Two Must-See Star Clusters in Scorpius
Today, we look at two splendid star clusters between the stinger of Scorpius and the “spout” of the teapot-shaped constellation Sagittarius. Lovely in a small scope or binoculars, and visible to the unaided eye, Messier 6 and Messier 7 are must-sees for stargazers nearly anywhere in the world, though they are best seen from as far south as possible.
Messier 6
M6 is among the brightest of dozens of open star clusters that fleck the Milky Way in Scorpius and Sagittarius. Look for it about 4 degrees north of the bright star Shaula in the Scorpion’s tail (see map below). M6 is a respectable 4th magnitude, though its light spreads over an area as large as the full Moon. You’ll see the cluster without optics in dark sky; a full Moon or city lights make it harder to see, especially if the cluster is near the horizon.
A map of the tail of Scorpius showing M6 (the Butterfly Cluster) and M7 (click to enlarge).
M6 is often called the Butterfly Cluster, and a glance through a small telescope reveals why. At 40-50x, the cluster has 3 bright stars running through the center (the body of the butterfly), with two irregular loops of stars on either side (the wings). A little imagination reveals the butterfly’s “antennae” to the northeast. Experiment with different eyepieces to get the best view.
The cluster lies some 1,600 ly away from us towards the galactic center. It holds a little more than 300 stars, though you’ll see just a few dozen in binoculars, and perhaps 100 stars in a 6-inch scope. It’s a young cluster… about 100 million year old… so it contains mostly blue stars, with one orange-giant star (BM Scorpii) in the northeast corner.
Here’s an image of M6… lovely, yes?
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Messier 7
Look 3.5 degrees southeast of M6 to find the cluster M7 set in one of the richest sections of the Milky Way (see image at top). Though they’re closeby in the sky, the two clusters are not physically associated. M7 is closer, just 780 light years away. To the unaided eye, in the words of Stephen J. O’Meara, the spray of light from M7 looks “like the eruption of distant fireworks.”
M7 appears larger and brighter than M6, though both were known in antiquity. But M7 is the southernmost Messier object, so it’s rarely seen well at northern latitudes, and it presents a real challenge for observers in northern Europe. The view of this cluster from the southern hemisphere, however, where it’s high overhead this time of year, is jaw-dropping.
The cluster spans more than a full degree of sky, twice the size of the full Moon. So stick with binoculars or a low-power eyepiece. At 30x, the center of M7 looks square, or to some, cross-shaped. Since M7 is twice as old as M6, some of its 80 stars have begun to evolve off the main sequence and turn orange-red. The hottest and most massive blue stars have, presumably, burned out long ago.
Make sure you include M6 and M7 on your observing list this month and next. They prove the point that, like diamonds, no two open star clusters are alike, yet all are beautiful in their own way.
