BNGC 18. A Crowded Tour Of Ara
By Steve Mencinsky, 1994
The constellation Ara the altar, lying as it does between the contrasting wonders of galactic Scorpius and extragalactic Centaurus, is indeed an appropriate place to behold the wonders of the early winter sky. Its chief deity would doubtless be the giant globular cluster NGC 6397, about twice the diameter of its poor cousin-near-the-meridian, M 13. However, as our esteemed Editor has pointed out, in this column it is inappropriate to comment at length on NGCs, so we shall move directly to the BNGC items of interest. Ara boasts more than its fair share of these; all three of this month's targets are in that constellation and all on page of Uranometria II (U In, map 433.
The first chart shows a small cluster of galaxies near Eta Arae. In the 18" of Johannes, NGCs 6215 and 6221 are immediately obvious, even at lowest power. NGC 6215A has not been seen despite the closest scrutiny by a number of prominent ASNSW co-observers, at both Ilford and Mt White. However, as if to demonstrate that serependity is where you find it, (as Yogi Berra may have said), the vain search for 6215A succeeded in finding two more uncharted items. The one labelled as "anon-1" is the more difficult of the two. The chief impediment lies in that it is entangled in a line of four stars very close together. Thus, high power is necessary to separate the stars sufficiently to allow the object to peep from behind the glare. Fortunately, it is fairly small and quite sharp-edged, so that once located it can be readily held by direct vision.
The other, "anon-2", was much more straightforward, significantly fainter than either of 6215-6221, but not extraordinarily so. Andrew Murrell charts three more galaxies only two degrees away to the southwest, where Ara dips into Triangulum Australe near NGC 6156; but even the most careful scrutiny here at high power revealed nothing.
Moving only a degree and a half to the north, the much-mentioned ESO book promises an edge-on irregular spiral galaxy ESO-179-IGl3, near "an 8th magnitude star" . Not only are the ESO book's positions given in 1950 coordinates (shame, shame!), they are also occasionally a little, well, vague. So, after applying the simplified precession formula given to your correspondent by Geoff Pearson, four possible candidates emerged. It was the work of only a few minutes to scan all four, to reveal a tiny thin faint streak of light as indicated on chart 2. The faint star to the north of the observed object is the only one on this chart not shown in U II. The relative positions, brightnesses and orientations of what was seen and drawn suggests a good match with the ESO galaxy.
Note that the galaxy has been drawn out of proportion, much larger than its observed size; were it drawn to correct scale its length would be less than the drawn diameter of the 5th magnitude star to the south of it. This is indeed a tiny galaxy.
Chart 3 takes us to an object type rarely seen in this column, the reflection nebula. As a class, these are the most challenging objects. By their very nature, most of their light is in the blue-violet in a more-or-less continuous spectrum and so would seem to be beyond the assistance of even in today's high-performance interference filters. [An intriguing suggestion as to filtering reflection nebula put forward by Chris Ross has, your correspondent believes, yet to be tested at Ilford]. Furthermore, again by their nature, reflection nebulae are always closely associated with one or more bright stars, the glare from which makes the sighting of the nebulae - which is almost always very faint compared to the star(s) - that much more difficult.
Nevertheless, the opportunities are there for those who are prepared to take them, and so this last chart shows us the very small cluster/nebula van den Bergh - Herlst (vdBH) 81. U II suggested only a double star with nebulosity, so your correspondent was delighted to see so much more. The first impression was that of a stick-figure rocket on take-off. The two "rocket engines", marked with crosses, are U II's double star; while the thin swirls of nebulosity are the "rocket exhaust". What also impressed - and hopefully this is all brought out on the chart - is the astonishing degree of symmetry, both in position of the stars within the cluster and in their respective brightnesses. The scale of the chart is an approximation, but even with that clearly this is indeed a very very small cluster.
To end on a speculation: is vdBH-81 similar to a Pleiades (with the Merope nebula) or even an M 16? True, there were no traces of fainter cluster members, although a moment's thought will reveal that the Pleiades, too, is to most people only a half-dozen or so stars.