M7 in Scorpius is one of the brightest, most beautiful open clusters in the sky. It’s also “home” to a half-dozen other delectable deep-sky sights.
I live at latitude 47° north and really have no business talking about M 7 in the tail of Scorpius. With a declination of –34.8° it stands just 8.2° above the southern horizon at culmination. Despite this fact, the star cluster is routinely visible with the naked eye as a round powder puff of light with a brighter center. My view may be compromised, but I'm not complaining.
In 10×50 binoculars M7 is an amazing sight, with a packed, X-shaped core surrounded by bright outliers that bring the total diameter to 75′. The cluster's true extent is a little less than 25 light-years, and it's located 990 light-years away in the Orion Arm of the Milky Way. If an orb spider spun stars instead of silk this is what its web would look like.
Ptolemy was here
M7 is also known as Ptolemy's Cluster after the Greco-Roman astronomer Claudius Ptolemaeus who first recorded it as "a nebula following the sting of Scorpius" in AD 130. The cluster is a relatively youthful addition to the galaxy with an estimated age of around 200 million years.
Because of its low altitude at my location, M7 is greatly affected by atmospheric seeing, which bloats and smears the stars to varying degrees. I stick to low magnification (64×) with the 15-inch reflector on most nights. The blazing core suns pop like flashbulbs at a press conference, but my favorite feature here is a 9 arc-minute-long chain of somewhat fainter stars — magnitudes from 7.5 to 10.5 — that snakes east of the center. Stellar pairs and triads also dot the field.
As eye-catching as M7 is, it also serves as a jumping-off point for other journeys. Within and along its borders are a half-dozen additional deep-sky sights which happen to lie along the same line of sight as the cluster. Let's explore them.
Peeling back the layers
- Globular cluster NGC 6453 (magnitude 10.2, diameter 7.6′) — I see a little ball of haze with a moderately compressed core at 64× magnification along M7's western border. It isn't particularly bright but easy enough to find. Increasing the magnification first to 142× and then to 245× I was happily surprised to see numerous faint stars (~14th magnitude) resolved in the cluster's halo. The globular appears quite a bit smaller than its official diameter, closer to 2′ by my estimate. A brighter star, possibly a member, lies along the cluster's west-southwest edge.
- Open cluster NGC 6444 (magnitude ~10.0, diameter 12′) — What a curious shape. Fifty suns of 11th magnitude and fainter form a clumpy bar 12′ long extended east to west that look as if someone spilled them from a carried sack. With moderate magnification (142×) the view is rich with minute diamonds. Very nice!
- Open cluster Trumpler 30 (magnitude 8.8, diameter 20′) — Although only 20 stars are listed as members, this attractive, compact gathering seems to possess twice that number. The field here is so rich it's sometimes difficult to visually determine what belongs to a cluster and what doesn't. Tr 30 stands out well, and its quiet sparkle makes a wonderful contrast to M7's bright splash.
Annie Jump Cannon was here, too
- Planetary nebula Cannon 2-1 (magnitude 12.2, diameter 2″) — Abbreviated Cn 2-1, it looks exactly like a dim star although about a magnitude fainter than the listed brightness, most likely due to atmospheric extinction. Located 20,125 light-years away, the tiny nebula really shines with an O III or UHC filter. If you interpose the filter between your eye and the eyepiece the planetary will appear to brighten by about two magnitudes. Moving the filter in and out of view will make Cn 2-1 flash and fade like a strobe light. It's also a useful way to quickly identify it in a busy field.
This nebula was first studied by American astronomer and Harvard Computer Annie Jump Cannon, who's best known for devising the OBAFGKM spectral classification still in use today. She discovered several planetary nebulae while manually classifying about 350,000 stars.
- Dark nebula Barnard 287 (25′ x 15′) — Chocolaty clouds of interstellar dust abound in M7's vicinity. In deep photos with south up, B 286's shape resembles a pterodactyl in flight, but the best I could do at 64× was a vague, dark presence bounded by an isosceles triangle of 7th- and 8th-magnitude stars. Jiggling the telescope to increase its visibility, I could see that the nebula's western end was darker, wider, and more obvious that the rest. You'll find this shadowy reptile 25′south-southeast of the cluster's center.
- Mira variable SY Scorpii (magnitude range 8.5 to 14.5:) — Lucky us! This pulsating red giant with a period of 234.9 days is currently at maximum. It lies within the boundary of the cluster 25′ north of the center. Be aware that an unrelated 10th-magnitude star shines ~15″ immediately northeast of the variable, giving the two the appearance of a double star. I observed SY on July 9.2 UT at magnitude 8.9, when its orange-red color was very apparent. Click here for an AAVSO star chart to locate and follow the star's ups-and-downs. Cannon discovered SY Sco, too! It's one of about 300 variable stars she unearthed.
I can only imagine how much better the view of M7 must be from the Caribbean . . . not to mention the added benefit of being able to stand up to observe it instead of crouching on one's knees!