Nebula and Cluster of the Month Archive 2024

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.

• Stock 8 and IC 417 in Auriga

  December 2024 - Nebula and Cluster of the Month

  As it draws to a close, we experience the year’s longest and coldest nights. Riding high is the celestial charioteer, Auriga. Historical images of Auriga show the charioteer in his chariot, holding the reins with his right hand and with a goat and two kids under his left arm. The identity of the charioteer is in some dispute, and several semi-mythical (or wholly mythical) individuals have been claimed as the charioteer. There are many, quite diverse, stories about how he came to be placed in the heavens. Strangely, none of these include a goat, and nobody seems to know where that came from.

  The constellation straddles the galactic equator and is rich in open clusters, three of which are in Charles Messier’s list – M36, M37 and M38. The NGC adds another 14 open clusters to the tally, and the IC a further three. That’s not the final word, though. There are many catalogues of open clusters and the Basel, Berkeley, Collinder, Czernik, Dolidze, King, Lund and Stock open cluster catalogues (no doubt amongst others) all contain objects within Auriga. Some of these are relistings of others. For example, all the Collinder clusters in Auriga are also NGC objects. Three of them are the Messier objects. It all gets a little confusing.

  Stock 8 is an open cluster with (as far as I can tell) four aliases. In addition to its Stock listing, it is also Lund 178 and bears the modern designations of OCl 436 and the ungainly C0524+343. Stock 8 it is, then.

  Stock 8 is located just to the east of the 5th magnitude star φ Aurigae, which lies near the middle of the southern half of the constellation’s ‘kite’ asterism.

  Stock 8 is associated with the emission nebula IC 417. The cluster is very young, with measured ages all giving an upper limit of five million years. IC 417 is the stellar nursery where the cluster was born. A photometric study¹ of the cluster, published in The Astrophysical Journal in 2019, revealed 51 short-period variables that were found to be members of the cluster. Of these 53, 28 were Main Sequence stars of β Cephei- and δ Scuti-type. Variable stars of β Cep-type are usually young, B-type stars with small variations caused by pulsations. δ Sct-type stars (also sometimes known as ‘dwarf Cepheids’ are also pulsating stars, still young but with spectral types typically in the range A0-F5. The remaining 23 variables were Pre-Main Sequence T Tauri-type stars. These are stars in the late-formation phase, still working their way towards the Main Sequence of the Hertzsprung-Russel diagram. Typically, they have masses of less than three solar masses. The Sun probably spent some time early in its career as a T Tauri star.

  

  Streams of material within IC 417 have been found to contain many Young Stellar Objects (YSOs) that are not associated with Stock 8, showing that IC 417 is still an active star-forming region and may be in the process of birthing another open cluster.

  As usual, there are slight differences in authors’ estimates of the cluster’s Trumpler type. Uranometria (as far as I can tell from their somewhat verbose expansions of Trumpler types) rates it I2m n, with which the Night Sky Field Guide agrees, whereas Archinal & Hynes rate it I3m n. The only difference here is the range of magnitudes ascribed to the member stars. Both describe it as well detached with a strong central condensation, fairly rich in stars and associated with nebulosity.

  The cluster is around 5’ across and contains 40 stars or thereabouts, the brightest of which is magnitude 9.0.

  To observe this young open cluster, centre your telescope field on the fifth-magnitude φ Aurigae. The cluster is located about 6’ slightly north of east of it. Note that in atlases, it is nearly always placed in the wrong location, 4’ north of φ Aurigae. This positional inaccuracy is a not-uncommon problem with some of the more obscure open cluster catalogues, and one that the observer should always be aware of. In this case, the cluster is located 5.6’ E and 2.6’ N of the usually-given location. For those with Goto capabilities, use the RA and Declination given in the table below.

  

  The cluster may not be immediately obvious. Although the brightest star is magnitude 9, the magnitudes of the other members stretch down into the mid-teens, so you may not see all 40 stars. Nevertheless, the impression is of a fairly rich, compact cluster. The brighter stars shine against a backdrop of fainter sparkles. On exceptionally good nights, a hint of the emission nebula IC 417 can be seen as a faint haziness around the cluster. On my observation shown here, Stock 8 is the clustering of stars east of the brightest star in the field (which is φ Aurigae).

  May I take this opportunity to wish you all best wishes for Christmas or whatever Midwinter festival you choose to observe.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  Stock 8	05h 28m 04s	+34° 27’ 06”	Open cluster
  IC 417	05h 28m 13s	+34° 25’ 10”	Emission nebula

  References:

  1. Sneh Latah et al, Short-period Variable Stars in the Young Open Cluster Stock 8, 2019 AJ 158 68

  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 1514 in Taurus

  November 2024 - Nebula and Cluster of the Month

  In the mid-1970s, George R Kepple produced a series of postcard-sized ‘Astro Cards’, finder charts for deep-sky objects. If I remember correctly, there were five sets, detailing the Messier Objects, the Finest NGC Objects (two sets), Double Stars and NGC Objects for Larger Telescopes.

  For Christmas 1977, I was delighted to receive a set of these cards, the Finest NGC Objects (part one), which provided finder charts for 117 NGC and IC objects. I still have them. At the time, I was also a keen member of my local Astronomical Society’s Messier Objects Section, a group which consisted almost entirely of enthusiastic, though somewhat unruly, teenage boys.

  Occasionally, our leader, the slightly older Ken Irving, would bring a particular object to our attention, and we would all then try to make an observation of that object. One such object was NGC 1514, a bright planetary nebula in Taurus. I was delighted to see that NGC 1514 had its own finder chart amongst my shiny new Astro Card set.

  

  Ken had made an observation of this object with his 8” reflector, and so I set to with my 4.5” reflector. Despite my enthusiasm and my Astro Card, I have to report total failure. Despite several attempts and encouragement from Ken, I never saw it with my little 4.5” telescope.

  It wasn’t until 36 years later that I actually saw the object and made an observation of it. Of course, I didn’t spend every clear night in between searching for it, in fact I probably never even looked for it again after my initial failure.

  

  The planetary nebula was first observed by William Herschel on 30 November 1790. It was this observation that finally convinced Herschel that, even given sufficient aperture and magnification, not all ‘nebulae’ could be resolved into stars. He wrote A most singular phenomenon; an eighth-magnitude star with a faint luminous atmosphere of a circular form, about 3’ in diameter. The star is perfectly in the centre, and the atmosphere is so diluted, faint, and equal throughout, that there can be no surmise of its consisting of stars, nor can there be a doubt of the evident connection between the atmosphere and the star. Another star, not much less in brightness, and in the same field with the above, was perfectly free from any such appearance.

  He placed the object in his fourth class (‘planetary nebulae’) as IV.H 69.

  Perhaps surprisingly, NGC 1514 does not make it into the Herschel 400 observing list. Or maybe not so surprisingly, because this is not a particularly easy object to see. When I finally got around to looking for this object again, in September 2013, armed this time with a 12” reflector, I found that without an OIII filter, all I could see was the bright central star, which shines at magnitude 9.4. I searched carefully with averted vision, but the nebula itself eluded me, though I knew I was looking at the right star. Once the OIII filter was fitted, however, the nebula popped out of the blackness very clearly. The nebulosity is smooth and forms a bright circular disc, brightening towards the centre star and fading away at the periphery. A UHC filter will produce a very similar effect.

  

  The Field Guide to Uranometria 2000.0 gives the diameter of the nebula as >114”, which is fairly meaningless. The Night Sky Observer’s Guide (another fine production written in part by George R Kepple) simply repeats this figure. Herschel’s estimate of 3’ could easily fall within that upper-limit-free figure but is probably a little on the large side. Visual Observations of Planetary Nebulae by Kent Wallace gives a far more precise 136 x 121”. My own estimate of the longest diameter (the nebula is very slightly elongated) is 130”.

  The central star is unusually bright at magnitude 9.4 and is fairly easy to locate. The overall magnitude for the object is usually given as 10.9, but bear in mind that it is quite difficult to see and that some filtering may be necessary.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  NGC 1514	04h 09m 17s	+30° 46’ 33”	Planetary nebula	10.9

• NGC 637 in Cassiopeia

  October 2024 - Nebula and Cluster of the Month

  During October, the northern Milky Way rides high, close to the zenith for British observers. Taking centre stage are the rich galactic star fields of Cassiopeia, a constellation bursting with open clusters.

  William Herschel, the doyen of deep-sky observers, discovered 14 open clusters in Cassiopeia, including this month’s object. He catalogued it as 49 H.VII, that is the 49th entry in his seventh catalogue (‘Pretty much compressed clusters of large or small stars’). He found the cluster on the night of 9 November 1788 and described it as A cluster of some considerably large stars and many extremely small so as hardly to be seen. The large stars [are] arranged in circular order, 3 or 4’ diameter.

  The cluster was added to the New General Catalogue by J. L. E. Dreyer in 1888 and was assigned the catalogue number NGC 637. It is included in the Herschel 400, one of the 99 open clusters included in that list.

  NGC 637 is 1.3° west and slightly north of ε Cassiopeiae (Segin), the easternmost and, at magnitude 3.3, the faintest of the five stars that make up Cassiopeia’s ‘wonky W’.

  It transits close to midnight towards the end of October at an altitude of 80° from mid-Britain.

  The cluster is relatively bright at magnitude 8.2. Most listings agree that it contains 20 member stars and that the brightest member has a magnitude of 9.97. Archinal & Hynes, however, in their Star Clusters argue for 55 stars, the brightest of which, they say, is magnitude 8.0. There are, however, no stars as bright as magnitude 8.0 within, or even very near, the cluster.

  

  Estimates of the Trumpler type vary, as they so often do. Two of my sources opt for I2m (detached with strong central condensation, moderate range in stellar brightness, medium rich (50 – 100 stars)). I am aware that this is at odds with the assertion that the cluster has only 20 members. The Night Sky Observer’s Guide goes with I3p (detached with strong central condensation, wide stellar magnitude range, poor (< 50 stars)).

  The cluster is young, estimated to be about ten million years old. Its distance is reported as 2400pc (7800 light-years), placing it in the Perseus arm of the Milky Way.

  With four (or five) known examples, NGC 637 is one of the clusters best endowed with β Cephei-type variable stars. β Cephei stars (also known as β Canis Majoris stars) are short-period, small-amplitude pulsating stars. These pulsations are particularly useful for asteroseismology, the study of the interiors of stars from their pulsations.¹

  There are several published visual observations of NGC 637. From these observations, several features stand out: in small telescopes, the five brightest stars, which are arranged in a short arc, give the cluster a crescent-shaped appearance. Observations with mid-range telescopes (8—10”, 200—250mm) frequently mention the double star near the centre of the group whose components are magnitudes 10.2 and 10.9 (usually quoted as 10 and 11) with a separation of nine seconds of arc. I can find no observation that records any colour for these stars.

  I made my observation of this cluster in 2017, from my home location using a 12” (300mm) Newtonian reflector. The conditions were poor, as they always are. The sky was bright and snow on the ground enhanced the brightness. I should add that my observation was made in December, not in October.

  

  I found the cluster to be small and fairly compressed. It was best seen at fairly high power (x300). It is well detached from the background. The stars that I could see ranged in brightness from magnitude 10 to about 14.5. I estimated the size of the cluster to be a little under 10’. Within that 10’, I counted 27 stars. It is attractive and pleasing to the eye.

  For what it’s worth, my visual take on the Trumpler classification for this object is I3p.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  NGC 637	01h 43m 08s	+64° 02’ 06”	Open cluster	8.2

  References:

  1. New β Cephei stars in the young open cluster NGC 637, G. Handler & S. Meingast, Astronomy & Astrophysics vol 53, September 2011.

• NGC 7662 in Andromeda

  September 2024 - Nebula and Cluster of the Month

  Our object this month is, by contrast to recent objects, very bright and very easy to see. Let’s relax into autumn with a nice easy target.

  Discovered, of course, by William Herschel, NGC 7662 in Andromeda is one of the brightest planetary nebulae in the sky, and is particularly well-placed for northern hemisphere observers in September. At declination +42.5°, it should be visible to most southern hemisphere observers, too, barring those below about latitude 30, where the object only rises 18° above the horizon.

  It lies at the western end of Andromeda, in a field fairly rich in 5th and 6th magnitude stars. The closest ‘named’ star is 13 Andromedae (magnitude 5.7), 26’ to the planetary’s NNE.

  On discovering the object on the night of 18 October 1784, Herschel described it as Bright, round. A planetary. Pretty well-defined disk. 15” diameter.

  

  NGC 7662 has the Vorontsov-Velyaminov classification of III + IV, which indicates that there are two components, an irregular disc (III) and a ring structure (IV).

  The magnitude of NGC 7662 is widely quoted at 8.2, and this must be about correct. The object can be glimpsed as a stellar point in 10x50 binoculars. A matter that often puzzles me is the quoted magnitudes of planetary nebula central stars. The Deep Sky Field Guide to Uranometria quotes a magnitude of 12.5 for the central star. The Night Sky Observer’s Guide Volume 1 states that it is 13.17 (var). This seems very precise, especially for a variable star. Some observers claim to have seen the central star visually. The late Steve Coe, in his published observations, says that using a 17.5” reflector at a magnification of x320, ...it appears annular or ring-like, central star suspected in good seeing.

  With a smaller instrument (a 13” reflector), however, he sees ...several rings around a central star. At x330, the central star Comes and goes about 30 percent of the time.

  Even with a 6”, he reports that ...the central star is seen some of the time.

  Are these differing reports with different telescopes showing the actual variation in the star’s brightness? It seems odd that a 6” telescope would show the star some of the time, a 13” shows it 30% of the time, whilst in a 17.5” telescope, the star is merely suspected in good seeing.

  I found another reference to the central star in The Night Sky Observer’s Guide. Here it is stated that in an 8” or 10” telescope, the central star is not visible. In 12” or 14” scopes, the star is very faint, whereas with 16” or 18” scopes, the central star is very difficult but a twinkle may be glimpsed in periods of good seeing.

  I have never seen the central star. On my 2015 observation of NGC 7662, reproduced here, the star shown to the east of the planetary nebula is magnitude 13.5 and was very clearly visible. If the central star is magnitude 12.5 (or even 13.17), then it would also have been very clearly visible.

  

  Wide-field images of NGC 7662 in which the central star is visible show that the 13.5 magnitude star to the east is very much brighter than it.

  As a last word on the central star, in a study published in the Astrophysical Journal in 2004, Martín A. Guerrero et al determine that its mass is likely to be 0.605 solar masses.¹

  I made my first observation of NGC 7662 in 1984 with my 4.5” reflector. I wrote the following in my notebook. At low finding power (x22.5) this object is indistinguishable from a star. x45 reveals a very small disc when compared with stellar points. No detail is visible at this magnification. At high power, x150, the nebula could be seen to be elongated, a faint outer envelope was visible and the object was suspected of being annular. A very bright planetary nebula.

  This is a deep-sky object that rewards the user of a small telescope.

  The 2015 observation shown here has these notes attached: Very, very bright. Quite elongated. Pale blue in colour. Darker in the middle, making it appear annular. Faint extensions are visible outside the brighter part of the nebula. There is a brighter ring structure within the bright nebula.

  Whatever size telescope you have, then, will reveal some detail in this object, making it a prime target no matter your instrument or experience.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  NGC 7662	23h 25m 53s	+42° 32’ 08”	Planetary nebula	8.2

  References:

  1. Physical Structure of Planetary Nebulae: II NGC 7662, arXiv:astro-ph/0407029, https://doi.org/10.48550/arXiv.astro-ph/0407029.

• NGC 7094 in Pegasus

  August 2024 - Nebula and Cluster of the Month

  During August, we get a full return to astronomical dark, with darkness lasting about an hour and a half at the beginning of the month and extending to nearly six hours by the end. This is the time to get back out under the sky and begin enjoying observing again.

  Along with the increasing darkness, the autumnal constellations begin to make their presence known in the east, rising to prominence by the end of the month.

  This month's object is a challenge and a test of your sky conditions. I will demonstrate my own attempts (ultimately successful) to see this object.

  For once, this object was not discovered (or even seen) by William Herschel, which gives an indication of its difficulty.

  NGC 7094 was discovered by Lewis Swift, a discoverer of deep-sky objects second only to the Herschels, with 1,248 new objects plus 13 comets to his name. He first saw what was later to be catalogued as NGC 7094 on 10 October 1884. He described it as Nebulous star. Bright star; in most extremely faint nebulosity; very difficult; nearly pointed to by 3 stars in a line. It was first published in his second catalogue, under the designation Sw II 88. The object was included in the NGC four years later, with the description remaining materially unchanged, if a little briefer: star in most extremely faint nebulosity, very difficult.

  To be honest, this object is very difficult, but not impossible, as I will show.

  

  NGC 7094 is a planetary nebula, situated 1.8° east-north-east of the bright globular cluster M15 in Pegasus. It lies against a background of extensive nebulosity labelled LBN 152, though this is not visible without long exposures. The planetary’s Vorontsov-Velyaminov classification is IV, indicating annularity, although the ring structure in it is fairly weak.

  In The Night Sky Observer’s Guide: Volume 1 – Autumn & Winter by Kepple and Sanner, the description of the nebula through a 12-14” telescope (there is no observation from a smaller scope) says [NGC 7094] is a very faint, round, uniform glow surrounding a 13.7 magnitude star.

  In his Visual Observations of Planetary Nebulae, Kent Wallace includes an observation of this object made with an 8” SCT. He describes it at x62.5 as a ...faint, large disc, requiring the OIII filter and averted vision. At x100, it is clearer but still needs the filter and averted vision.

  My own quest to observe this object started in July 2014 when, towards the end of an observing session from home, I noticed the planetary nebula close to M15 on the chart for the region I had been observing in. I brought up a chart showing stars to 15th magnitude and positively identified the field I was looking at from that. Even with an OIII filter in place and the use of averted vision, I could not see the object.

  I looked again on 5 August of that year. This time I wrote in my journal Faintly glimpsed at x83 with the OIII filter in place. Lost again. Later barely suspected at x150 with the OIII. I have to record this as not seen.

  In September, I tried a third time to spot the object, this time from a dark-sky site in the Lake District. I had a little more success this time, but the view was impaired by the atmospheric conditions, which I described as Very wet. Very heavy dew, mist making bright torch beams. Otherwise quite good. Milky Way visible horizon to horizon. Mild. No wind.

  My description of the object on that night was Extremely faint but quite large. A very dim aura of nebulosity around a faint star. The OIII filter does nothing except render the object quite invisible. The reaction to the OIII filter was quite the opposite of what I expected.

  Thus the matter stood for over a year, until October of 2015, when from the same dark-sky site, I tried for a fourth time. This time I met with much more success. The air was a lot drier and made the stars much crisper than they had been on the previous attempt. This time, I wrote Quite large and immediately visible at x81, but very dim. Round, smooth, quite circular grey disc. Totally invisible with the OIII filter. Bright central star (magnitude 13.7).

  

  I don’t know what was different between my observations and Kent Wallace’s. My observations were all made with the same 12” Newtonian, and his with an 8” SCT. Why the OIII filter should be necessary for him to see it but render the object quite invisible to me is a mystery. I have always found the OIII filter to be an essential piece of kit when looking at planetary nebulae. Sometimes, however, when the object is just too dim, the elimination of any light (which is what a filter does) is too much and renders the object invisible.

  As a further test of your sky conditions and equipment, there is a second star superimposed on the disc of the nebula, clearly seen in the image above. This star is 16th magnitude and is too faint for my 12” telescope.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  NGC 7094	21h 36m 53s	+12° 47’ 13”	Planetary nebula	13.4

• NGC 6857 in Cygnus

  July 2024 - Nebula and Cluster of the Month

  Cygnus rides high in the sky during July nights. The Milky Way, as it runs down through the body of the swan, is packed with myriad deep-sky wonders. Open clusters, planetary nebulae and bright nebulae (emission and reflection) abound.

  This month we’re going to look at one of the lesser-known objects. It can be considered as a brighter knot in the masses of swirling gaseous nebulae that wreath the centreline of the constellation.

  NGC 6857 was first catalogued by William Herschel, who found it on the night of 5 September 1784. He placed it in his third category (very faint nebulae) and gave it the catalogue number 144 H.III. He described it that night as Some extremely faint stars with nebulosity. Irregularly extended.

  The field is indeed scattered with many faint stars, a fairly typical Milky Way background. Herschel may have believed there was some connection between his ‘extremely small stars’ and the nebulosity. This seems to have been refuted by the time the object made it into the NGC (1888) where its description reads Faint, amongst Milky Way stars.

  

  This little patch of brighter nebulosity sits amongst many knots and swirls of nebulae, with a bewildering number of designations. NGC 6857 is the brightest spot in the nebula known as Sh2-100, which in turn is part of a larger complex called GN 19.59.6. NGC 6857 itself once bore the planetary nebula designation PK 70+1.2, though this has now been expunged from the catalogue, as it has been definitively shown to be an emission nebula and not a planetary.

  The Revised New General Catalogue of 1973 maintains its reputation as an unreliable source by listing the object unequivocally as a planetary nebula.

  This does give a clue to its appearance, though. Through the eyepiece it is easy to believe that this is a small planetary nebula, similar to many others in the area.

  In the Night Sky Observer’s Guide: Volume 2 – Spring & Summer by Kepple and Sanner, descriptions of the visual appearance of the object are given for 8—10” telescopes at x75 and for 12—14” telescopes at magnification x125. Both use the word ‘faint’, and seem more concerned with the distribution of small stars around the object. The entry for the 12—14” telescope finishes with ‘The nebula rather resembles a planetary.’

  Parameters for this object are very thin on the ground. Even that massive repository of astronomical data, the Deep Sky Field Guide to Uranometria 2000.0, although it lists the nebula, gives no parameters at all for it. I have only ever seen one magnitude for it, and that was 11.4. This agrees with my experience of the object, and I’m happy to quote that. As to its size, my measurements suggest that this triangular object is about 40” long on its longest side.

  NGC 6857 can be found slightly to the east of the centre point of the line joining γ Cygni (Sadr) and β Cygni (Albireo) or 1.9° southeast of η Cygni (magnitude 3.9). Be prepared to do some hunting. It’s not always immediately obvious. An OIII or UHC filter can help – adding to the earlier confusion about its true classification.

  

  I observed it in June 2014, on a strongly twilit night, but with no moon. The sky was very bright and pretty blue, near white at low altitudes. If I can see it under those conditions, you have to be in with a good chance.

  On that night, I found NGC 6857 to be fairly bright. It was visible to direct vision even without the OIII filter in place. The filter improved the visibility of the nebula and revealed more of it. It appeared as a triangular smudge apparently leading away from an 11th magnitude star. The middle is brighter and also triangular.

  This is another under-observed object, and is worth a few moments of your time.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  NGC 6857	20h 01m 48s	+33° 31’ 30”	HII Region	11.4

• NGC 6309 in Ophiuchus

  June 2024 - Nebula and Cluster of the Month

  Here we are again, in the time of the lightest and shortest of nights. It’s not great for visual astronomy, but as most of us live under the perpetual twilight of light pollution, there’s no good excuse not to get out and have a look. If you are plagued by ever-bright skies anyway, it can be difficult to detect that the Sun is contributing some of the brightness in the sky.

  One type of object that is surprisingly good at punching through bright skies is the planetary nebula. Now planetary nebulae come in all shapes and sizes, and I’m not saying that all planetary nebulae are like this, but the smaller, more compact ones generally are. So this month, I’m going to provide a target that you at least have a chance of seeing through the glow – though it won’t necessarily be easy.

  NGC 6309 was discovered in 1876 by Wilhelm Tempel (1821—1889). Tempel was a prolific observer, discovering 21 comets, six asteroids and several galaxies. He published his discovery of what was to become NGC 6309 in 1878.

  Photographs and images reveal the object to have a very unusual shape. The blue plates from the Palomar survey show a rectangular object, tilted at 45° from north-south, placing one corner of the rectangle at each cardinal point. From the eastern corner, an arm extends and curves up towards the north, whilst from the western corner, an arm extends and curves down towards the south.

  

  This peculiar shape is down to the convoluted history of the nebula. A study by G. Rubio et al published in 2015¹ shows that the first calculable event in the life story of this object was a mass ejection from the central star about 150,000 years ago. This created a large structure which is detectable today by the presence of two diffuse blobs of material, positioned some 55” and 78” from the centre. The central star is surrounded by a halo of material, 60” in diameter, which must have formed no later than 46,000 years ago, whilst an inner ring and the quadrupolar outflow that now defines the rectangle probably date from about 4,000 years ago. A complex object indeed.

  

  Little of this structure is available to the visual observer. The rectangular shape has been reported, but most observers (including myself) see it simply as elongated. I am not aware of any visual sightings of the ‘arms’.

  It seems widely reported that the central star is of magnitude 13. The Deep-Sky Field Guide to Uranometria gives this figure, and Kepple and Sanner’s The Night Sky Observer’s Guide – Volume 2 states that in 8—10” telescopes ...its magnitude 13 central star [is] intermittently visible. I would dispute that. Images of the nebula clearly show a 12th-magnitude star just to the north of the nebula – this is bright and easily seen. To the south-west is a 15th-magnitude star, which is very difficult with a 12” telescope – I have never seen it. The central star, when visible on images, can be clearly seen to be much fainter than this 15th-magnitude star. The UCAC3 plots a star in the centre of the nebula and assigns it a magnitude of 16.83, which seems far more realistic. The catalogue number of the star is 3UC155-173189.

  

  NGC 6309 lies in Ophiuchus, in a rather barren patch of sky 1.6° west of fourth-magnitude ν Ophiuchi. Once located, it presents as small and moderately bright. It initially appears round, but a little patience (or a higher power) will reveal an elongation in the same direction as the long axis of the rectangle, 45° from north-south. With an OIII filter in place, the nebula becomes brighter and some structure can be made out. It can be seen to be brighter in the middle and an area of faint outer nebulosity appears. There is a brighter spot at its southern edge. With the OIII filter in place, the 12th-magnitude star to the object’s north is not visible.

  A challenge, then, for these over-light nights. Best of luck hunting it down.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  NGC 6309	17h 41m 04s	-12° 54’ 38”	Planetary nebula	11.5

  References:

  1. ‘NGC 6309, a planetary nebula that shifted from round to multipolar’, Monthly Notices of the Royal Astronomical Society, Volume 446, Issue 11 January 2015, pages 1931-1938 https://doi.org/10.1093/mnras/stu2201

• Messier 5 in Serpens Caput

  May 2024 - Nebula and Cluster of the Month

  May is another very poor month for nebulae and clusters. Galaxies still reign supreme in the target lists of northern hemisphere deep-sky observers.

  This month we’re going to have a quick look at one of the few objects mentioned in these articles not discovered by William Herschel. This object was discovered in 1702 by Gottfried Kirch (1639—1710). After a varied and not always successful series of career choices, Kirch became director of the Berlin Observatory. His other discoveries include Messier 11 (an open cluster in Scutum) and the Mira-type variable star, χ Cygni. He has a crater named after him on the Moon and asteroid 6841 Gottfriedkirch is named in his honour.

  Charles Messier observed this globular cluster, which was to be catalogued by him as Messier 5, on 23 May 1764 and described it as A fine nebula which I am sure contains no star. Round; seen well in a good sky in a telescope of 1-foot (focal length). Superior telescopes in later years corrected Messier’s impression that the object contained no stars. In May 1791, William Herschel inspected it with his 48” reflector. Although very famous and for many years the largest telescope in the World, the 48” was never very satisfactory. It was very cumbersome to use and the mirror (Herschel made two, only one of which was usable), was less than optimal. Nevertheless, Herschel described seeing about 200 stars involved and stated that ...the middle was so compressed that it is impossible to distinguish its components.

  Our own Reverend T W Webb described it as A beautiful assemblage of minute stars, 11—15 mag. Greatly compressed in the centre.

  

  When it was included in the 1888 New General Catalogue, it was given the designation NGC 5904.

  M5 lies in the constellation of Serpens Caput, in an area devoid of bright naked-eye stars. It is about 7¾° south-west of 2.6 magnitude α Serpentis, and even less usefully, 29° south-east of Arcturus (α Boötis).

  M5 has an apparent diameter of about 20’ (well over half a full moon) and shines at a very respectable magnitude 5.7, making it technically a naked-eye object under good skies. At declination 2°N, however, it never gets more than 37° above the horizon in mid-Britain. It is, however, an easy binocular object.

  Over 100 variable stars have been identified within M5, 97 of which are the familiar ‘cluster variables’, RR Lyrae-type variables. RR Lyr stars are similar to Cepheid variables and also act as ‘standard candles’ for distance measurement. The distance derived from these standard candles is 7,500 parsecs or 24,500 light-years.

  Studies of the stellar population of M5 have provided some unexpected results. It is often assumed that all the stars in a globular cluster are of the same age; that is, they all formed at the same time. It’s not an unreasonable assumption, given the relative lack of potentially star-forming gas in globular clusters. Studies showed that in fact, M5 hosted not one, but two populations of stars. A more recent study¹ has revealed that there are actually three populations. The majority of stars, spread evenly through the cluster, form the first population. The second and third populations are concentrated close to the centre of the cluster and display higher-than-normal abundances of nitrogen.

  How did these later populations of stars form? It seems that they may be formed from the gaseous wreckage left by the supernova explosions of the most massive members of the first generation of stars. The stars in M5 are literally recycling themselves.

  M5 is a glorious sight in a modest amateur telescope. One feature that stands out is that it is noticeably elliptical – an unusual feature in globular clusters. Stars are scattered across the field like salt on a black cloth. The brightest stars in the cluster are around 12th magnitude. As I have noticed in other bright globular clusters, the individual stars at the centre are lost against the brilliant glow.

  If you tire of little fuzzy galaxies this month, give your eyes a treat and seek out this staggeringly beautiful object.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  M 5	15h 18m 34s	+02° 04’ 58”	Globular cluster	5.7

  References:

  1. Formation of Multiple Populations of M5 (NGC 5904), Jae-Woo Lee 2021 ApJL 918 L24.

• Melotte in Coma Berenices

  April 2024 - Nebula and Cluster of the Month

  This month, we look at the question ‘what does an open cluster look like close-up?’ We have a small collection of nearby star clusters to choose from. The closest is the Ursa Major Moving Group. Arguably no longer a true open cluster, it appears that this stream of stars, all moving in the same direction and at the same speed, were at least once part of an open cluster. At the centre of the group, what would originally have been the core of the cluster, are five of the seven stars of the Plough. The stars at the ends of the asterism, α UMa (Dubhe) and η UMa (Alkaid or Benetnash) are not members, and can be measured as moving in entirely different directions.

  In total, there are 35—40 naked-eye stars (depending on your sky conditions and visual acuity) which are members of the stream, the brightest of which is β Aur at magnitude 1.9. In total, nearly 60 stars are known to be part of the group. The brightest star in the sky, Sirius, was once thought to be a member, but this has now been disproved. The sun lies at the edge of the moving stream but is not itself a member, being many times too old. The stream stretches from Cepheus to Triangulum Australe, so cannot be said to have a great visual impact.

  The closest still-intact open cluster is the Hyades, in Taurus. Lying around 150 light-years from Earth, the Hyades contains about 380 stars, the brightest of which is magnitude 4.0. It covers 5.5° of sky, 11 times the diameter of the full moon. It’s a familiar sight, representing the face of the bull. The first magnitude star α Tau (Aldebaran) is not a member, lying as it does, less than halfway between us and the Hyades.

  Perhaps a better example of what an open cluster looks like close-up is the next closest, Melotte 111. This open cluster lies 100 light-years further away from us than the Hyades, at 260 light-years distant. However, even at this remove, it spans a greater amount of sky, being 7.5° across, giving it a true diameter of close to 35 light-years.

  

  Melotte 111 is of course the asterism that makes up the most prominent part of the constellation of Coma Berenices, and is sometimes referred to as the ‘Coma Star Cluster’. In today’s constellation, it represents the hair of the Greek-Egyptian Queen Berenike II Euergetis, who reigned in her own capacity as queen of Egypt from 258 to 246 BC and as co-ruler with her husband, Ptolemy III Euergetes from 246 to 222 BC. The legend attached to the constellation relates how after the safe return of Ptolemy III from battle, Berenike made an offering of her famed hair (which was amber in colour, apparently) on the altar in the temple of Aphrodite at Cape Zephyrion. This offering subsequently disappeared, most likely stolen. In order to assuage the fury of the king and queen, the court astrologer, a man by the name of Conon convinced the royal couple that the hair had been accepted by the gods and placed in the heavens.

  Before the hair incident (and occasionally for some time afterwards) the open cluster represented the tuft of hair at the end of Leo’s tail or a sheaf of wheat held in the left hand of Virgo. The name ‘Coma Berenices’ was fixed by Tycho Brahe in his catalogue of 1602, and has been in use ever since.

  Current star counts give Melotte 111 some 273 member stars. The brightest, designated 12, 13, 16, 17, 21 and 22 Comae are all visible to the naked eye under good conditions, with the brightest, 12 Com, shining at magnitude 4.8. A curious feature of this open cluster is that it seems to contain no faint stars. There is a surprising cutoff at about magnitude 10.5 (for comparison the Sun at this distance would shine at magnitude 9.2), leading to the conclusion that, most unusually, it contains no red dwarf stars. There are also no giant stars, though the H-R diagram shows some stars just beginning their journey to the giant stage.

  The age of the Coma cluster is calculated at 450 million years, barely a tenth of the age of the Solar System.

  The best option, then, if you want to know what an open cluster looks like close-up, is to step out on an April evening and look up towards Coma Berenices. There, lying midway between α CVn (Cor Caroli) and β Leo (Denebola) lies a twinkling, misty patch. The darker your skies, the better you will see it, of course. See how many individual stars you can discern. If you can see four or five, you’re doing well.

  Then look through a pair of binoculars. It’s a wonderful sight, a northward-pointing arrowhead fills the field of view of standard 10x50s. There it is: a lovely open cluster that is, at least cosmically speaking, right on our doorstep.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  Melotte 111	12h 25m 21s	+25° 59’ 28”	Open cluster	1.8

• NGC 4147 in Coma Berenices

  March 2024 - Nebula and Cluster of the Month

  March is the month when the nighttime side of Earth begins to face away from the Milky Way. Ahead of us are the Spring months, stuffed with distant galaxies. This is good, of course. We all like a nice galaxy.

  It’s not so great for those of us who have to present a cluster or nebula of the month. They are vanishingly rare in the northern hemisphere at this time of year. The southern hemisphere is far better served. Due south of Virgo, and permanently below our southern horizon, lie the glorious Milky Way fields of Crux Australis and Centaurus.

  Here in the north, March offers but four objects relevant to this column, and one of those is a mere asterism (NGC 3231 in Ursa Major).

  Our object for this March is a globular cluster in the constellation of Coma Berenices - more famous for its galaxies. It is somewhat isolated from its fellow globular clusters, being about 70,000 light-years from the galactic centre. The object is NGC 4147 (also catalogued as NGC 4153). It was discovered by William Herschel on the night of 15th February 1784 (and then again, accidentally, on the night of 14th March of that year, accounting for the double entry in the NGC). He thought it to be bright, placing it in his first class (bright objects) as 11H.I and as 19H.I. He anticipated the descriptions of many observers to come by describing it as Very bright, pretty large, gradually brighter towards the middle.

  A century later, the NGC description had barely changed: Globular, very bright, pretty large, round, gradually brighter towards the middle, easily resolvable. From a visual perspective, that about says it all.

  

  A recent study¹ by Sneh Lata of Arybhatta Research Institute of Observational Science in India et al identified 42 variable stars in the cluster, 28 of which were new discoveries. Most were RR Lyrae stars (pulsating variables of spectral type A and F). RR Lyr stars are often called ‘cluster variables’ and are used as standard candles, like delta-Cepheid stars. They are common in globular clusters. The study enabled the team to reassess the distance of the cluster from Earth. Previously believed to be 61,000 light-years, this figure has now been reduced to 56,000 light-years.

  The location and relative remoteness of NGC 4147 suggests that it was once a member of the Sagittarius Dwarf Spheroidal galaxy, which was once a satellite galaxy of the Milky Way before being drawn into our Galaxy and tidally disrupted. Most of its former mass now lies as an elongated stream in Sagittarius. The Sagittarius Dwarf Spheroidal galaxy was believed until quite recently to have contained at least nine globular clusters, the largest and brightest of which is Messier 54. M54 may, indeed, be the actual core of the galaxy. Analysis of Gaia data has, however, revealed at least twenty more.

  For those who like contrasts, NGC 4147 is well placed, not quite equidistantly, between two galaxies, NGCs 4064 (1.4° to the west) and 4293 (2.6° to the east). Whilst these three objects are of comparable magnitude, they are not equally easy to see. I briefly quote here from my own observations:

  NGC 4064 (barred spiral galaxy, mag 10.7) ‘Pretty faint, though... visible to direct vision at x83.’

  NGC 4147 (Class VI globular cluster, mag 10.4) ‘Very bright... brighter middle.’

  NGC 4293 (barred spiral galaxy, mag 10.3) ‘Only very dimly visible to direct vision.’

  This is largely due to a quantity known as ‘surface brightness’. Surface brightness is measured in units of magnitude per square minute or magnitude per square second. I find the former far more useful, though unfortunately, magnitude per square second is widely quoted. Magnitude per square second tends to be down in the 20s, whereas using square minutes seems far more meaningful. For example, the surface brightness of NGC 4064 is 13.3, and of NGC 4293 is 13.2.

  I have noted before that the most important measurement of how visible a globular cluster is, is its concentration class. The classes run from I to XII, in decreasing order of concentration (note that this is incorrectly reversed in the Field Guide to Uranometria).

  NGC 4147 is a class VI globular cluster, placing it right in the middle of the concentration classes. It has a small angular diameter (just about 4’). Taken together, these suggest that NGC 4147 should be fairly easy to see, and so it proves.

  

  My observation of this object was made on a particularly cold night in March 2015 (the object glass of my finder scope was iced over) from my home location in the happy days before the advent of LED street lighting. I recorded a NELM of 5.8 that night. I can barely get to 3.0 these days.

  I followed the path trodden by many previous observers, describing the globular cluster as Pretty bright but not particularly large. Brighter middle. Not resolved but spangly. No resolution even at x375.

  The brightest star in the cluster is of magnitude 14.5, so resolution was a little beyond my equipment and conditions. Can you do better?

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  NGC 4147	12h 10m 06s	+18° 32’ 32”	Globular cluster	10.4

  References:

  1. Photometric observations detect 28 new variable stars in NGC 4147 (2019, May 29) retrieved 26 February 2024 from https://phys.org/news/2019-05-photometric-variable-stars-ngc.html

• Messier 44 in Cancer

  February 2024 - Nebula and Cluster of the Month

  February is a month particularly devoid of clusters and nebulae in the northern hemisphere. Apart from the two Messier-listed clusters, there is virtually nothing to be seen. Those of us with access to the southern skies have a much better deal of it this month. If you are in the UK but have exceptionally good skies and a clear southern horizon, you may just be able to glimpse the lovely NGC 2627 in Pyxis, though at a declination of -30°, this will be available to very few in the UK.

  We UK-bound observers therefore have a choice of two open clusters to observe this month. I covered M67 last year, so this year we turn to the second and final object, the wonderful M44, called since ancient times ‘Praesepe’, a strange-sounding Latin word that means ‘manger’ or ‘cot’. The two stars that nestle alongside the cluster, one to the north and one to the south, γ and δ Cancri, are called Asellus Borealis and Asellus Australis, respectively. The word ‘Asellus’ is Latin for donkey or ass. Apparently, these two stars and the cluster (which remember can only have looked nebulous to pre-telescopic eyes) represent the two donkeys and the manger from where they came, on which Dionysus and Silenus (both presumably terribly drunk at the time) rode into battle against the Titans in Greek legend. The braying of the terrified donkeys so scared the Titans that they scattered and were easily defeated by the gods (at least according to Eratosthenes). Make of that what you will.

  

  The object is listed by Hipparchus in his star catalogue of 130BC as a ‘cloudy star’. Other prominent pre-telescopic observers who catalogued it were the Greek Ptolemy, the Persian Ulugh Beg, the Danish Tycho Brahe and the Polish Johannes Hevelius.

  Galileo turned his little telescope to it in 1609. He was the first to see its true nature as a cluster of stars, writing ‘The nebula called Praesepe contains not one star only but a mass of more than 40 small stars. We have noted 36 beside the Aselli.’ This demonstrates that the small telescope constructed by Galileo could reach down at least to magnitude 9.5.

  Charles Messier published his first catalogue in 1771. It contained 45 objects, the final two of which seem to have been added (some may say cynically) to make up the numbers to a multiple of five. Messiers 44 and 45 had both been known since antiquity and there really was no need to include two such obvious objects that no comet-hunter worth their salt would ever confuse for a comet.

  On to the cluster itself, then. M44 is nestled between and slightly to the west of γ and δ Cancri, magnitudes 4.7 and 3.9, the aforementioned ‘Aselli’. These two stars in turn lie at the centre of the wonky-K shape of Cancer. Even if it wasn’t so bright, M44 would still be easy to find.

  M44 shines at magnitude 3.1, making it a naked-eye object from all but the worst light-polluted sites. Diameter estimates vary from 70’ to 95’, so this object will only fit in the lowest-power fields. Rich-field telescopes show it best, but it looks fabulous in binoculars as well, especially if they are mounted.

  The number of member stars varies considerably from source to source. Uranometria states a mere 50, Archinal & Hynes stretch to 161, whereas Wikipedia plumps for a far more impressive 1,000.

  The Trumpler classification is II3m (detached with little central condensation, wide magnitude range of member stars, moderate richness (50 – 100 stars)).

  The brightest star in the cluster is magnitude 6.3.

  The distance has recently been refined by the Hipparcos satellite to 182 parsecs, or about 590 light-years, making this one of the closest open clusters to us. Interestingly, another nearby open cluster, the Hyades in Taurus, shares the same proper motion and age profile as M44, though the Hyades might be slightly older (625My as opposed to 600My for M44). This suggests to some that the two clusters have a common origin.

  Visually, M44 is one of the more impressive open clusters, although as mentioned, it does need a rich-field or very low power, as it’s so big.

  The first impression I get is always that the centre of the cluster is defined by a keystone-shaped asterism, very much like a smaller version of the Hercules keystone. In fact, the whole cluster looks a little like a smaller version of Hercules. The brightest stars, of sixth and seventh magnitude, are brilliant, but as the Trumpler classification suggests, there are stars of all brightnesses. The brightest stars are of spectral type A, F and G, giving the cluster a little colour. Doubles and multiples abound, making this object a delight to spend a little time studying. The higher power eschewed for the overall view can now be employed to investigate the many doubles and multiples.

  Richard Hinkley Allen in ‘Star Names and Their Meanings’ reports that the Chinese name for M44 translates as ‘The Exhalation of Piled-Up Corpses’. Which is nice.

  M44 sits in front of a rich stream of galaxies, though the brightest of these, NGC 2624, is just magnitude 14.9. All the others are below 15th magnitude. If you have good skies and a large telescope, there is much here to investigate, though of course, that’s beyond the scope of this article.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  M 44 (NGC 2632)	08h 40m 22s	+19° 40’ 17”	Open cluster	3.1

• NGC 2281 in Auriga

  January 2024 - Nebula and Cluster of the Month

  Let me start by wishing you all a very happy new year. Let’s hope for dark, clear skies and a complete ban on unnecessary outside lighting. You may say I’m a dreamer, but I’m not the only one.

  As the new year opens, we’re going to look at a modest but pleasing little open cluster in Auriga. It culminates at midnight on the 5th January, when it is at an altitude of 77° from mid-Britain.

  Like so many of the objects featured in this column, it was discovered by William Herschel. Herschel discovered over 2,500 deep-sky objects, and naturally, these tend to be the brightest and most easily distinguishable. Hence so many of the objects we look at are his discoveries.

  He first saw this cluster on 4th March 1788. At that time, Herschel described it as A cluster of coarsely scattered, pretty large stars. Pretty rich. He placed it in his class VIII, ‘Coarsely scattered clusters of stars’ as no. 71.

  

  A study¹ was made of the cluster during 2020, during which the distance from Earth was calculated as 500—560 parsecs (1630—1830 light-years) and its age as 630 million years.

  The study also identified an eclipsing binary star, GSC 2945-01857. This (generally) 12.3 magnitude star lies 17’ from the centre of the cluster, and the authors of the paper admit that the probability of this star actually being a member of the cluster is only 18%, statistically speaking.

  NGC 2281 lies in a patch of sky rather barren of bright stars. It forms an isosceles triangle with and to the east of β and θ Aurigae, magnitudes 1.9 and 2.6 respectively, and sits amid the scattering of stars that all bear the designation ψ Aurigae. Specifically, the cluster is just about quarter of the way between ψ7 and ψ3. These two stars are magnitudes 5.0 and 5.3.

  Most authorities agree that the combined magnitude of the cluster is 5.4, and generally concur that the Trumpler classification should be I3m (well detached, range of bright and faint stars, moderately rich (50—100 stars), though The Night Sky Observer’s Guide assigns it a ‘poor’ richness rating (fewer than 50 stars).

  Uranometria and The Night Sky Observer’s Guide both assign the cluster 30 member stars, of which the brightest is magnitude 7.3, whereas Star Clusters by Archinal and Hynes give it 119 members, the brightest of which is given a magnitude of 8.0.

  Estimates of its angular size on the sky also vary, with Uranometria and The Night Sky Observer’s Guide giving 14’ whilst Archinal and Hynes nearly double that at 25’.

  It’s always worth checking the proper motions of stars in and around open clusters. This can give a much clearer insight into how large the cluster is and, indeed, whether it is a true cluster or simply a coincidental line-of-sight arrangement.

  The proper motions around NGC 2281 (below) show clearly that this is a true cluster of gravitationally-bound stars and that the bounds of the cluster lie well outside that marked on the chart.

  

  Visually, NGC 2281 is bright and despite the lack of bright signpost stars, is simple enough to find. At magnitude 5.4, it should be easily visible in a finder scope.

  Moderate telescopes (8”—16” aperture) will reveal two to three dozen stars, the brightest four clumping at the centre.

  I made a written observation of NGC 2281 in November 2013 with my 12” (300mm) Newtonian. The description in my records reads A pretty little cluster maybe 15’ across. There seem to be two dozen members or so. It looks like a cavorting fish with the snout pointing north and the thrashing tail pointing east. The brightest stars (9m) form a rhomboid at the junction of the body and the tail. The stars of the body are about mag. 10.

  Although I didn’t make a drawing of the cluster at the time, it has remained quite vivid in my memory because of this fish-like appearance.

  Have a look. See if you can see the fish.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  NGC 2281	06h 48m 20s	+41° 04’ 48”	Open cluster	5.4

  References:

  1. Terrell D, Gross J, Cooney WR Jr. Analysis of the Open Cluster NGC 2281. Galaxies. 2021; 9(1):7. https://doi.org/10.3390/galaxies9010007