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Scorpion treasures

Scorpius contains dazzling open star clusters for binocular users.
Harrington

Two of my favorite summer star clusters can be found scraping the southern horizon this month. M6 and M7 trail after the stinger stars that mark the tail of Scorpius the Scorpion. Each would be a lovely sight if viewed alone, but when teamed together in such rich surroundings, the scene becomes singularly beautiful.

To spot them, place the stinger on the southern edge of your binoculars’ field of view, and then look to the north. The clusters should fit into the same field. In fact, you might not even need binoculars to spot them. Both are bright enough to see with the naked eye if the sky in that direction is dark and clear.

M6 is the smaller of the pair. At first glance, it strikes most observers as sharply rectangular, making it unique in a universe populated with ovals and circles. But take a closer look, and let your mind play connect-the-dots with the stars. See anything besides a rectangle? Many observers can trace the outline of a butterfly among the stars. Look for two wings outstretched from a centered body. The butterfly appears to be headed toward the northwest.

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The Butterfly Cluster (M6) in Scorpius is beautiful through binoculars, with dazzling stars and a curious shape.
Bernhard Hubl
M6 is the smaller of the pair. At first glance, it strikes most observers as sharply rectangular, making it unique in a universe populated with ovals and circles. But take a closer look, and let your mind play connect-the-dots with the stars. See anything besides a rectangle? Many observers can trace the outline of a butterfly among the stars. Look for two wings outstretched from a centered body. The butterfly appears to be headed toward the northwest.

M6 is about 1,590 light-years from Earth, spans about 10 light-years, and is believed to be between 90 million and 100 million years old. More than two dozen cluster stars can be seen using 50mm binoculars, while 70mm and 80mm glasses add another 10 to 15 fainter points. Most are hot blue-white infernos.

One of its most massive stars, however, has left the main sequence and evolved into an orange stellar ember. That star, set east of the cluster’s center and known as BM Scorpii, is an irregular variable that fluctuates slowly and erratically from 6th to 8th magnitude across an average of 850 days.

Steer to the southeast of M6 to find the next member of Charles Messier’s catalog. M7 is larger and brighter than M6, so it should be even more obvious through binoculars. In fact, it is one of the few deep-sky objects that was known to the ancient world. Ptolemy was first to mention it in his epic volume Almagest, published in A.D. 130.

Today, many refer to this as Ptolemy’s Cluster, although he had no way of knowing its true nature. That revelation had to wait 15 centuries. In 1654, Italian astronomer Giovanni Battista Hodierna published the first telescopic observation, recording 30 stars. In 1764, Messier included it as object No. 7 in his catalog, describing it as “a cluster considerably larger than the preceding” (M6).

One reason M7 appears considerably larger is because it’s considerably closer, some 980 light-years away. It also spans twice the space — 23 light-years versus 10 light-years. However, both include about the same number of stars.

At nearly 35° south of the celestial equator, M7 is the southernmost object in Messier’s listing. With a clear view, you’re in for a real treat. Even through modest pocket binoculars, M7 explodes into a striking assortment of stars covering an area larger than the Full Moon. Eighty stars have been identified as belonging to the cluster, with dozens of nonmembers either in the foreground or lying beyond also contributing to the scene. More than 30 cluster stars shine brighter than 10th magnitude, and as such, should be visible through 50mm binoculars. Several also show delicate hues of yellow and blue, with the brightest being a yellow beacon close to the group’s center.

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The enormous open cluster M7 is a fine naked-eye sight. In binoculars, it explodes with bright stars.
Allan Cook/Adam Block/NOAO/AURA/NSF
I know it’s an illusion, but my 16x70 binoculars create a three dimensional effect that makes many of the brighter stars in M7 look like they’re floating in front of a field of fainter stardust. This visual impact can never be duplicated in a photograph.

This region of Scorpius also holds a pair of asterisms visible through binoculars. Both were created by the imaginative mind of Massachusetts amateur astronomer John Davis, who, sadly, passed away earlier this year.

The first, which he christened the Hockey Stick, lies about a 7° binocular field west of M7. Look for a north-south line of four equally spaced 7th-magnitude stars. A fifth sun southwest of the line forms the stick’s heel. I can squeeze this, plus both open clusters, into my 10x50s. When you place M7 at the easternmost edge of the field and M6 toward the north, the hockey stick will be visible along the western edge.

Another Davis creation is about a binocular field northwest of the hockey stick. He dubbed this one the Garden Trowel. Three 7th-magnitude stars point southward to form the trowel blade, while another four or so create its northward-meandering handle. Most of the Trowel’s stars are white, but a couple of them shine with a subtle golden glint.

We will explore more of Davis’ asterisms in my next column. Meanwhile, do you have a favorite binocular target that you’d like to share? I’d love to hear about it and possibly feature it in a future column. Drop me a line through my website, philharrington.net. 

Until next time, remember that two eyes are better than one!

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