BNGC 10. A Beautiful, Large Planetary Nebula
By Steve Mencinsky, 1993
As has been pointed out by many observers, hunting down planetary nebulae is a particularly satisfying form of deep-sky observing for city dwellers. (And in contradiction to our worthy Secretary's outrageous statement that the only form of observing possible from the city is that of variable stars!) This is so because the planetary nebulae's very strong emission lines mean that they can be easily located and successfully observed through narrow-band interference filters such as the UHC or OIII, from even heavily light-polluted skies.
A quick scan through the table of contents for the last few issues of "Universe" suggests that with the possible exception of our worthy secretary, your correspondent would appear to be the regular contributor most affected by this particularly noisome urban blight. (Is this really true? Or is this an example of the proverb that the sky is always darker on the other side of the fence?) Through the late 12 inch telescope, your correspondent has successfully observed almost all of the NGC/IC planetary nebulae listed in Uranometria Volume II, in most cases from one or both of his Cremorne/Newtown backyards; and has now moved on to more challenging targets.
Most of the planetary nebulae thus observed are reasonably bright although quite small - in fact, even under the best seeing many of them are totally stellar. Presumably, the only way that the Herschel's could have distinguished them from stars with any certainty was to exploit the line emission spectrum: planetary nebulae would have shown single images through an eyepiece-prism, whereas the stars would have shown streaks. A useful side effect of this is that since the stars' light would have been thus smeared out over a larger area, but the planetary nebula's would not, the apparent relative brightness of the planetary nebula would have increased: exactly as in the modern (but much more, convenient?) technique of "filter-blinking". With the OIII filter, identifying planetary nebulae by filter-blinking is almost trivially simple: Tony Buckley's favourite planetary nebula, NGC 3918 in Centaurus, has such a high surface brightness that it could be easily filter-blinked through the 7 X 60 finder from Camperdown! Indeed, your correspondent was able to sustain a rate of about 6 NEW planetary nebulae per hour during the winter (and not from Ilford - from his Cremorne backyard!), when Ophiuchus, Scorpius, Sagittarius and Serpens were overhead.
This month's feature object is, however, (to mix a metaphor) a ghost of a different colour. PK329+2.1, aka Shapley 1, is a large (1.1' diameter) planetary nebula, with a very low surface brightness. With the "old" 12" and against a Cremorne sky, therefore, it was utterly beyond the unfiltered eye, and barely visible with the OIII filter. The best (if one could call it that) view was obtained at 164X, while at 306X it was undetectable, which seemed somewhat odd - possibly due to the large size. As to the object's shape, this was quite difficult due to the low surface brightness, but the most certain impression (obtained, as usual, at a winter 3am culmination) was that of a wide 'C'-shaped lobe, partially encircling a faint, sharp-edged round glow. It was supremely gratifying to later note that the photo of Shapley 1 shows it as a classic Ring Nebula, with one bright section in the ring - presumably the aforementioned C-shaped lobe.
With the 18-inch telescope, naturally observations are much easier, the ring shape is quite clear - even from Cremorne. The 18-inch from Ilford clearly shows the thin inner ring, an outer, larger more "fuzzy" ring; and therefore a (proportionally) very large centre hollow. For a planetary nebula so apparently symmetric and round, one would expect "its" star to be located close to the apparent centre. Indeed, the 18-inch shows at least one star near the apparent centre; it is not known whether that is "the" star.
The location as specified in Uranometria is very accurate. The diagram shows the location of Shapley 1 with respect to a single segment of the Uranometria chart, to which have been added the four faint stars closest to the planetary nebula. These stars are those not marked with a cross. The planetary nebula itself (as observed) has been drawn approximately to scale.
The ESO book "Observing the Southern Sky" has an excellent photograph of Shapley 1 (plate 159). So does Burnham, Volume 2, page 1175; he labels it "Sp1". The designation PK329+2.1 means that the planetary nebula is contained within the Perek & Kohoutek catalogue [of planetary nebulae] and is positioned at 329 degrees galactic longitude, +2.1 degrees galactic latitude. Since all/most of the PK planetary nebulae are within our galaxy, they are generally found close to the galactic equator, so P & K obviously thought the galactic coordinate system the most natural to use in their catalogue.
Finally, speaking of Burnham; in its best tradition your author would now like to branch off at a tangent to the main line of discussion. For many years he has dabbled in the more theoretical aspects of astronomy, in particular the calculation . of orbital elements from observations and the subsequent prediction thereof. While doing this, something totally unexpected and surprising has emerged:
Duffet-Smith shows with great clarity Kepler's method of performing the above calculations, yet even with the help of the ubiquitous and almost omnipotent computer the work involved in performing them is extremely tedious. That Kepler performed this work manually is almost beyond belief! This is why Johannes Kepler is number one, without any competitors, in your author's personal list of "Great Astronomers". It was personally very gratifying when Scott Mellish in his series of articles devoted more space to Kepler than anyone else!. As a result, your author has christened his 18 inch telescope, probably the best and largest telescope he will ever own: Johannes.
Shapley 1 / PK 329 +2.l:
Position (J2000.0) RA: 15h 51.1' , Dec: -51° 30'