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 457 and Cz 3 in Cassiopeia and NGC 604 in Triangulum
October 2020 - Nebulae or Cluster of the Month
This month we’ll again be looking at three different objects. Each of these objects presents a different observing challenge.
We’ll start with this month’s first open cluster – NGC 457 in Cassiopeia. From the UK, this cluster is almost at the zenith at midnight in mid-October, so for those of you with Dobsonian-mounted telescopes, that in itself is something of a challenge! Good binoculars will show the cluster, and it’s easy to find, being attached to the 5th magnitude star φ (phi) Cas.
Phi Cas itself is a multiple star. It has five components, labelled A-E. Phi Cas (sometimes referred to as phi-1 Cas) is the brightest at magnitude 5.0, with component C (sometimes referred to as phi-2 Cas) the second brightest at magnitude 7. These two stars are well separated, being about 2.2' apart. They are the brightest stars in the field of NGC 457.
Is the phi Cas system actually part of NGC 457? This is a much-debated point. The two main stars far outshine the other members of the cluster, and phi-1 is an extremely luminous F5-type star, very close to being categorised as a yellow hypergiant. Its luminosity exceeds 100,000 suns. Phi-2 is a more regular, B5-type supergiant. The proper motion of the two supergiants is close to that of the other cluster members, and the parallax from Gaia data is broadly the same as that of cluster members. So phi is very close, spatially, to NGC 457, but the massive difference between the phi stars and the true members of the cluster usually tilt the balance of belief away from genuine membership.
NGC 457 itself was discovered by William Herschel on 18th October 1787. He described it as
A brilliant cluster of large and very small stars. Considerably rich. He placed it in his class VII – Pretty much compressed clusters of large or small stars – as no. 42.
NGC 457 lies around 2400pc (7800 light-years) away. It’s a fairly young cluster, its age being estimated at 21 million years. Its Trumpler classification is II3r (detached, slight concentration, bright and faint stars, rich). Archinal & Hynes give the membership as 204 stars, though the Deep-Sky Field Guide to Uranometria gives 80. The brightest star (ignoring the two phi stars) is V466 Cas, an M-type variable star of unknown type. It varies by about 0.08 of a magnitude and is around magnitude 8.6.
This is a lovely open cluster in almost any sized instrument. Its diameter is around 15 – 20’ and so requires only moderate magnification. My own observation of it, made with a 12” (305mm) Newtonian reads as follows: A visually striking, elongated cluster, dominated by 5th magnitude φ Cas and to a lesser extent by magnitude 7 SAO 22187 next to it. These two stars lie to the south-east of the cluster which stretches away north-west for about 20'. Two straggly arms of stars lead away from the body of the cluster south-west and north-east. Very well detached with a wide range of magnitudes, many bright, some very faint. About 80 stars were seen across about 20'. Nice.
Our second cluster could not present much more of a contrast. Czernik 3 lies 5° north-north-west of NGC 457, just the other side of Cassiopeia’s ‘wonky W’. This open cluster is very old and well into the process of disintegration. A recent study (Saurabh Sharma et al, 2020) has determined that the distance to the cluster is 3.5±0.9 kpc (8500–14300 light-years) and its age is around 0.9Gyr. The study found that the cluster was highly elongated (0.5x1.2pc) and 45 members were identified from Gaia data. It is believed that this is a rump cluster, with several, maybe even most, of its members already scattered.
Visually, it is not very prepossessing but presents an interesting challenge. Archinal & Hynes give a combined visual magnitude of 9.9, which I find to be hopelessly optimistic. In my 12” reflector, it is difficult to see. One or two very faint stars can be seen on the edge of a tiny patch of indistinct twinkles. The stars that were definitely seen were magnitudes 14.2 – 14.5. To the north-west of the cluster is an 11th magnitude star, which is clearly a foreground object.
The final offering for this month is an HII emission nebula, one of the largest known. It is NGC 604, and it lies within the galaxy M33 in Triangulum. This massive complex is over 1500 light-years in diameter (over 100 times the size of the visible region of the Orion Nebula, M42). It shines more than 6000 times brighter than the Orion Nebula, which is why, even at 2.7 million light-years distance, it is still readily visible in amateur telescopes.
NGC 604 was first noted as a discrete object by William Herschel, who discovered it on 11th September 1784, describing it as
Near V.18. Very faint. Small. Round. Brighter in the middle.
The reference to ‘V.18’ is to the 18th entry in his class of ‘large nebulae’. This is surely a mistake. V.18 is now referred to as NGC 205 or M110, the satellite galaxy of M31 in Andromeda. The entry should probably read
Near V.17…as V.17 is M33.
By the time the object was included in the NGC in 1888, the description had been updated to
Bright, very small, round, very, very little brighter towards the middle.
NGC 604 lies 11’ north-east of the nucleus of M33. For most of us living under urban or suburban skies, this places it outside the visible region of M33 itself. Under such conditions, it looks for all the world like a smaller, fainter field galaxy. Whilst it is very difficult to give meaningful magnitudes for emission nebulae as a whole, with NGC 604 being so compact and galaxy-looking, I would estimate its magnitude at about 11.5.
Under excellent conditions, I have seen it with a 4½” (113mm) reflector. When I made that particular observation (way back in 1979) I hadn’t recognised NGC 604 for what it was, but when I wrote up the observation and checked the stars I had plotted, I found one which did not coincide with a star, but it did coincide with NGC 604.
Below is a later observation, showing NGC 604 masquerading as a field galaxy.
Object RA Dec Type Magnitude NGC 457 01h 19m 33s +58° 17’ 00” Open cluster 6.4 Cz 3 01h 03m 09s +62° 47’ 03” Open cluster 9.9(?) NGC 604 01h 34m 33s +30° 47’ 00” Emission nebula about 11.5
IC 4996 and NGC 6888 in Cygnus, and IC 4997 in Sagitta
September 2020 - Nebulae or Cluster of the Month
This month we’ll be looking at three different objects. Two because they’re great objects, and the third as a bit of a challenge and a talking-point.
We’ll start with this month’s open cluster – IC 4996 in Cygnus. Although apparently observed by William Herschel in 1786, IC 4996 did not make its way into his catalogue, nor into the NGC. It had to wait until 1908 to be included in the Second Index Catalogue of Nebulae Found in the Years 1895 to 1907. The IC credits the discovery to Frank Bellamy (1863 – 1936). The description given is accurate, if brief: Cluster, stars magnitude 8–13.
IC 4996 is a young open cluster, about 8 to 10 million years old. Its distance is not known precisely, but lies somewhere around 1915 ± 110 pc, or between 5900 and 6600 light-years. It lies deep within the Milky Way, with extinction values from galactic dust ranging between 1.3 and 2.4 magnitudes across the cluster. This would be a spectacular object if it were located closer to us. It appears to be connected with the Cygnus-OB3 association, which, according to Gaia data, lies at the same distance.
Archinal & Hynes give a Trumpler classification of II3p n (II = detached, slight concentration, 3 = both bright and faint member stars, p = poor (fewer than 50 stars), n = involved in nebulosity) for the object and yet give the number of member stars as 56. If this is true, then the type should be II3m n, though it should be noted that the Deep-Sky Field Guide to Uranometria gives the number of member stars as 15!
Visually, despite its distance and extinction, IC 4996 still appears a bright little cluster. The centre is marked by a tight little grouping of seven stars, the brightest of which form a trapezium approximately 1’ across. Three of the four stars that make up the trapezium are the brightest stars in the cluster, with magnitudes of 8 or 9. The other stars in the cluster are fainter but there is a good mix of brightnesses. The visual impression is much closer to 50 stars than 15.
All in all, IC 4996 is a nice, bright, compact little open cluster. Definitely worth a look.
This month’s nebula is much more of a challenge; NGC 6888 in Cygnus. Something like 300,000 years ago, the massive star HD 192163 evolved into a red giant and ejected some of its mass in a slow-moving stellar wind. The red giant phase was brief, and since then the star has contracted and become a Wolf-Rayet star with a much more vigorous and fast-moving stellar wind. The more recently ejected material has now caught up with the slower-moving, previously ejected material. This has energized both winds and also caused a rebound of some of the faster-moving material, resulting in two shock waves, one expanding and one contracting. The result is this delicate-looking shell of nebulosity which nevertheless is energetic enough to emit X-Rays.
The object was discovered my William Herschel on 15 September 1792. He describes it as
A double star of 8th magnitude, with a faint south-preceding milky ray joining to it, 8’ long and 1½’ broad.He placed it in his fourth class, planetary nebulae, as 72 H.IV.
I have to confess that whilst writing this article, I read Herschel’s description for the first time, and was struck by how close to my own observation of the object it is. The double star Herschel mentions is ΟΣ 401 (HD 192182). The components are magnitudes 7.2 and 10.2 and are separated by about 13”. The position for NGC 6888 given below is for ΟΣ 401.
My observation (reproduced here) was made with a 12” (305mm) reflector under sub-optimal conditions, using a power of x83 (36’ field) and an OIII filter. This is an object that responds well to UHC or OIII filters, and except under very good skies would most likely be unseen without them. The notes that accompany this drawing read
Just a very small segment of this huge nebula was seen. It appeared faint, though use of the OIII filter helped. Wisps of nebulosity stretch away SW from a 7th mag star with small traces around another. A little difficult but quite certain.
The magnificent images of this object which can be seen in many books, magazines and online give no hint as to the actual visual appearance of this ephemeral object. It’s a challenge, but it’s not an impossible one. I would be very interested in observations (successful or otherwise) of this object under different conditions, apertures and filters.
The Wolf-Rayet star which produced this nebula is located 5.5’ south of ΟΣ 401 and is magnitude 7.5, making it the second-brightest star in the field, and the southernmost of an obvious ‘lozenge’ of 7th and 8th magnitude stars.
Finally, as if the last object wasn’t challenge enough, let’s try something very different. IC 4997 (only coincidentally the next entry in the IC after our cluster of the month) is a tiny, stellar planetary nebula in Sagitta. These stellar planetary nebulae are fascinating objects, though it could be argued that they are less than spectacular to look at. The same could be said for quasars, of course.
IC 4997 appears stellar at all powers. Its true nature is revealed by the use of an OIII filter (a UHC will serve). It is bright, its magnitude normally given as 10.5. 1’ to the south-west of the planetary is a 10th magnitude star. Without an OIII filter, the star is the brighter of the pair. With the filter in place, the planetary is brighter.
So far, so so-so. What I find interesting – and it was the subject of a discussion of the Webb Society Forum a while ago – is that as soon as some observers have the field of a stellar planetary nebula in sight, they can almost immediately spot which ‘star’ is the nebula. I am one of those observers, though I know I’m not alone. There is something about the object. Maybe it’s just not quite stellar or maybe there’s something about the quality of the light itself. Stars emit a continuum, planetary nebulae do not.
Here, then, is the challenge. Direct your telescope to the field of IC 4997 and see if you can spot which star is the nebula without use of a filter. If you can, try to determine what it is about the object that makes it different from the actual stars. I’d be fascinated to hear your thoughts (click here to email me).
Object RA Dec Type Magnitude IC 4996 20h 16m 31s +37° 38’ 44” Open cluster 7.3 NGC 6888 20h 12m 13s +38° 26’ 34” Emission nebula IC 4997 20h 20m 09s +16° 43’ 54” Planetary nebula 10.5
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