Messier 6 and Messier 7
July 2009 :
The song “Love and Marriage,” contains a line that goes, “you can’t have one without the other.” The words aptly describe the open clusters M6 and M7 in Scorpius. This cosmic “horse and carriage” lies in the southern sky above the Scorpion’s stinger.
M7 is the brighter and larger of the two. With an overall magnitude of 3.3, it spans 80’ – over twice the moon’s apparent diameter. Readily seen with the unaided eye in the absence of bright moonlight or city lights, M7 was first reported by the Greek astronomer Claudius Ptolemy nearly two millennia ago. “Ptolemy’s Cluster” is a dazzling sight in binoculars and small rich-field scopes – a striking aggregation of some 80 stars between magnitudes 6 and 10, immersed in a sparkling background of Milky Way stars. Because of its large size, M7 appears rather sparse in large-aperture scopes. Current studies indicate that M7 is 800 light-years away and is approximately 200 million years old.
Just five degrees northwest of M7 is its partner M6. This cluster, which is a magnitude fainter than M7 and one-third as large, lies just outside the glow of the Milky Way. Like M7, M6 is visible to the unaided eye and was recorded by Ptolemy. Rather than be saddled with the nick-name “Ptolemy’s Cluster II,” M6 was dubbed the “Butterfly Cluster.” The outline formed by its brightest stars does indeed resemble the outstretched wings of this insect. M6, like M7, is at its visual best when viewed with binoculars or small RFTs. With the latter, you can see about 80 stars brighter than 11th magnitude. The most luminous of the cluster’s member stars is the reddish-orange semiregular variable BM Scorpii, whose magnitude fluctuates from 5.5 to 7.0 in a cycle of roughly two years. M6 is twice as remote as M7 and half its age.
Though the splashier M7 seems to get better reviews, I found M6 to be a more attractive sight when recently observed in the 1.5° field encompassed by my 4-inch f/4 Astroscan at 35X. M6 appeared as a tight little group, while M7 seemed sparse. Oddly enough, I was more impressed by M7 when I first viewed the two clusters with a 3-inch f/10 reflector at 30X back in the summer of 1977. Compare M6 and M7 and see what you think. Now if I can just get that “Love and Marriage” melody out of my head!
Your comments on this column are welcome. E-mail me at gchaple@hotmail.com.
