cross, of course) is on fantastic display this month. It’s Scorpius the Scorpion. (Note: not ‘Scorpio’ which is the name used only in astrology.) This scorpion is well known in Greek mythology as the one sent by Gaia, the Goddess of the Earth, to sting Orion as punishment for his boast that he would hunt and kill every creature on Earth. Foolish man.
Scorpius is virtually high overhead this month at mid-evening so you choose which way you want to face, north or south, You’ll still end up craning your neck, so a comfortable lay-back chair is recommended – oh yes, and warm clothing.
There are quite a few objects suitable for enjoying with binoculars in Scorpius, starting from near the scorpion’s head, working down its spine and around to its tail. It’s a veritable binocular smorgasbord up there. Here we will just concentrate on two very popular and distinct open clusters, Messier objects M6 and M7. These are also sometimes known as The Butterfly and Christmas Tree Clusters respectively. In all honesty, I can see the butterfly immediately but I usually have to struggle to recognise the shape of a Christmas tree. Maybe it’s all in the viewer. You look and decide what you see.
They are embarrassingly easy to find, as you’ll see from the chart below. Both are visible to the naked eye on a clear dark sky, though M7 can tend to be lost in the Milky Way, looking like a brighter knot in the star field. M6 happily is located in a dark rift in the Milky Way and stands out more obviously to the naked eye.
First locate the end of the scorpion’s stinging tail. You’ll notice there is a sort of ‘barb’ at the end of the tail, marked by a 3rd magnitude star, G Scorpii. Start from there. If you now move North by 2.5°, (about half your binoculars’ field of view) you’ll land on M7. It is a huge open cluster, about twice the diameter of our Full Moon. It has about 80 stars of 6th magnitude of fainter, but its overall brightness is around 3rd magnitude making it a very attractive sight in binoculars. M7 is much closer to us than M6 and is about 950 light years away.
Happily, M6, which at 1600 light years lies much further away than M7, is in the same binocular field of view as M7, being only 3.5° away to the NW. So you can enjoy and compare them both at the same time. M6 also has about 80 stars in it and most of these are blue stars, with the exception of one orange giant. It has an apparent diameter of about that of our Moon, about half that of M7. But then it is almost twice the distance so in actual size, M6 and M7 are approximately the same.
In both M6 and M7, their main stars can be resolved and seen with binoculars so you can enjoy the patterns and decide what they look like to you.
For a detailed list of more binocular objects in Scorpius, see my book “Heavens Above – A Binocular Guide to the Southern Skies”.