Nebula and Cluster of the Month Archive 2022

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 6756 in Aquilia

  July 2022 - Nebula and Cluster of the Month

  July brings the glories of the northern Milky Way to our midnight skies. Not so great for the obsessive galaxy-hunter, though there are still plenty shining through gaps in our local galaxy’s dust. It’s a good time for open cluster enthusiasts, though.

  Our open cluster for this month, although listed in the Herschel 400 observing list, is a little-known object in Aquila, NGC 6756. It was discovered by our hero William Herschel on 21 August 1791. He placed it in his class VII – ‘Pretty much compressed clusters of large or small stars’, as entry number 62. On that occasion, he described it as A pretty compressed cluster of stars not very rich. (Phil. Trans. R. Soc. Lond. 1802 92)

  

  William’s son John Herschel, a deep-sky observer at least as good as his father in my opinion, made four observations of the cluster which were published in his 1834 Observations of Nebulae and Clusters of Stars in which it received the designation JH 2031.

  • Sweep 77: Pretty compressed; irregular figure; 2’ diameter; stars 15th magnitude.
  • Sweep 78: Pretty compressed; Small; Round or irregular.
  • Sweep 79: Pretty compressed cluster; irregular figure.
  • Sweep 278: Pretty rich; Small; Much compressed; oval or rather fan-shaped. The stars 11..12 magnitude, 4’ in extent the north following side most compressed.

  These observations are pretty consistent with the glaring exception of the estimates of the constituent stars’ magnitudes. Somehow, they brightened from magnitude 15 to magnitude 11-12, an increase in brightness of at least 16 times!

  The cluster made its way into John Herschel’s General Catalogue (GC) of 1864 as number 4471, where its description was given as Cluster; small; rich; little compressed; stars 11..12. It has degraded from the Herschels’ description of it as ‘pretty compressed’ to ‘little compressed’, and the magnitudes have been transferred as the much brighter estimate.

  When the General Catalogue was updated to the New General Catalogue in 1888, the cluster was listed as number 6756, and the GC description was retained.

  What accounts for these seemingly irreconcilable differences? It depends on how you look at the object. As the photograph shows, NGC 6756 as a whole is about 4’ in diameter and generally is not very compressed. The two brightest stars in this 4’ diameter patch of sky are magnitudes 11.5 and 12.4. No other star is brighter than magnitude 13.3. These two stars are markedly different in their colour from the other stars in the cluster, though, and are therefore probably non-members lying between us and the cluster itself.

  Closer inspection reveals a highly concentrated patch of stars to the northeast. Note John Herschel’s comment ...the north following side most compressed. This area of the cluster is 1’ in diameter. This may be coincident with John Herschel’s earlier description of a cluster 2’ in diameter comprising 15th magnitude stars. The stars in this region are 14th to 16th magnitude and are very compressed.

  Modern catalogues give the cluster’s diameter as 4’ and the number of stars as 40, though there are over 100 stars brighter than magnitude 16.0 within the 4’ circle. Its distance is quoted as 1507pc, or a little short of 5000 light-years. Galactic dust absorbs 1.2 magnitudes, reducing the whole object to a magnitude of 10.6. I have found different age estimates for this cluster. WEBDA gives 62 million years, a figure widely quoted. However, in a paper¹ investigating the occurrence of Be stars (young, non-supergiant B-type stars that display emission lines in their spectra) in open clusters, the age of NGC 6756 (which contains two Be stars) is given as 125-150 million years. According to the authors, this would make it the oldest open cluster known to contain Be stars. In this same paper, however, NGC 7039 is quoted as having one Be star and having an age of 1000 million years. I’ll leave you to figure that out for yourself.

  

  Visually, this object is quite striking. I observed it through my 12” Newtonian reflector in June 2015, under less than perfect skies that were also suffering from the onset of morning twilight. I noted a bean-shaped curve of nebulous, unresolved stars. Some member stars were seen superimposed. These stars range from magnitude 14.2 to 14.5, about my limit for the brightening sky. The background of unresolved stars is quite bright and looks very much like a low-power view of a globular cluster.

  It’s a cracking little object.

  Object	RA	Dec	Type	Magnitude
  NGC 6756	19h 08’ 44”	+04° 42’ 51”	Open cluster	10.6

  Patrick Maloney

  Reference

  1. Be phenomenon in open clusters: results from a survey of emission-line stars in young open clusters. Blesson Mathew, Annapurni Subramaniam and Bhuwan Chandra Bhatt. Mon. Not. R. Astron. Soc. 388, 1879–1888 (2008)

  If you'd like to try out the Clear Skies Observing Guides (CSOG), you can download observing guide for the current Cluster of the Month without the need to register. CSOG are not associated with the Webb Deep-Sky Society but the work of Victor van Wulfen.

• NGC 6356 in Ophiuchus

  June 2022 - Nebula and Cluster of the Month

  June brings the shortest of our nights, but also the promise of better things, as the glorious northern Milky Way comes into prominence in the east, moving to its overhead passage in July. Although the June sky (defined for this column as objects that are at the meridian at midnight at some point during the calendar month) is not as packed as the July sky, there are still some objects worth seeking out.

  June is particularly rich in globular clusters, with 30 NGC objects and one IC higher than declination -30° visible at meridian during the month, though it must be confessed that several of these are quite low in British skies.

  Our object for this month is one of these globular clusters – not too low, not too high; not too easy, not too difficult. NGC 6356 lies in Ophiuchus, at a declination of close to -18°, and culminates at midnight on the 30th of the month. It is 2.8° south-east of the magnitude 2.5 η Ophiuchi, and 1.2° north-east of Messier 9.

  

  It lies about 50,000 light-years (15,000 pc) away and has a mass of around 600,000 suns. NGC 6356 is a relatively metal-rich globular cluster and was believed for some time to be particularly dust-rich, too. A study published in 1998 by Hopwood, Evans, Penny and Eyres (Mon. Not. R. Astron. Soc. 301, L30-L32 (1998)) determined the quantity of dust in the cluster to be of the order of 0.004–0.017 solar masses, less than was expected for the cluster’s escape velocity, which they quote as 35.6 kms^-1.

  As with so many of the objects featured in this column, it was first discovered by William Herschel, who spotted it on 17th June 1784. On that occasion, he described it as Bright. Large. Round. Gradually brighter towards the middle. Easily resolvable.

  When reading Herschel's descriptions, we need to take two things into account. Firstly, he made most of his observations with an 18” (500mm) reflector, considerably larger than most amateur instruments today, and he did not suffer from light pollution. Both these factors tend to make his descriptions slightly ‘optimistic’ from the point of view of a light-polluted amateur observer of today. However, NGC 6356 is fairly bright, though not very large (depends what you’re comparing it to, I suppose). One thing it certainly isn’t is ‘easily resolved’. The brightest star is magnitude 15.1, making this a tough one to resolve unless you have good skies and a telescope of over 14” (350mm) diameter.

  The magnitude quoted for this object in nearly all sources is either 8.2 or 8.4. That does, indeed, make it fairly bright. Even more importantly, it is very concentrated, being classified as a II on the I–XII Shapley-Sawyer concentration scale. This indicates a dense central condensation. For comparison, the bright globular cluster M2 has this same level of concentration.

  The diameter of the cluster is usually given as 7.2’, though this takes into account the furthest stragglers. It actually appears considerably smaller than this. Through the eyepiece, expect a diameter of 2’–3’.

  

  I made an observation of NGC 6356 on 10th June 2015 with a 12” Newtonian reflector from my suburban site. I noted that it appeared bright, small and round. I also recorded that I thought I was only seeing the central section (which was true) and that there was no sign of resolution. This is hardly surprising with the brightest stars being 15th magnitude; the faintest stars I recorded on my drawing were between 11th and 12th magnitude.

  The resolution of this object is a good test of your conditions and your instrument.

  Object	RA	Dec	Type	Magnitude
  NGC 6356	17h 23’ 35”	-17° 48’ 47”	Globular cluster	8.2

  Patrick Maloney

• IC 4593 in Hercules

  May 2022 - Nebula and Cluster of the Month

  May brings with it, we hope, warmer weather and the real promise of sunny days. What is more certain is that for those of us in northern temperate zones it brings the end of astronomical darkness for some weeks to come. The centre of Britain sees astronomical darkness for only 22 minutes on the night of 12/13 May. The following night there is none, and it will not be seen again until 31 July, when we get 26 minutes.

  The occurrence of astronomical darkness is these days more theoretical than practical. Most of us live all year under perpetual artificial twilight, so we should be used to dealing with it, and unless you live in the far north, you really have no excuse for putting the telescope in storage for 10 weeks.

  This month, then, we’re going to concentrate on an object that is easy to see, even under twilight, but can be difficult to pick out in a low-power field. High power is needed to get the best view.

  May’s object is a small planetary nebula in Hercules, IC 4593. It is very bright – magnitude 10.7 – so should be easily seen by all but the smallest telescopes. Recognising it for what it is will require higher power.

  

  It was discovered on Harvard photographic plates by Williamina Fleming in 1907 and bears her designation Fleming 108. The entry in the Index Catalogue is typically terse: Planetary, stellar.

  NASA quotes a distance for IC 4593 of between 7000 and 8000 light-years (2150 - 2450pc), though other estimates range from 3000 to 12000 light-years. Assuming a diameter of one light-year (about average for a planetary nebula), the distance should be about 5000 light-years. Planetary nebulae distances and diameters are notoriously uncertain.

  Images of the nebula show a bright inner region and a much fainter, much larger outer envelope. The object also displays what appear to be bipolar jets with brighter blobs of material at the ends, which look very similar to the famous ‘ansae’ (Latin; ‘handles’) of NGC 7009.

  In 2012, observations made with the Chandra X-Ray telescope led to the discovery of a ‘hot bubble’ around the central star of IC 4593 (Monthly Notices of the Royal Astronomical Society, Volume 494, Issue 3, May 2020, Pages 3784–3789). This is not the only hot bubble detected in a planetary nebula by Chandra, but is so far the most distant.

  IC 4593 is not the easiest object to locate. It lies in the southwest of Hercules, close to the border with Serpens, in a relatively barren region of sky.

  The brighter central part, which is all that you’re going to see visually, is about 29” in diameter. The central star, of spectral type O0, is magnitude 11, and easily visible, and can be well seen in low finding powers. You may have to inspect a few stars in the field on high power before picking out the right one. Some observers of planetary nebulae can pick out a stellar planetary nebula even with low power. There’s something about the quality of the light from these objects that draws the eye. I’ve found this to be so on several occasions. This is a good one to test the effect out on.

  

  High power reveals a circular bright disc, with an elongated outer envelope (not the really faint one visible in images) that stretches away from the bright inner part like wings. I found that the object was still stellar at x83, but when I cranked the magnification up to x450, a fair amount of detail could be seen. To my eyes, the effect of the OIII filter was marginal. At high power, the object is isolated in the field with no visible field stars. There is a 9.4 magnitude star 5’ to the northwest, just outside my high-power field. In common with several other observers, I see a very clear blue colour in this object. The bright central star appeared to be oddly off-centre.

  Object	RA	Dec	Type	Magnitude
  IC 4593	16h 11m 44s	+12° 04’ 17”	Planetary nebula	10.7

  Patrick Maloney

• NGC 5053 in Coma Berenices

  April 2022 - Nebula and Cluster of the Month

  In the northern hemisphere, April is a time for observing galaxies as the great galaxy fields of Leo, Coma and Virgo straddle the meridian. Yet April also brings some examples of another class of objects into view. Globular clusters. The brightest of these for northern hemisphere observers (for in the south, the glorious Omega Centauri also culminates this month), is Messier 53. This is a lovely object, well seen even in small telescopes.

  However, it is not M53 that we’re going to concentrate on, but its neighbour, NGC 5053. First observed by William Herschel on 14 March 1783, he described it as An extremely faint cluster of extremely small stars with resolvable nebulosity. 8 or 10’ diameter. Verified [with a magnification of] 240. Beyond doubt. He placed it in his sixth class of objects – ‘very compressed and rich clusters of stars’ as entry no. 7.

  

  NGC 5053 lies less than a degree southeast of M53. The distances of the two globulars are very similar, from which it has been calculated that they lie only about 2kpc (6,500 light-years) apart. Evidence that the two globular clusters have interacted in the past is presented by a tidal stream of stars that connects the two.

  Chemically, the stars of NGC 5053 have low metallicity, that is, they are poor in elements heavier than Helium. The chemical composition of the member stars is very similar to that of the stars of the Sagittarius Dwarf Spheroidal Galaxy (SDSG) and far less akin to those of stars in the Milky Way’s halo. Studies of the spatial velocity of the globular cluster add weight to the argument that NGC 5053 was originally a globular cluster of the SDSG, and not of the Milky Way at all.

  Whilst M53 may present little difficulty to the visual observer with a small telescope, NGC 5053 is a completely different matter. It is fainter than M53, magnitude 9.9 as opposed to M53’s 7.7. It is also slightly smaller, with a diameter of 10.5’ compared to M53’s 12.6’.

  Finally, and much more importantly, whilst M53 has a concentration class of V, NGC 5053’s concentration class is XI, making it a very loosely compacted cluster. I’ve said it before and I’ll say it again, the most important metric for determining the visibility of a given globular cluster is its concentration class.

  All this means that seeing NGC 5053 can be tricky. When searching for a low-concentration globular cluster, it’s best not to expect to see a fuzzy ball, as brighter ones tend to appear at low magnification. It’s far better to search for a scatter of very faint stars, although on a good night NGC 5053 itself does present as a faint fuzzy patch. With my 12” Newtonian reflector, from an admittedly poor sky location, I certainly did not see it the first time I looked. When I looked for it after the sky had been washed by a particularly heavy rainfall, I finally saw it.

  

  I saw a large, circular patch of dim nebulosity. There was no indication of brightening towards the centre (typical of low-concentration globulars). It looked more like a ghostly grey patch. Three stars were bright enough to be plotted. It turns out the brightest star in the cluster is magnitude 13.8. Other stars were seen, or glimpsed, but never clearly enough or for long enough to plot.

  Object	RA	Dec	Type	Magnitude
  NGC 5053	13h 16m 27s	+17° 41’ 55”	Globular Cluster	9.9

  Patrick Maloney

• Messier 97 in Ursa Major

  March 2022 - Nebula and Cluster of the Month

  For the same reasons as outlined in last month’s feature, I find myself somewhat bereft of objects to write about for this month.

  I’m therefore going to concentrate on a single object, NGC 3857, alternatively known as Messier 97, PK 148+57.1 and PN G 148.4+57.0.

  

  M97 lies in Ursa Major, 2.3° ESE of β Ursae Majoris (Merak), the bottom right-hand star of the bowl of the Plough.

  It was first seen by Pierre Méchain on 16 February 1781. His friend Charles Messier then observed it for the first time on 24 March of that year. He wrote of his observation: Nebula in Ursa Major, near beta. “It is difficult to see”, reports M. Méchain, “especially when one illuminates the micrometer wires: its light is faint, without a star.” ... Near this nebula he saw another which has not yet been determined, also a third near gamma Ursae Majoris. Diam 2’.

  The two other ‘nebulae’ discovered by Méchain are almost certainly NGC 3556 and NGC 3992. In fact, so secure are these identifications that these two galaxies have now been designated as M108 and M109, even though they never actually made it into Messier’s catalogue.

  The Herschels, father and son, both observed the object, William describing it as A globular body of equal light throughout and John as Diam. 2’40”, light equable, with a softened edge and fairly bi-central.

  Lord Rosse observed it with his 72” Leviathan, writing that it was Two stars considerably apart in the central region; dark penumbra around each spiral arrangement. Kenneth Glyn Jones notes of this observation that on many occasions only one star was seen and the spiral form was doubtful.

  

  The reference to two stars is problematic. There are actually three stars visible in the nebula. One is the central white dwarf, and the others are line-of-sight objects. They are not in the locations shown on Rosse’s charming and somewhat comical illustration. Thomas Romney Robinson, Director of the Armagh Planetarium, and inventor of the four-cup anemometer, observed M97 through Rosse’s telescope in 1848. He describes A most intricate group of spiral arcs disposed round two starry centres, looking like the visage of a monkey.

  Our own Rev. T. W. Webb in his Celestial Objects for Common Telescopes, adds something to the story: Large, pale planetary nebula. Very remarkable object: two stars, one in each opening – only one seen since 1850.

  Are these just mistaken observations? The ‘voids’ in the planetary nebulae are not centred on stars, as these 19th-century observers would have us believe. It’s easy to misplace a star, especially an extremely faint one, when making an observation. They fade in and out of sight and it can be very difficult to place them directly where they actually lie. The brightest of the three stars in the nebula is the central white dwarf, magnitude 16 (or is it? – see later) and the second brightest is magnitude 17.

  M97 is one of the closer planetary nebulae to us, lying about 2000 light-years away. It consists of (at least) three concentric shells of nebular material, and expansion data indicates that it began expanding, at the time of the central star’s collapse into a white dwarf, about 8000 years ago. Its current diameter is a little less than one light-year and it contains 0.13 solar masses of material.

  I have made two observations of this object, both reproduced here. Firstly with a 4½” (11cm) reflector in 1979 and then again in 1995 with a 12” (300mm) reflector. The observation made with the smaller telescope was made under dark sky conditions and demonstrates that the planetary is visible in small telescopes if the conditions are right. Kenneth Glyn Jones (KGJ) in Messier’s Nebulae and Star Clusters writes M.97 is a fairly difficult object to find as it is small and faint and will probably elude instruments smaller than 6-inches aperture. He was observing with an 8” (200mm) reflector, but his observation shows a field of 65’. Higher power is much better for planetary nebulae. The tired old maxim that deep-sky objects demand wide-field low powers is a much-regurgitated myth.

  Through my little 4½” ‘scope, I saw ...a large, bright disc of non-homogeneous light. Indistinct dark and light patches appear briefly before disappearing once again into the overall glow. A very noticeable feature of the nebula is the absence of a faint periphery. ... M97 has a very definite edge. A star of about mag 11 is very close Nf.

  

  The central star is widely quoted as having a magnitude of 16 (very specifically in Uranometria as 16.01). In my observation of 1995, I have placed the central star where I saw it. KGJ states that the central star is of magnitude 14. If it was really a 16th mag. object, I doubt I would have seen it. My estimate was about 15, as it was just occasionally glimpsed. I noted the planetary’s large size and brightness. The brightness over the disc was irregular, and darker incursions were seen on both sides of the object.

  

  The Night Sky Observer’s Guide – Volume 2 Spring & Summer states that This planetary’s disk is about the same diameter as Jupiter’s disk...

  The diameter of M97 is 194” as opposed to Jupiter’s maximum of about 50”. This jarringly incorrect statement, made almost immediately after the diameter of the object is correctly quoted, had me scratching my head for a while until I came across the description given by Admiral W. H. Smyth in his Cycle of Celestial Objects (1859). He says This very singular object is circular and uniform and after long inspection, looks like a condensed mass of attenuated light seemingly the size of Jupiter.

  It seems that this is another astro-myth being repeated without being checked. It just goes to show that even the most highly-regarded texts can sometimes be misleading.

  Object	RA	Dec	Type	Magnitude
  M97	11h 11m 48s	+55° 01’ 09”	Planetary nebula	9.9v, 12.0p

  Patrick Maloney

• Open Cluster Classification

  February 2022 - Nebula and Cluster of the Month

  The criteria I use for nebulae and clusters to be included for a particular month are that they should culminate within an hour of midnight on some date during the month. This effectively means that the Right Ascension (RA) of the object must lie within a two-hour band. I also tend to favour objects north of -30° declination.

  I have in the past struggled with February. There are really only two open clusters of any note in February’s RA band – Messiers 44 and 67, in Cancer. Both these objects are so well known that they fail to make my other criteria – that they should be good but obscure objects, or that I have something interesting to say about them.

  Nebulae are equally absent in February. I find myself therefore in a bit of a quandary, so this month I’m going to discuss how we categorize open clusters. I frequently quote the Trumpler classification for the open clusters that I discuss, but what exactly do those classifications mean?

  The first person to attempt to classify open clusters was probably our old friend William Herschel, who gathered his deep-sky discoveries into eight different catalogues. Catalogues VI, VII and VIII were for open clusters of differing visual appearance. VI was for ‘very compressed and rich clusters of stars’, VII was for ‘pretty much compressed clusters of large or small stars’, VIII was for ‘coarsely scattered clusters of stars’.

  A later, though barely more complex, system was introduced by Harlow Shapley. He suggested eight classes: (a) – field irregularities, (b) – star associations, (c) – very loose and irregular, (d) – loose and poor, (e) – intermediate rich, (f) – fairly rich and (g) – considerably rich and concentrated.

  A class (a) object could simply be a statistical irregularity within a certain patch of sky, slightly more stars than would be expected on average. Class (b) objects would include such systems as the Ursa Major moving group. Shapley included the Hyades and the Pleiades in his class (c), M21 and M34 in class (d), M38 in class (e), M37 in class (f) and M71 (a globular cluster) in class(g).

  Enter Robert Trumpler, who recognised that open clusters had more complex morphologies than could be described in Shapley’s system. He broke his classification system into three distinct properties: concentration/detachment, the range of brightnesses of stars within the cluster and the richness (number of stars) of the cluster.

  The word ‘detachment’ refers to how easily differentiated from the background star field the cluster is. Obviously, this is closely allied to, but not wholly dependent on its concentration.

  The concentration or detachment part of the classification is given first, in Roman numerals.

  I

  Detached clusters with strong central condensation

  II

  Detached clusters with little central condensation

  III

  Detached clusters with no discernible central concentration

  IV

  Clusters not well detached from the surrounding star field

  The range of star brightnesses is given next, in Arabic numerals:

  1

  Most stars are of similar magnitudes

  2

  There is a moderate range in brightnesses

  3

  The cluster is composed of both bright and faint stars

  The third part of the classification refers to the richness of the cluster, defined by the number of members. This is given as a lower-case letter:

  p

  poor (fewer than 50 stars)

  m

  medium (between 50 and 100 stars)

  r

  rich (more than 100 stars)

  These categorizations are determined usually by a visual inspection of a photograph of the cluster. If there is nebulosity associated with the cluster, then an additional ‘n’ will be placed after the richness indicator.

  The system is flexible but is beset with problems. How for example, does one define how strong the central condensation is? What exactly does ‘moderate range’ mean for brightnesses, and how can you be sure how many stars in the field are cluster members?

  This inevitably leads to differences in quoted Trumpler types from different sources. I’ve pointed some of these out in previous months. When I make an observation of an open cluster, I tend to have a go at assigning a Trumpler classification based on my observation, rather than on what may be seen on a photograph or image. This can be a fun addition to the process, and I recommend it to you. It can improve your observing technique as you try to classify the object in your field of view, making you look ever more closely and critically.

  Have a look at this image of NGC 1245 in Perseus.

  

  The following Trumpler classifications are given to it just in books that happen to be on my desk at the moment:

  • II2r
  • III1r
  • III2r
  • III3r

  At least everyone agrees that it’s a rich cluster, but does it have little central condensation (II) or no discernible central condensation (III)? Where, for that matter, is the centre, exactly? Are the stars all the same magnitude (1), of moderate range (2) or with both bright and faint stars (3) (but the range isn’t ‘moderate’, whatever that means)?

  I have my own opinion; I’d be interested to hear yours.

  Patrick Maloney

• NGC 2264 in Monoceros

  January 2022 - Nebula and Cluster of the Month

  Firstly, I would like to wish you a happy new year. We’ve had a couple of tough ones, so let’s hope for some highly visible improvements this year.

  To start the new year, I’m going to be looking in some detail at a single object that incorporates both nebula and cluster. Located in Monoceros, nine degrees north of the celestial equator, it is easily accessible to observers in both the northern and southern hemispheres.

  The nebula and the cluster, both now referred to as NGC 2264, were first observed by William Herschel. He found the cluster first, on 18 January 1784, describing it as Double and attended by more than 30 considerably large stars.

  Evidently, Herschel did not see the nebulosity in which this cluster is wreathed until he revisited the object nearly two years later, on 26 December 1785. On that night, he noted Some bright stars 7 or 8' south preceding 15 Mon are involved in extremely faint nebulosity which loses itself imperceptibly. It reads as though he was unaware that he had previously observed the cluster. He gave the cluster the designation 8 H.VIII, including it in his eighth class – ‘Coarsely scattered clusters of stars’, whilst giving the nebula the designation 27 H.V, placed in his fifth class – ‘Very large nebulae’.

  

  The first word in the description of the open cluster – ‘Double’ – refers to the star he used as the basis for the position he gives for the cluster, 15 Monocerotis. 15 Mon is a double, but more than that, it’s the principal star in a complex multiple star system, the relationships between which are the subject of no fewer than 28 entries for 17 different stars in the Washington Double Star Catalogue. Seven of these stars are double or multiple stars in their own right.

  15 Mon also bears the variable star designation S Mon. The variations are slight, from magnitude 4.62 – 4.68. S Mon is a young star with a spectral type of O7e. It has an absolute magnitude quoted from -2.6 to -5.1, which is confusing and unhelpful. Its mass is around 30 solar masses. The distance is problematic, with ground-based dynamical parallax and isochrone measurements consistently giving a distance of between 700 and 900 parsecs (2300–3000 light-years), whereas Hipparcos parallax measurements suggest a distance only half this.

  The age of the cluster is between three and five million years.

  Stripped of its nebulosity, the cluster itself is fairly unremarkable visually, though its resemblance to a Christmas tree has been noted, with S Mon forming the short trunk of the tree, and 7.2 magnitude HD 47887 (26’ almost due south of S Mon) representing the top of the tree. The Trumpler type of NGC 2264 is variously given as III3m n and III3p n. This indicates a cluster that is detached from its background but does not have a noticeable concentration towards the centre, that the stars have a wide range of magnitudes and that it is either medium rich or poor. The number of stars listed as members by various authorities range from 40 (Archinal & Hynes) to 222 (Luginbuhl & Skiff). One can only shake the head.

  The overall magnitude of the cluster is consistently quoted as around magnitude 4.0, making it a naked-eye object under good dark skies, and otherwise an easy binocular object. Through my 12” (300mm) reflector, I found a large, bright cluster dominated at the north by 4.6 magnitude S Mon, which appeared to be surrounded by a nebulous haze. Two arms of stars stretch to the south where they meet at another fairly bright star, though there is a paucity of stars in the triangular area that they delineate.

  The two stars at the extremes of the cluster, S Mon and HD 47887, are also the best spots to look at more carefully to see the nebulosity associated with the cluster. The whole of this nebulosity is given the designation LBN 912, but in addition, the brighter part by S Mon is also designated LBN 911. Under good conditions, both the nebulosity around S Mon and some hint of the nebula by HD 47887 (which is often called the Cone Nebula) can be detected visually. My observation of the area around HD 47887 (labelled LBN 912 on my observation) shows the brighter ‘cap’ to the Cone (which is itself a dark nebula). At the time of this observation, I noted that a tiny nebulous spot is visible where the tip of the Cone Nebula lies. Fainter nebulosity was barely visible around it.

  

  Around S Mon lies the brighter nebulosity called LBN 911. This is the brightest and most easily seen part of the nebula. It is quite bright around S, making it look like the star is shining through very thin, high cloud. The other two patches seen, to the south-west, were much fainter.

  

  If you are fortunate enough to have good dark skies, a tougher challenge is to see the Cone itself. The use of an OIII or UHC filter will considerably enhance your view of the nebulosity round here, sometimes the whole field has a gentle glow, and darker lanes can be glimpsed passing roughly east-west. If you have truly good skies, the Cone can be seen as a dark intrusion, often just a ‘nick’, in the very faint nebulosity south of HD 47887.

  Object	RA	Dec	Type	Magnitude
  LBN 911	06h 40m 48s	+09° 51’ 00”	Bright nebula	-
  NGC 2264	06h 41m 04s	+09° 43’ 06”	Open cluster + emission nebula	Cluster 4.0

  Patrick Maloney