NGC 6934 and NGC 6905 in Delphinus
August 2020 - Nebulae or Cluster of the Month
August brings with it the return of astronomical darkness and the best of the Milky Way. This month I have two great objects for you. Both are in the little constellation Delphinus, in the northern sky, but low enough to be relatively easy targets for southern hemisphere observers.
This month’s featured cluster is a globular, NGC 6934. It was discovered by William Herschel on 10th September 1785. He described it as
Very bright, large, gradually much brighter towards the middle. It is small but bright at magnitude 8.7. It has a diameter of around 6’, and its concentration class is VIII, on the Shapley-Sawyer scale that runs from I for the most concentrated clusters to XII for the loosest. Class VIII is defined as ‘Rather loosely concentrated towards the centre’. Note that in the first edition of the Deep-Sky Field Guide to Uranometria, where they use Arabic numerals as opposed to the more traditional Roman numerals, the listings for globular clusters all give concentration as ‘1=lowest, 12=highest.’ This is the wrong way round.
NGC 6934 is an easily-seen globular, though resolving it is a more difficult proposition. The brightest individual star is magnitude 13.8, with several more around 14. Given a good night, it should be speckly in a 12” (305mm) telescope. Superimposed on the western edge of the cluster is a 9.1 magnitude K-type star. The field has several 12th–13th magnitude stars.
I first observed this cluster with a 4½" (114mm) reflector. I noted at the time that it was bright and easy to see, appeared perfectly round and was unresolvable at any power available to me.
In a later observation, made with a 12” reflector, I noted that at x81, the cluster appeared speckled, with a slightly ragged edge and when magnification was increased to x244, individual stars could be glimpsed with averted vision.
This month’s second object is a planetary nebula, NGC 6905. If you haven’t seen this object before, you’re in for a treat. Discovered by William Herschel on 16th September 1784, when he described it as
Pretty bright. Perfectly round, pretty well defined. ¾' diameter. Resolvable. At this time, Herschel believed that all 'nebulae' would resolve into stars under sufficient magnification, an opinion he later refuted. His use of a single 'r' (for resolvable) indicates that he thought the object mottled, rather than actually resolved.
This planetary nebula shines at magnitude 11 and has a diameter of close to 40”. It is very bright and I found it immediately visible (without any filter) at x83. It stands high power very well. It’s basically round, but brighter and fainter regions within the disc give it a waisted appearance, which I found surprisingly reminiscent of M27 in Vulpecula.
The central star is usually listed as being magnitude 15, and is a variable star bearing the designation NT Delphini. It’s a ZZ Ceti class star, which are DA-type white dwarfs showing variations from a few thousandths to about a fifth of a magnitude, typically with periods from just a few seconds up to about 20 minutes. Some observers have reported seeing the central star with apertures as small as 8” (200mm), despite its low reported magnitude. Although faint, I have caught the central star with a 12” telescope at very high magnification (x450 in this case). Reports of the visibility or otherwise of this star through various apertures would be happily received.
|NGC 6934||20h 34m 12s||+07° 24’ 12”||Globular cluster||8.7|
|NGC 6905||20h 22m 22s||+20° 06’ 16”||Planetary nebula||11.1|