You Simply Must See M6 And M7, Summer’s Brightest Star Clusters

M7 (left) and M6 along with a portion of the southern Milky Way poke out from between trees earlier this week. I took this photo near Duluth, Minn. around midnight on June 12-13 with a 35mm lens at f/2.8, 6 second time-exposure and ISO 6400. The view is very similar to what you’d see in a pair of binoculars from a dark sky. Bob King

In fall and winter, the Pleiades and Hyades star clusters brighten our nights. Both are plainly visible without optical aid even from the suburbs. And every spring, the Beehive Cluster in Cancer the crab looks like a fuzzy glow resembling a comet. But when it comes to summer you can’t do better than M6 and M7. Never heard of ’em? I’m not surprised.

Better known as the Butterfly Cluster (M6) and Ptolemy’s Cluster (M7) they aren’t nearly as well known because they’re not as bright as the Pleiades and rather poorly placed in the sky for skywatchers in mid-northern latitudes. Both are located in in the tail of the fishhook-shaped constellation Scorpius the scorpion.  From the northern U.S. and southern Canada they stand about 10° (one fist) high at best, but the further south you live the higher they climb: 15° in Des Moines, Iowa (central U.S.) and 25° in New Orleans (southern U.S.).

This map shows the sky facing south around midnight in mid-June (11 p.m. by month’s end) local time. The Butterfly Cluster (M6) and Ptolemy’s Cluster (M7) each lies about 5° off the tail end of the Scorpius between the scorpion’s stinger stars, Shaula and Lesak, and the Teapot’s spout. They are summer’s brightest open star clusters, M6 and M7. Stellarium

Every increment in altitude makes them that much brighter. Why? Because the clusters have to battle less and less with the dimming effect caused by our atmosphere. Dimming is strongest nearest the horizon because we peer through the bottom of the atmosphere where the air is thickest, compared to looking overhead.

Extinction not only dims stars near the horizon it also reddens them. In this photo the Milky Way appears relatively colorless higher up (middle and left) but pale orange near the horizon (right) for the same reason the sun and moon appear orange there — violet, blue and some of the green light from the stars is scattered away, leaving only the “warm” colors. Bob King

Vega, one of the brightest stars of the spring-summer sky, shines at magnitude 0. But when it first rises it’s only as bright as magnitude 2 even on the clearest of nights because of the denser air (along with the extra humidity and dust) its light must slice through. Astronomers call the dimming of celestial objects near the horizon extinction.

Both the Butterfly and Ptolemy clusters suffer from extinction plus they’re tucked low in the sky, playing hide-and-seek with trees or buildings. Otherwise they’d be better known to northern observers. So while it takes a little bit more effort to seek them out you’ll be glad you did.

I can spot both clusters faintly with the naked eye from my rural observing site. They look like puffs of light. Closer to the city I lose sight of them but either way they’re wonderful in a pair of binoculars. Depending on where you live you may have to deal with various levels of light pollution, so be sure to have binoculars on hand. My 10x50s turn these fuzzy patches into nests of tiny, twinkling stars.

M7 is the brightest star cluster in the summer sky and easily visible with the naked as a smoky patch off the tail of Scorpius. The “M” in M6 and M7 refers to Charles Messier, an 18th century French astronomer, who cataloged these and 108 additional star clusters, galaxies and nebulae. Known as the Messier catalog, many skywatchers try to find all 110 objects. ESO

Both are bright, with M7 shining at magnitude 3.3 and M6 at 4.2. One advantage of their low altitude is that it’s easier to hold the binoculars steady for a good view. Looking at objects higher up in the sky will often give you a case of shaky arms! Find M6 first by shooting a line a fist and a half in length from the bright, red-orange star Antares toward the scorpion’s tail. Once you’ve examined M6, slide one binocular-field down to the lower left, and you’ll run in to M7. You can also find the clusters by starting at the stinger stars and moving a binocular-field to the left for M7 and above for M6.

M7 is also known as the Ptolemy Cluster since it was first recorded by 1st-century Greek-Roman astronomer Ptolemy in 130 A.D. Located about 980 light years away it’s a gravitationally-bound group of stars that coalesced from clouds of gas and dust millions of years ago. Individual stars within this and other clusters vary in size, but they’re all about the same age and move together around the galactic center as a group. Mr. Ptolemy’s stellar stash looks like a sprawling spider in my binoculars. At its core I can see a tiny “K-shaped” pattern of stars tipped on its side.

This telescopic view of the Butterfly Cluster shows the bright orange giant at its east (left) edge. Can you make out the butterfly? Ole Nielsen

The Butterfly Cluster (M6) was first recorded by Italian astronomer Giovanni Battista Hodierna in 1654 but it’s hard to believe it wasn’t noticed by earlier skywatchers. M6 is about half the apparent size of M7 and located about 1,600 light years from Earth. In spite of its greater distance, binoculars still show a lots of “tiny” stars packed into a space about as wide as a full moon. To see the butterfly shape clearly, you might need a small telescope and low magnification. Be sure to look for the colorful orange giant star along the cluster’s east side — it stands in striking contrast to the backdrop of the cluster’s white and blue-white stars.

With the moon out of the sky I hope you’ll take the opportunity to drive to the country not only to soak up the Milky Way but also to appreciate the beauty of our featured star clusters.

NGC 6231 in the tail of Scorpius joins M6 and M7 to form a trio of the brightest summertime star clusters. While right next to the horizon from the northern U.S., NGC 6231 is visible from the central U.S. (very low in the southern sky) and points south. Stellarium

Addendum:  Savindu, one of our readers, pointed out to me that I’d overlooked NGC 6231, another bright star cluster in Scorpius. It’s located 5,900 light years away in the bottom of the scorpion’s tail just north of the star Zeta. I intentionally left it out because the cluster is so close to the horizon where I live that it’s impossible to see. But you may live further south where the cluster is better placed for viewing, so let’s include it. At magnitude 2.6 NGC 6231 is even brighter than M7, making it the brightest of the trio.

From latitude 40° north the cluster stands just 8° high when it’s due south and is faintly visible with the naked eye as a fuzzy spot next to Zeta. Binoculars and small telescopes reveal a pretty spray of blue-white suns described by Savindu’s friend as “an unfinished spangle work on a black dress.” If you live in the central and southern U.S. and further south I encourage you to include NGC 6231 in your tour of Scorpius star clusters.