Nightfall

1998 June

Newsletter of the Deepsky Observing Section of the Astronomical Society of Southern Africa

Reprinted from the Mon.Notes.astron.Soc.South.Afr., vol 57, June 1998

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1998 April

RAINY CAPE WINTER NIGHTS remind me of Scorpio. When these conditions finally let up, the night sky seems particularly black, the stars blazing and the air crisp. Perhaps its just my senses that are made keener by the anticipation of a clear night.

From my home town of Stellenbosch, the Scorpion’s curved body can be seen rising over a mountain - Stellenbosch Mountain - in the southeast. From the pincers (beta, delta, pi ) to Antares (alpha Sco) sweeping upwards through epsilon, mu and zeta, curving down to eta, these stars fit the mountains silhouette almost exactly. Perhaps there is some Great Cosmic Significance in the fact that these stars form a celestial crown around the mountain. Maybe this Alignment indicates that some ancient civilization built Stellenbosch Mountain as a monument to future generations. Or possibly its Stellenbosch itself - after all, its over 300 years old - that is significant, and needs to be landmarked. [Is it mere coincidence that, when seen on a map, popular pubs in the town are arranged in a figure resembling the constellation Ara, which rises with the Scorpion? Perhaps authors Hancock and Bouval should be alerted?]

While you ponder on these and other mysteries, take time out to examine some of Scorpio’s gems.
Three open clusters noted by Abbe Lacaille lie near the Scorpion’s tail. Near the western extreme of the tail, where it joins the body, lies two very different clusters, NGC 6124 and NGC 6231.

Catalogued by Lacaille in 1755, he saw NGC 6124 as a “fairly big tailless comet” which is a pretty good description considering the optics he used. In modern binoculars, this cluster is a superb half-degree moderately rich swarm of faint and very faint stars. It appears vague and nebulous while scanning the area, but is clearly resolved into tiny stars when closer attention is paid. The stars appear to group together somewhat towards the northwest of the cluster.
Its worth a look in larger instruments, too. John Herschel called it “loosely scattered, not much compressed in the middle, fills nearly a field, consists of about 50 or 60 stars.” I used a 15.5-inch reflector at 220x to see it as a large, scattered cluster having about a dozen bright members and many smaller stars. It fits snugly into a 23' field of view, and invites the eye to “connect-the-dots” and trace out asterisms, of which there are many. For example, you can find a tiny Southern Cross, a Triangulum and a Musca. Hartung commented that “it contains several orange stars as well as numerous pairs, triplets and small groups.”

The much different NGC 6231 lies to the east. Halley observed it from his island observatory on St Helena, and Lacaille, too, recorded it; he called it a “close group of seven or eight close faint stars.”
Its an easy naked eye object, seen as a fuzzy spot where the curve in the Scorpions’ tail starts. Binoculars show a stunning, brilliant cluster of bright, white stars, about 9’ across. Burnham fittingly described it as “a handful of glittering diamonds displayed on black velvet.” Phil Harrington writes that “one glance will immediately tell you that this object is something special ... when viewed with an 8-inch telescope at low power, about a quarter of its stars shine between magnitudes 5 and 13. The remaining suns, all fainter and unresolved, surround these in a wedge of celestial mist. Larger apertures and higher magnifications resolves some of the cloudiness into even more stars.”
Take time out to scan the immediate surroundings in binoculars; veteran observer Tom Lorenzin writes: “add the Zeta Sco crowd and this area makes a terrific binocular field; see the field as the “Baby Scorpion” which clings to the back of its mother’s tail.”

The other end of the tail marks the position of NGC 6475, better-known as Messier 7. This huge stellar gathering is easily visible with the naked eye, even on poor nights, or with the interfering Moon, and was known to the ancients as “The Cloudy Ones which Follow the Sting”
Lacaille noted it as a “group of 15 to 20 stars in a square” and Hartung too picked up on the linearity evident in this cluster: ““a remarkable sight in a large field with its structure of quadrant and straight lines. With outliers it is more than 40' wide, and very effective for small telescopes.”

Just south of the Scorpion lies Ara the Altar, sometimes identified as the Altar of Noah, an interpretation that is all too fitting with the wet Cape winters. Although smaller than Scorpio, this grouping also has a fine collection of deepsky objects. I’ll discuss three of the most splendid, starting with the two nearest Scorpio. NGC 6193 is probably the most prominent open cluster in Ara, and was first noted by James Dunlop.
The cluster is well spread out, and in modest-sized telescopes will reward the observer with remarkable star chains and stellar knots. Interestingly, the brightest star in the cluster is a close double star — see if you can split it.
Far more elusive is the nebulosity (NGC 6188) associated with the cluster; John Herschel wrote: “consists of about a dozen stars 10..11 magnitude, and perhaps as many less, with stragglers, which fill field. In its preceding part is a fine double star ... and yet more preceding is a very large, faint nebula, in which the preceding part of the cluster is involved.” Although spectacular on long-exposure photos, I’ve never seen the nebulosity through a telescope. If you try to spot it, let me know how you fare.

An altogether easier target lies to the east and a degree further south. This open cluster, IC 4651, was oddly enough missed by John Herschel but is readily visible in binoculars. It was catalogued by Solon Bailey of Harvard College Observatory, who noted it on a photograph of the southern Milky Way.
The cluster is readily found one degree west of Alpha Arae. In binoculars, it shares the field with a number of brightish paired stars, and appears as a large (16’), faint, irregularly-round patch, mottled, with three small stars made out. It is a surprisingly large but delicate object, and quite beautiful. Funnily enough, it reminds me of the Helix nebula!
Hartung calls it “an irregular gathering of numerous stars fairly uniform in brightness, about 15' across with stragglers to 20'. The stars are in lines, curves and chains enclosing dark spaces so that the pattern is open without much condensation.” He recommends at least a 6-inch telescope to view the cluster well.
I noticed Hartung’s “dark spaces” in the 15.5-inch; at 220x there appears to be a roughly triangular nucleus made up of about 15 stars; surrounding it are dark star-less patches (one west, one northeast) which almost isolate the nucleus from the rest of the cluster. These two voids look somewhat like a pair of lungs!

Ara is also home to NGC 6397, about 4° further south. This globular cluster forms a 90° triangle with two bright stars (one of which is pi Arae) as seen in binoculars, which show it as a pretty bright, large, round stellar gathering, with three small stars seen in the outer reaches. It grows gradually brighter to the middle, to a broad dense bright disc, 6’ across. With averted vision, the cluster expands to delicately cover a 14’ circle of sky.
With a telescope, however, the odd nature of the clusters nuclear region is apparent. Dunlop recorded an interesting description: “a pretty large nebula, extended nearly in the parallel of the equator, brightest and broadest in the middle; a group of very small stars in the middle give it the appearance of a nucleus, but they are not connected with the nebula, but are similar to other small stars in this place which are arranged in groups.”
John Herschel noted that “in the middle is a more compact group of much smaller stars” and on another occasion wrote: “very much compressed in the middle where, however, the stars are very small, while every where else they are 13th magnitude.”
In the Union Observatory Circulars (Nos. 45-76, p 50) it is called “a fine cluster, 15’ in diameter, the very centre is confused and looks nebulous, there are about 150 stars in all.”
Prolific observer Magda Streicher, of Pietersburg, viewed the cluster with a Meade 8-inch from the Kruger National Park and called it “very large (about 19 arcminutes), bright, round, with a hint of a core. Well-resolved close stars form arms and trails randomly, with dark patches in-between.” See if you can notice the dark patches she mentions.

On the same Uranometria chart (U 434), but across the border into Telescopium, lies NGC 6584. This cluster was missed by Dunlop, but picked up by John Herschel. He noted it as “bright, round, gradually brighter towards the middle, entirely resolved into stars; easily seen . . . easily resolved with left eye into stars 15th magnitude.” [‘An interesting personal characteristic appears in Herschel’s notes. He frequently referred to observations made with his left eye In his Cape Observations of Nebulae and Clusters we have for example: 2405, a difficult object but certain after long attention with the left eye; 3152 all finely resolved into perfectly equal stars like the finest dust, which are seen with the left eye without effort, but the right requires to be somewhat strained to discern them; 3690 the right eye does not resolve or barely makes it resolvable, the left eye resolves it completely into stars 17...20m. Whether or not he customarily observed with the left eye, it was certainly superior to the right in resolution and sensitivity.’ — Lindsay, 1964, IAJ, 6, 286-289]
NGC 6584 is reasonably easy in 11x80s, where it is easily pinpointed as one corner of a triangle with two 8th mag stars north. Binoculars show it as a delicate round glow, confirmed with averted vision as a glow up to 4’ across. The cluster does not appear to grow brighter to the middle.

This quick trip through some of winter’s highlights must necessarily come to an end. However, I would like to discuss a group of objects brought to my attention recently by Gabriel Giust of Buenos Aires, Argentina. Gabriel is busy working towards his Bennett Certificate, and had been observing the Bennett objects in the congested region of the Large Magellanic Cloud. The key to successful observing in the Clouds is to use a good map. Oftentimes one reads about the complexity of the Virgo galaxy cluster, and how difficult it is to not get lost. Well, the Virgo cluster should be regarded as a practice run for the LMC. In the 1920s, R T A Innes wrote that, within the boundaries of the LMC, are contained, “with the utmost profusion, irregular patches of nebulae, spiral nebulae, nebulous stars, nebulous clusters, loose clusters of stars, and fine globular clusters.” He was planning to resurvey the area with the 26.5-inch refractor; “a start was made in 1926, but a few nights’ work indicated that a survey such as contemplated would take several years and would seriously interfere with the double-star program to which this observatory is committed.”
The brightest object within the LMC is the well-known Tarantula Nebula, NGC 2070, one of the southern skies showpieces. NGC 1763 - Bennett 27 - is part of a busy field of clusters and nebulosity, and was recorded by Herschel on a number of occasions: “very bright, very large, irregularly extended, irresolvable but thickly dotted with many distinct stars.”; “a very bright, very large nebula with stars (the chief of which in the anterior part of the nebula taken) of a crooked rounded oblong shape. A fine object.”; “very bright, very large, irregular oval figure with stars in it.”
Gabriel observed it with a 13.1-inch f/4.4 Newtonian: “With 37x, I can see a group of three oval nebulae in the field, north-east of a 6th magnitude star. With 98x - a very beautiful view! The field is very rich in faint stars between many nebulae. The brightest of these is extended NE-SW, and has a straight line in its interior. Examining the Uranometria chart, I conclude this one is NGC 1763.”
The chart below, taken from “LMC Selected Areas” shows this region and identifies a number of NGC objects. I recommend this set of charts to any observer who wishes to delve into the treasures of the LMC; write to M. Morel, 18 Elizabeth Cook Drive, Rankin Park, NSW, 2287, Australia, for details.

As always, correspondence is welcomed at PO Box 12838, Stellenbosch, 7613, or via email to auke@global.co.za

Do you have any comments on the objects discussed here? If so, please submit them!

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