BNGC 11. Towards Trantor

By Steve Mencinsky, 1993

With the centre of the galaxy poised overhead and Herschel's "cloven grindstone" easily visible to anyone who cares to look up, the attention and telescope turn to matters galactic. The galactic centre provides enormous viewing opportunities for a variety of BNGC objects. This month concentrates on the Barnard dark nebulae, planetary nebulae, and a couple of faint globular clusters.

The most spectacular of the Barnard dark nebulae for visual observers is without doubt B86. This is a tiny, utterly black "comma" shape, set between a brilliant orange star and the open cluster NGC 6520. The cluster appears most unusual - it consists of a cloud of fainter stars, surrounded by an almost perfect circle (circlet?) of brighter ones, an even brighter one at the centre, all on a (if you'll excuse the pun) "milky" background. This aspect is not well shown on photos (Maybe Richard Berry's CCD can do better???). As mentioned in previous BNGC's, the blackness of the dark nebulae is much more obvious if the eyepiece is slightly ("gently does it", as Max Gardner has said, in a different context) defocussed, apparently due to the de-emphasising of the individual bright stars.

The next dark nebula worthy of mention is B72, the famous S-shaped "Snake" at the northern end of the Pipe nebula. Your author was unable to convince himself of the visibility of this nebula with even the 18" telescope, until a serendipitous accident at Mt. White involving a Nagler eyepiece and a melting bar of chocolate required the immediate washing of said eyepiece with soap and water. The now clean eyepiece was pressed into service and the nebula was immediately seen! The difficulty here was that visually, B72 is an extremely low-contrast object.

Near B72 are three small dark nebulae - B68, B69, and B70. Again, these are low-contrast nebulae, visible only after the washing of the eyepiece episode. Even then, much experimentation with defocusing was required before the nebulae could be held with any certainty.

Plate 157 on the previously-mentioned ESO book shows all of the above-described "B" nebulae. The low contrast is very well shown in this particular plate: the photo appears quite dense, the contrast appears excellent, yet the dark nebulae are barely visible! Plate 161 shows but does not identify B68,69 and 70 on a much larger scale and a much deeper plate, upon which they are indeed (deep photographically) very dense and very black. Moving along, another noteworthy large "B" nebulae is B75, a bifurcated "tail" projecting from the northern portion of B262. Even from the good-but-not-great skies of Mt White, the double tail is quite well seen.

Readers who care to apply a little logic may well see that on any given map, there will be some size limit to features, below which it becomes impractical to draw said features to correct scale. For Uranometria II, this limit is 30" for planetary nebulae and 5' for all others. Despite their small size, there are several dark nebulae below this limit which are well worth tracking down. B68, B69 and B70 have already been mentioned. Another is B83a, quite close to a whole bunch of objects. It boasts as neighbours the globular cluster NGC 6440, the bright, large, complex planetary nebula NGC 6455, and the dark nebula B84. B83a itself is a very pleasing item, shaped like a "bent almond" or perhaps a "grevillea seed", nicely outlined by a dozen or so stars.

Moving away from the dark nebulae, a class of objects popular near the galactic centre that your author has only recently explored in any detail is the huge number of non NGC/IC planetary nebulae, which Uranometria II usually numbers by their PK number. A particularly striking one is PK 6-2.1 (at the southern end of NGC 6559, an emission nebula near the Lagoon), whose sighting was totally unexpected. While observing from Mt White, your author noticed an unexpected bright spot that responded well to the OIII filter. He assumed that this was IC 4684, a small emission nebula near the north end of 6559. This really upset the apple-cart, for he immediately thought

that one or more of the chart, the field of view, or his perception of reality had been turned upside down! Fortunately, coolness of head eventually prevailed and the real identities sorted out.

The final class of objects in this vicinity and of interest to BNGC is the sprinkling of non-NGC globulars. These are (like comets) named after their discoverers: UKS 1751-24.1, Terzan 9, 10, 11, Palomar 6. Only two of these (in this area of sky) are "easily" visible in the 18" reflector. The one named HP1 was first seen (quite easily!) in the late 12" reflector from Ilford. The other is Ton 2, close to another globular, NGC 6380. Both of these are very faint, but what makes them difficult to find is that they are both incorrectly charted. 6380 is, as Steve Quirk has noted, incorrectly placed in the RNGC and therefore in several atlases, including Uranometria II (Editor's note - see my article, "In Search Of The Elusive Globular Cluster - Part 1" Universe August/September 1991, for an independent discovery of this error by myself, Andrew Murrell, and Steve Mencinsky).

Once your author had procured himself a copy of Steve's photo, both globulars fell in very short order to the combination of the Ilford dark sky, the 18" reflector, and the (clean) 9mm Nagler. It should be noted that without a doubt, 6380 is the most difficult of all NGC globulars observed by your author thus far.

For those who have not been able to source the correct positions for themselves, this month's mandatory chart shows 6380 and Ton 2 in their correct positions relative to the appropriate segments of Uranometria II.

Finally, in defence of Uranometria: for all the scathing remarks levelled at it by some of our august deep-sky observers, your author has always believed it to be an extremely accurate and informative observing guide, with the odd emission or misplaced object not detracting from it at all. Indeed, a trip to Ilford without Uranometria is a trip wasted: one may as well have stayed at home and looked at planets, an activity which doesn't require an all-hemisphere chart!