How to find them
The Pleaides cluster, perhaps the most celebrated star cluster in all the heavens, is always absent from the summer evening sky. On summer evenings, a gorgeous pair of star clusters – M6 (Butterfly Cluster) and M7 (Ptolemy’s Cluster) – finally has its turn to take center stage. M6 and M7 may well be summertime’s finest clusters.
To star-hop to these summer delights, first find the Cat’s Eyes stars on the stinger of the J-shaped constellation Scorpius the Scorpion. Draw an imagainary line from the star Lesath through the star Shaula to find M7, the brighter and larger of these two 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 custom-built for binoculars. All of a sudden, these faint objects taking ambush in the jungle of night spring out like creatures from the wild. The brilliance erupts in your binoculars.
This pair of clusters is highest up in the sky when due south. As seen from mid-northern latitudes, these clusters never climb very high in the southern sky. A southern horizon obstructed by trees or mountains may put M6 and M7 out of reach.
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.
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 1600 light-years away, whereas M7 is about half that far, at 800 light-years.
Each cluster of stars was born from the same 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.