Nebula and 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 1893 in Auriga and Abell 12 in Orion

  December 2020 - Nebula and Cluster of the Month

  After raised eyebrows from James last time, I’m restricting myself to a single nebula and a single cluster this month!

  For our cluster, I’m turning to Auriga, a constellation richly endowed with open clusters, no fewer than three of which are in Messier’s catalogue. The triangle of stars delineated by the stars θ Aur, ι Aur and β Tau is the heart of this concentration of clusters. We’re going to look at a somewhat overlooked example, NGC 1893.

  

  NGC 1893 lies about 50’ east of the little asterism formed by the 5th and 6th magnitude stars 16, 17, 18 and 19 Aur. This makes it an easy target.

  It was discovered in 1827 by John Herschel, who gave it the number 351 in his catalogue of 1833. He describes it as a rich coarse cluster of scattered stars, 9 – 15 magnitude; more than fills field.

  The cluster is embedded in the extensive nebula IC410, a star-forming region in which NGC 1893 itself formed. The cluster is fairly young, and observations in X-rays from the Chandra satellite suggest that it contains between 4000 and 5000 stars, though most of these are not detectable visually, of course.

  The Deep Sky Field Guide to Uranometria gives the cluster an overall magnitude of 7.5 and credits it with 60 stars, the brightest of which is magnitude 9.3. It assigns it a diameter of 12’. Archinal & Hynes, on the other hand, state that there are 270 stars and that the diameter is 25’. They agree on the magnitude of the brightest star. My observations of this cluster suggest a diameter of around 20’.

  The Trumpler classification is II3r n, which indicates a detached cluster with little central condensation (II), a wide range of magnitudes, both bright and faint stars (3), that it’s rich (r, more than 100 stars). and that it’s involved in nebulosity (n). My own visual description reads A large, bright cluster. Over 60 stars counted in a diameter of about 20', many in a long north-south chain. Some concentration, moderate variation in the stars' magnitudes and pretty well detached.

  In a moderate telescope (say 12”/300mm), the cluster is seen to lie in a rich Milky Way field, with about 60 stars visible in a 30’ field. It’s unlikely that all of these are cluster members. The cluster itself takes on a ‘Y’ shape, with the downstroke heading almost due south, this being the most prominent feature of the cluster.

  The nebula, IC 410, in which the cluster lies, is difficult but can be viewed on a good night with 12” or more of aperture, especially if a UHC or OIII filter is used. It appears as a slight mist around the stars. The most easily visible portion of the nebula is to the north-west of the cluster.

  This month’s nebula presents a bit more of an observing challenge. The object in itself would be much easier to see if it were somewhere else. The planetary nebula PN G 198.6-06.3 (or Abell 12) lies a mere 50” from magnitude 4.1 μ Orionis. μ lies 2.8° north-east of Betelgeuse, and thus it is easy to get your field of view to the right place, but actually seeing the object is another matter.

  

  Magnitudes given for this object vary considerably and I have seen estimates from 12.4 to 13.9. My thoughts are that it’s likely to be about magnitude 13, but it appears more difficult because of the presence of the bright star.

  The problem is that on most nights, the nebula lies within the halo of the star. To see Abell 12 at its best, you will need a still night with low humidity. Having said that, it is visible on sub-optimal nights, but is much more difficult. On poorer nights, I find the nebula virtually swamped by the aura from μ Ori, and never visible without the OIII filter in place. I haven’t tried a UHC filter on this object.

  

  The nebula fades in and out of visibility with micro-changes in the seeing, and when visible, presents a featureless circular patch of dim, grey light. Images of Abell 12 do show some structure, but it’s subtle and unlikely to be detected by a visual observer. If you know differently, please let me know.

  I’ll finish 2020’s offerings by wishing you as merry and as enjoyable a Christmas / Midwinter / Yuletide as possible under the current strained circumstances. Here’s wishing for clear skies in a pandemic-free 2021.

  Object	RA	Dec	Type	Magnitude
  NGC 1893	05h 22m 47s	+33° 26’ 21”	Open cluster	7.5
  Abell 12	06h 02m 20s	+09° 39’ 15”	Planetary nebula	about 13

  Patrick Maloney

• Around the Heart Nebula (IC 1805) in Cassiopeia

  November 2020 - Nebula and Cluster of the Month

  This month, we’ll be looking at a compact region of sky in Cassiopeia to inspect four open clusters all wreathed in the same nebulosity, and then we’ll move south to Fornax with an interesting object for more southern observers.

  

  IC 1805 is a large and spectacular region of emission nebulosity beloved of imagers. This is understandable. It’s high in the sky for northern observers and presents a wealth of structure and detail. The area covered by the nebula (about 3° x 3°) contains many differently-labelled nebulae and at least four open clusters. Each little knot of the nebula seems to have its own catalogue number, but for the purposes of this discussion, I’ll assume that ‘IC 1805’ refers to the whole lot.

  IC 1805 was discovered by E. E. Barnard. The designation refers to both the nebulosity and the central open cluster. The description in the Index Catalogue reads Cluster, coarse, extremely large nebulosity extends following. Having said Barnard was the discoverer, the brightest portion of the nebula, on its western edge, which bears the designation NGC 896, was discovered over a century earlier, predictably by William Herschel, on 3 November 1787, when he described it as Extremely faint, small, irregularly formed. My own observation of NGC 896, the only part of the nebulosity that I find clear enough actually to draw, reads A small amount of nebulosity around a faint star, with a slightly brighter patch to the west. Formless and of uniform surface brightness.

  

  At the centre of the nebulosity is the open cluster also designated IC 1805, though sometimes referred to as Melotte 15. This is a large, bright, loose, splashy cluster. It contains several fairly bright stars. About 30-35 stars are easily visible in a 20' field. With a UHC filter in place, faint nebulosity can be seen across much of the field.

  Just about 1° following this cluster is another, NGC 1027, which lies right on the edge of the IC 1805 nebulosity. I observed this cluster through a 12” (305mm) Newtonian and found it to be A fairly loose cluster of mixed magnitude stars. The brightest, at the centre, is magnitude 7.0, though it's so much brighter than the others that it may be foreground. Some stars at around 9th - 10th magnitude, more at around 13th magnitude.

  Later reading showed that my assumption about the bright central star (SAO 12402, HD 16626) to be correct, it is a foreground object, coincidentally almost dead-centre of the cluster.

  Although apparently involved in the IC 1805 nebula, NGC 1027 actually lies much closer to us. The distance to 1027 is about 4000 light-years, whilst the nebula is about 7200 light-years away.

  In February 1980, a team of astronomers led by Helmut Abt of Kitt Peak Observatory published a paper in the ‘Publications of the Astronomical Society of the Pacific’ entitled ‘A Small Cluster Near IC 1805’. The paper describes an investigation into a ‘small cluster’ apparently located within the IC 1805 nebula. Although the cluster is not named at any point in the paper, it is clear that it is the bright but poor little group known as both Markarian 6 and Stock 7.

  

  Abt’s team set out to investigate whether this was a true cluster, and whether or not it was physically associated with IC 1805. They concluded that it was a true cluster consisting of at least four members, the two brightest of which were shell stars (rapid rotators like γ Cas and Pleione in the Pleiades). Fainter stars in the field may not be truly associated with the group. The distance they measured was 485 pc (1580 ly), placing it well in the foreground of IC 1805. A more recent measure of the distance places it slightly further away at 511 pc (1660 ly). The entry for Markarian 6 in Archinal & Hynes ‘Star Clusters’ gives the number of member stars as 29.

  Visually, this is a pleasing little group, looking like a bent cross, or a slightly disjointed version of Delphinus. A good night will show 30–35 stars in a 20’ field, though the probability that all, or most, of these stars are true members must be considered remote.

  Our last port of call within this nebula is another open cluster, Tombaugh 4, which lies 24’ to the south-east of NGC 896, the brightest portion of our nebula. Tombaugh 4 is a very different beast from the other clusters we’ve looked at. Firstly, it is very faint, with the brightest members just about topping 16th magnitude. Secondly, it is very rich, though Archinal & Hynes credit it with only 40 members. Images of the cluster seem, to my eyes at least, to show far more stars than this. One for the 20”+ brigade, I think.

  

  This is a wonderful area to explore. It’s compact, just 3° across, yet it’s filled with good objects and given a good sky and a UHC filter, faint nebulosity can be seen stretching across much of it.

  Now to move south to the constellation of Fornax. Our object here is a tiny thing, NGC 1049. This very small object (visually less than 1’ in diameter) was discovered on 19 October 1835 by John Herschel, whose description in his ‘Cape Observations’ reads: Pretty bright, small, round, like a star 12m a very little rubbed at the edges, a curious little object and easily mistaken for a star, which, however, it certainly is not.

  

  He was quite right, of course. NGC 1049 is a globular cluster, but not one that belongs to the Milky Way. 102 years after Herschel discovered NGC 1049, Harlow Shapley discovered the galaxy now known as ESO 356-4, PGC/LEDA 10074 or, more commonly, the Fornax Dwarf. The Fornax Dwarf is a spheroidal galaxy, a satellite to our own. It currently has six known globular clusters, of which NGC 1049 is the brightest. The galaxy itself has a combined magnitude of 9, though as this is spread over about one square degree (four times the size of the full moon), it is exceptionally dim. NGC 1049 is superimposed on the face of the galaxy.

  The globular cluster, although very small, shines at magnitude 12.6, and as it is sort-of stellar, it should make for a relatively easy target for observers far enough south to see it.

  One aspect of visual observing is the drive to see further, fainter. Seeing globular clusters in external galaxies is one way of doing this. Stretch yourself. NGC 1049 is an easy place to start.

  Object	RA	Dec	Type	Magnitude
  NGC 896	02h 25m 31s	+62° 00’ 54”	Emission nebula
  Tom 4	02h 28m 51s	+61° 46’ 35”	Open cluster	Br * = 16.0
  Mk 6	02h 29m 42s	+60° 40’ 23”	Open cluster	7.1
  IC 1805	02h 32m 41s	+61° 27’ 17”	Open cluster + emission nebula	6.5 (cluster)
  NGC 1027	02h 42m 37s	+61° 35’ 19”	Open cluster	6.7
  NGC 1049	02h 39m 48s	-34° 15’ 47”	Globular cluster	12.6

  Patrick Maloney

• NGC 457 and Cz 3 in Cassiopeia and NGC 604 in Triangulum

  October 2020 - Nebula and 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

  Patrick Maloney

• IC 4996 and NGC 6888 in Cygnus, and IC 4997 in Sagitta

  September 2020 - Nebula and 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

  Patrick Maloney

• NGC 6934 and NGC 6905 in Delphinus

  August 2020 - Nebula and 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

  Patrick Maloney

• Stephenson 1 and NGC 6765 in Lyra

  July 2020 - Nebula and 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

  Patrick Maloney