In Heaven with M7, a Portal Into the Deep

M7 deep unlabeled
M7 in Scorpius is one among the brightest star clusters in the sky. Outlying stars body its jewel-like core, whereas darkish nebulae riddle the complete area. North is up.
Thomas Jäger
/ CC BY-SA 4.0

I stay at latitude 47° north and actually don’t have any enterprise speaking about M 7 in the tail of Scorpius. With a declination of –34.8° it stands simply 8.2° above the southern horizon at fruits. Despite this truth, the star cluster is routinely seen with the bare eye as a spherical powder puff of sunshine with a brighter heart. My view could also be compromised, however I’m not complaining.

In 10×50 binoculars M7 is a tremendous sight, with a packed, X-shaped core surrounded by vibrant outliers that carry the complete diameter to 75′. The cluster’s true extent is a little lower than 25 light-years, and it is situated 990 light-years away in the Orion Arm of the Milky Way. If an orb spider spun stars as an alternative of silk that is what its internet would appear to be.

Ptolemy was right here

M7 is also called 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 comparatively youthful addition to the galaxy with an estimated age of round 200 million years.

M7 Ptolemy's Cluster telephoto view
From my observing website in northern Minnesota M7 by no means will get far above the treetops, but it is readily seen with the bare eye at magnitude 3.3. Eighty stars reside there.
Bob King

Because of its low altitude at my location, M7 is drastically affected by atmospheric seeing, which bloats and smears the stars to various levels. I persist with low magnification (64×) with the 15-inch reflector on most nights. The blazing core suns pop like flashbulbs at a press convention, however my favourite characteristic right here is a 9 arc-minute-long chain of considerably fainter stars — magnitudes from 7.5 to 10.5 — that snakes east of the heart. Stellar pairs and triads additionally dot the subject.

M7 locator map
M7 is simple to seek out 4.9° east-northeast of Shaula and Lesath, the “Stinger Stars” in the tail of Scorpius. M7 culminates round 11:30 p.m. native daylight time in mid-July.

As eye-catching as M7 is, it additionally serves as a jumping-off level for different journeys. Within and alongside its borders are a half-dozen further deep-sky sights which occur to lie alongside the identical line of sight as the cluster. Let’s discover them.

Peeling again the layers

  • Globular cluster NGC 6453 (magnitude 10.2, diameter 7.6′) — I see a little ball of haze with a reasonably compressed core at 64× magnification alongside M7’s western border. It is not significantly vibrant however straightforward sufficient to seek out. Increasing the magnification first to 142× after which to 245× I used to be fortunately stunned to see quite a few faint stars (~14th magnitude) resolved in the cluster’s halo. The globular seems fairly a bit smaller than its official diameter, nearer to 2′ by my estimate. A brighter star, presumably a member, lies alongside the cluster’s west-southwest edge.
NGC 5453 globular cluster
Although in the identical line of sight as M7, NGC 6453 is rather more distant — about 37,000 light-years away however simply 3,260 light-years from the galactic heart. Intervening cosmic mud dims its gentle.
  • Open cluster NGC 6444 (magnitude ~10.0, diameter 12′) — What a curious form. Fifty suns of eleventh magnitude and fainter type a clumpy bar 12′ lengthy prolonged east to west that look as if somebody spilled them from a carried sack. With average magnification (142×) the view is wealthy with minute diamonds. Very good!
  • Open cluster Trumpler 30 (magnitude 8.8, diameter 20′) — Although solely 20 stars are listed as members, this enticing, compact gathering appears to own twice that quantity. The subject right here is so wealthy it is typically troublesome to visually decide what belongs to a cluster and what would not. Tr 30 stands out properly, and its quiet sparkle makes a fantastic distinction to M7’s vibrant splash.

Annie Jump Cannon was right here, too

M7 and environs
This map plots the featured objects on our deep dive into M7. North is up.
Stellarium with additions by the creator
  • Planetary nebula Cannon 2-1 (magnitude 12.2, diameter 2″) — Abbreviated Cn 2-1, it seems precisely like a dim star though about a magnitude fainter than the listed brightness, almost certainly attributable to atmospheric extinction. Located 20,125 light-years away, the tiny nebula actually shines with an O III or UHC filter. If you interpose the filter between your eye and the eyepiece the planetary will seem to brighten by about two magnitudes. Moving the filter out and in of view will make Cn 2-1 flash and fade like a strobe gentle. It’s additionally a helpful way to rapidly determine it in a busy subject.

    This nebula was first studied by American astronomer and Harvard Computer Annie Jump Cannon, who’s greatest identified for devising the OBAFGKM spectral classification nonetheless in use right now. She found a number of planetary nebulae whereas manually classifying about 350,000 stars.

B286 flying reptile
In this south-up view, B287 seems prehistoric. The Interstellarum atlas mislabels the nebula as B286.
Thomas Jäger
  • Dark nebula Barnard 287 (25′ x 15′) — Chocolaty clouds of interstellar mud abound in M7’s neighborhood. In deep images with south up, B 286’s form resembles a pterodactyl in flight, however the greatest I might do at 64× was a obscure, darkish presence bounded by an isosceles triangle of Seventh- and Eighth-magnitude stars. Jiggling the telescope to extend its visibility, I might see that the nebula’s western finish was darker, wider, and extra apparent that the relaxation. You’ll discover this shadowy reptile 25′south-southeast of the cluster’s heart.
M7 deep image
A deep picture of M7 exhibits all of the featured deep-sky targets.
Thomas Jäger
  • Mira variable SY Scorpii (magnitude vary 8.5 to 14.5:) — Lucky us! This pulsating red big with a interval of 234.9 days is at present at most. It lies inside the boundary of the cluster 25′ north of the heart. Be conscious that an unrelated Tenth-magnitude star shines ~15″ instantly northeast of the variable, giving the two the look of a double star. I noticed SY on July 9.2 UT at magnitude 8.9, when its orange-red colour was very obvious. Click here for an AAVSO star chart to find and observe the star’s ups-and-downs. Cannon found SY Sco, too! It’s one among about 300 variable stars she unearthed.

I can solely think about how a lot better the view of M7 should be from the Caribbean . . . to not point out the added good thing about having the ability to stand as much as observe it as an alternative of crouching on one’s knees!


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