The Pleaides cluster, perhaps the most celebrated star cluster in all the heavens, is always absent from the Northern Hemisphere’s summer evening sky. Instead, on summer evenings, we in the north look for a gorgeous pair of star clusters – M6 (Butterfly Cluster) and M7 (Ptolemy’s Cluster). M6 and M7 may well be northern summer’s finest clusters. To appreciate them, you need a dark sky.
How to find M6 and M7. These two star clusters are easy to spot in a dark sky near the curved Tail of the constellation Scorpius. Scorpius is shaped like the letter J. There are two stars at the end of the curved part of the J – the Scorpion’s Tail – known as the Cat’s Eyes, or the stinger of Scorpius. The two stars are Lesath and Shaula. Draw an imagainary line from the star Lesath through the star Shaula to find M7, which is the brighter and larger of the two star clusters. From M7, M6 is only a short hop away. In fact, the two clusters are close enough together to snuggle within the same binocular field of view.
Although M6 and M7 can be seen with the unaided eye on a dark, moonless night, these deep-sky beauties are wonderful in binoculars. As you scan for them, suddenly these faint objects will ambush your eyes in the jungle of night, springing out like creatures from the wild. Their brilliance and beauty erupts in your binoculars. In other words … you can’t miss them, assuming you have a dark sky.
One trick to finding them is to have an unobstructed horizon toward the south, especially if you’re in the northern U.S., Canada or a similar latitude. This pair of clusters is highest up in the sky when due south. As seen from northerly latitudes, these clusters never climb very high in the southern sky. A southern horizon obstructed by trees or mountains might put M6 and M7 out of reach. Meanwhile, from latitudes like those in the southern U.S., the clusters are easy to spot when highest in the south, above the Scorpion’s Tail.
In middle June, these clusters bedeck the southern sky around midnight (1 a.m. daylight saving time). Keep in mind that all the stars (and star clusters) return to the same place in the sky some 4 minutes earlier with each passing day, or 2 hours earlier with each passing month. Therefore, M6 and M7 appear highest up in the sky at about 10 p.m. (11 p.m. daylight saving time) in mid-July, and 8 p.m. (9 p.m. daylight saving time) in mid-August.
M6 and M7 science. Though the M6 Butterfly Cluster and the M7 Ptolemy’s Cluster appear close together on the sky’s dome, they are actually far apart in space. M6 is thought to be about 1,600 light-years away, while M7 is about half that far, at 800 light-years. Thus the clusters are not related to each other, but only appear near each other along our line of sight.
But, within each cluster, the stars are related. Each cluster was born from a single interstellar cloud of gas and dust. The hundreds of stars in each cluster are indeed sibling stars, in that they are gravitationally bound to one another and travel in the same direction through space.
M6 and M7 reside near the galactic equator, the region on the sky’s dome where star clusters, star clouds and nebulae most abound. Once you find M6 and M7, try locating other deep-sky binocular doubles, such as M8 and M20, and M16 and M17. Unlike M6 and M7, which reside within the Orion spiral arm of the Milky Way galaxy, these deep-sky wonders loom farther away, in the next spiral arm inward, the Sagittarius arm.
M6 and M7 are established star clusters. But M8 and M20, and M16 and M17 are still incubating clouds of star formation.