Nebulae or Cluster of the Month Archive 2020
In this series of articles we draw your attention to Nebulae, Clusters and other Galactic objects that are particularly worthly of an observer's time.
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.
Object RA Dec Type Magnitude NGC 6934 20h 34m 12s +07° 24’ 12” Globular cluster 8.7 NGC 6905 20h 22m 22s +20° 06’ 16” Planetary nebula 11.1
Stephenson 1 and NGC 6765 in Lyra
July 2020 - Nebulae or Cluster of the Month
In this new feature, I will be introducing an open cluster and/or a nebula (reflection, emission or planetary) each month.
In July, the bright little constellation of Lyra lies high in the sky and provides both our objects for this month.
Firstly, the cluster. This month I’m going to introduce an often-overlooked, though bright, open cluster. It was missed (or ignored) by the Herschels, and never found its way into the NGC. It’s known as Stephenson 1, or more prosaically as C 1851+368. It lies around the 4.3 magnitude star δ2 Lyrae, the brighter of the double δ Lyrae.
Stephenson 1 has had a chequered career, initially being thought a cluster, then later an asterism and now again is believed by most authorities to be a true open cluster.
Archinal & Hynes’ ‘Star Clusters’ (A&H) states that it contains 77 stars and has a diameter of 40’, though the Deep-Sky Field Guide to Uranometria grants it only 15 stars within a 20’ diameter. The visual appearance is somewhere between these.
The Trumpler classification of this cluster is IV3p, which translates as ‘Not well detached’ (IV), ‘wide magnitude range, a mix of bright and faint stars’ (3), ‘poor – fewer than 50 stars’ (p). If A&H is correct, this should be amended to ‘m’ for ‘medium, 50 – 100 stars’. Visually, it looks better than this.
My observation, made with a 12” Newtonian reflector and a field size of 36’ (x81) is as follows:
Clustered around δ1 and δ2 Lyrae, this is a very bright but poor cluster. Several bright stars are spread across the 36' field with fainter ones scattered amongst them. Although very loose, it stands out well from the Milky Way background on low power, so pretty well detached. About 70 stars were counted in the 36' field.
Although not a spectacular cluster, this is a very pretty object, the lovely colour contrast between the M-type δ2 and the B-type δ1 being a highlight. It’s worth a moment of your time.
Now to the nebula. This month we’re going to look at a planetary nebula in Lyra. No, not that one. There are several planetary nebulae in Lyra, but the second-brightest one is NGC 6765.
It lies in the south-east of the constellation, a little over a degree from the globular cluster M56.
NGC 6765 was discovered by Albert Marth (1828 – 1897) in 1864. Marth deserves to be better known. He discovered over 600 ‘nebulae’ during his career (584 of which are in the NGC) and was also the discoverer of asteroid 29 Amphitrite.
At magnitude 12.9, this object should be fairly easy to spot. I have found it difficult without a filter, though. A UHC filter will show it, but the best views I have had (with 12” and 16” reflectors) were with an OIII filter. Once found, it looks quite peculiar. Lower powers show a fairly large disc (about 40” diameter), but on higher power, it reveals itself to be very elongated. There are two tiny twinkles involved in it, more or less at each end. These may be stars or condensations. More careful examination reveals a dark area splitting the object along its longest axis, though the nebulosity on the ‘far’ side of this gap is very faint.
Object RA Dec Type Magnitude Stephenson 1 18h 54m 31s +36° 53’ 59” Open cluster 3.8 NGC 6765 19h 11m 07s +30° 32’ 45” Planetary nebula 12.9