Nebula and Cluster of the Month Archive 2023

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.

• Messier 4 and Messier 80 in Scorpius

  May 2023 - Nebula and Cluster of the Month

  Once more we must resort to the southern sky to provide us with material for this month. For May, we’re going to compare and contrast two globular clusters, both of which should be familiar even to northern hemisphere observers.

  Both lie in the constellation Scorpius, only three degrees apart, and both are in Charles Messier’s list of objects to avoid whilst comet hunting.

  The two globular clusters are Messier 4 and Messier 80. Fundamentally similar, but with interesting and illuminating differences.

  M4 was discovered by Philippe Loys de Chéseaux (1718-1751) in 1746. He wrote, of his entry no 19, Close to Antares... it is white, round and smaller than the preceding ones. I do not think it has been found before.

  Messier saw it on 8th May 1764, writing Cluster of very small stars: with an inferior telescope it appears more like a nebula; this cluster is situated near Antares and on the same parallel... diameter 2½’.

  

  It is interesting to note that Messier could resolve this globular cluster into stars, despite the allegedly poor quality of his instruments. The brightest stars in M4 nearly reach tenth magnitude (with a proviso that I’ll come to later), making the task of resolution somewhat easier than normal.

  M80 was discovered by Messier himself, on 4th January 1781. He described his discovery in these words: Nebula without star in the Scorpion, between the stars g [now called rho Ophiuchi] and delta; ... This nebula is round, the centre brilliant and it resembles the nucleus of a little comet, surrounded with nebulosity.

  William Herschel, on observing M80 and the star field around it, wrote: An opening in the Heavens... This opening is at least 4° broad but its height I have not yet ascertained. It is remarkable that [M80], which is one of the richest and most compressed clusters of small stars I remember to have seen, is on the western border of it and would almost authorise a suspicion that the stars of which it is composed, were collected from that place and had left the vacancy. A fascinating insight into his ever-theorising mind.

  

  On paper, M4 is the larger and brighter of the two. It is magnitude 5.8 and has a diameter of about 30’ or so, the same size as the Moon in the sky. Needless to say, barely half of this diameter will actually be visible to an observer at the eyepiece. M80, on the other hand, is considerably fainter at magnitude 7.3 and covers just 10’ of sky, though again, barely half of this will be visible to a visual observer.

  M4 is 5,500 light-years away (though estimates vary) and is the closest globular cluster to us. M80, on the other hand, is 32,600 light-years away – beyond the galactic centre (again, estimates vary).

  On the face of it, then, M4 would seem to be the better bet. In his 1968 book Messier’s Nebulae and Star Clusters, Kenneth Glyn Jones writes: ...in the amateur’s telescope it [M4] appears large and bright and much more conspicuous than near-by M.80.

  He clearly suffered from far less light pollution than most amateur astronomers today. I have seen M4 from a dark-sky site in Britain through 10x50 binoculars with no difficulty. However, from my home location I have never seen it at all. I can barely see Antares. I have, on the other hand, seen and made an observation of M80. The problem has two causes. Firstly, M4 is slightly lower in the sky than M80 (by about 3½°), but more importantly, M4 is a very loose globular, whilst M80 is one of the densest known. In the Shapley-Sawyer concentration classification system, M4 is a class IX object, whilst M80 is a class II. I have written before that the most important metric in determining the visibility of a globular cluster is its concentration, and this is a prime example.

  There is a chain of stars that passes North-South across the centre of M4. These stars are mostly 11th and 12th magnitude, but the brightest is magnitude 10.6. This feature is sometimes referred to as a ‘bar’. It was first noted by William Herschel and can be seen with modest telescopes quite easily, given a good enough sky. This will be what Charles Messier saw back in 1764. The Deep-Sky Field Guide to Uranometria 2000.0 gives the brightest star magnitude for M4 as 10.8 – the star I mentioned before. Archinal & Hynes’ Star Clusters, however, gives the magnitude of the brightest star as 13.4. It would seem, then, that the ‘bar’ is considered (at least by them) to be in the foreground and not part of the globular cluster itself.

  

  I’ve included my observation of M80. It isn’t exciting, but I was pleased to get it. I have observed objects – even a few globular clusters – more southerly than M80 (M30, M22, M79, M28), but it seems that my line of total invisibility lies somewhere between M28 (-24° 52’) and M4 (-26° 32’).

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  Messier 4	16h 23m 35s	-26° 31’ 34”	Globular cluster	5.8
  Messier 80	16h 17m 03s	-22° 58’ 29”	Globular cluster	7.3

  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 5286 and NGC 5307 in Centaurus

  April 2023 - Nebula and Cluster of the Month

  We remain in the southern skies for April, being banished from more northerly declinations by the great sweep of the Virgo galaxies.

  This month we’re looking at a cluster and a nebula – globular and planetary respectively, that lie just 44’ apart in the constellation of Centaurus.

  We’ll start with NGC 5826, a class V globular cluster which shines at magnitude 7.4. It lies about ⅓ of the way along the line joining ε and ζ Centauri, right next to a 4.6 magnitude orange-yellow star known as M Centauri.

  

  NGC 5826 was first catalogued (as Δ388) in 1827 by James Dunlop (1793 – 1848), a Scottish astronomer working from Australia. It is a class V globular cluster, on the concentration scale with I as the most compressed and XII as the loosest (contrary to the misprints in the first edition of the Deep-Sky Field Guide to Uranometria, which would have it the other way round).

  The globular cluster lies at a distance of approximately 9,300pc (35,000 light-years) and is one of at least eight that were inherited by the Milky Way when it consumed the dwarf Gaia-Enceladus-Sausage galaxy (yes, seriously, that’s what it’s been called) around ten billion years ago. This merger, the most recent major one, was vital in the evolution of the Milky Way, transforming the Galaxy from a metal-poor, thin-disc galaxy to a thick-disc galaxy, with dust from the dwarf galaxy triggering a new wave of star formation. Most of the metal-rich material in the halo of the Milky Way was donated by the Gaia-Enceladus-Sausage.

  The material gained from the merger is now spread all around the galaxy, as demonstrated particularly by the distribution of globular clusters. Globulars that have been identified as likely coming from the Gaia-Enceladus-Sausage are M2 (Aquarius), M56 (Lyra), M75 (Sagittarius), M79 (Lepus), NGC 1851 (Columba), NGC 2808 (Carina), NGC 2298 (Puppis) and NGC 5286. Of all of them, NGC 2808 is by far the most massive, containing about a million stars. It may be that NGC 2808 is the core of the old galaxy.

  Visually speaking, NGC 5286 is bright, with an overall magnitude of 7.4 making it an easy binocular target. It is variously quoted as having a diameter of between 9’ and 11’. The brightest star is magnitude 13.5. Brighter stars are projected against the cluster, and the triple star comprising three 12th magnitude stars in the centre of the cluster is not a member.

  44’ away from NGC 5286, slightly north of east, lies our second object, the planetary nebula NGC 5307. Although apparently much smaller and fainter than its neighbour, NGC 5307 is just one-third the distance from us as NGC 5286. It was discovered by John Herschel on 15th April 1836, its first designation being h3541. His discovery description from his ‘Cape Observations’ is worth quoting:

  ‘PLANETARY NEBULA. A very singular object. At first I thought it an ill seen double star; 12 . 13 [magnitudes] ... dist 2”; but not being able to get it into focus I applied 320 [magnification]; which showed it as a hazy, rather elongated planetary nebulous disc, as if a double * all but obliterated. It is positively not a star. The field is full of stars, two of which... are equal to this object in light, but 320 shows them both quite sharp. It is a difficult object to find, and unless in a good night for definition (this [night] is superb) it could not be recovered. ... It is the smallest and most difficult planetary nebula I have seen. (N.B. ...it would seem rather to belong to the class of double nebulae or double stellar nebulae of the utmost remoteness, than that of planetary nebulae, properly so called)

  

  NGC 5307, despite Herschel’s slight misgivings, is a planetary nebula, properly so called. A notable feature of the nebula is that it displays ‘point-symmetry’. This means, basically, that for every blob on one side of the nebula, there is a corresponding blob on the other. This can be explained by a precessing bipolar jet. As the jet spits out a large blob of material, it is expelled in opposite directions simultaneously, leading to the symmetrically placed blobs that are visible in the ESA/Hubble image.

  In his book, Visual Observations of Planetary Nebulae, Kent Wallace states that he has seen the blobs (which he calls ‘nubs’) in the nebula, through a 20” reflector.

  The central star displays weak emission lines and has a spectral type of O(H)3.5V

  In a paper published in 2003¹, Teresa Ruiz et al determined, amongst other things, that the expansion velocity for the nebula is 15 kms^-1, which is low.

  The nebula is elongated, with dimensions of about 20” in length and 15” in width. The visual magnitude is 11.2. As it is so small, it is likely that an OIII filter (or a UHC filter at a pinch) will be needed to identify the object as a planetary nebula, though it should be plainly visible, if stellar, without a filter.

  A very interesting pair of objects, then, that should both lie within the field of view of a medium-power eyepiece.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  NGC 5286	13h 46m 27s	-51° 22’ 28”	Globular cluster	7.4
  NGC 5307	13h 51m 03s	-51° 12’ 19”	Planetary nebula	11.2

  References:

  1. Very Large Telescope Echelle Spectrophotometry of the Planetary Nebula NGC 5307 and Temperature Variations. Astrophysical Journal, 595: 247-258, 2003 September 20

• NGC 3532 in Carina

  March 2023 - Nebula and Cluster of the Month

  Last month, I mentioned that this time of year is a poor one for open clusters and nebulae, the northern sky being dominated by the great galaxy fields of Leo, Coma and Virgo. This month is no different as far as the northern hemisphere is concerned, but things are very different in the southern hemisphere, where we must travel for this month’s object.

  During summer, the rich Milky Way constellation of Carina lies high in the sky of the southern hemisphere. The southern Milky Way is richer and brighter than in the northern hemisphere because the Sun is located slightly north of the galactic plane meaning, simply, that there is more ‘below’ us than ‘above’. The galactic equator runs right through eastern Carina (close by Crux Australis) and our open cluster for this month, NGC 3532, lies in amongst this bewildering array of stars and nebulae.

  

  NGC 3532 shines at magnitude three, so to claim that Nicolas-Louis de Lacaille ‘discovered’ it is an obvious fallacy. That he was the first European astronomer to catalogue its existence cannot be doubted. Lacaille travelled to South Africa in 1750 and remained there until 1754, making many important astronomical and geographical observations. He named several southern constellations, fourteen of which are still in use today.

  His observation of what is now known as NGC 3532 was made in 1755 and published in the second of his three catalogues, originally having the designation Lacaille II.10. He describes it as a Prodigious cluster of small stars, very compressed, filling the figure of a semi-circle of 20 to 25 arc min in diameter.

  John Herschel observed the cluster during his own sojourn in South Africa and was clearly impressed: A glorious cluster of immense magnitude, being at least 2 fields in extent every way. The stars are 8, 9, 10 and 11 mag, but chiefly 10 mag, of which there must be at least 200. It is the most brilliant object of the kind I have ever seen. This is the closest you can get to saying that this is the best open cluster in the sky.

  When Herschel’s deep-sky discoveries were condensed into the General Catalogue in 1864, the description was reduced to !! Cluster, extremely large, rich, little compressed, stars 8...12 which remained unchanged for the New General Catalogue of 1888.

  The Trumpler classification is almost universally given as II3r, meaning that the cluster is detached from its background but with only a small degree of central condensation, there is a wide range of magnitudes amongst the member stars and that it is rich (more than 100 stars). The diameter is quoted as being between 50 and 60 arcmin, making it twice the diameter of the full moon.

  The number of stars is (as always) a matter of much disagreement. The Deep-Sky Field Guide to Uranometria gives 150 members whilst Archinal & Hynes, with their usual precision, give 677. In a paper published in Astronomy & Astrophysics in 2019, Fritzewski et al.¹ describe their work to characterise 2230 stars within 17deg; of the centre of NGC 3532, and find a preliminary membership of the cluster of 660 stars, with a confident prediction that the reality probably lies in excess of 1000, readily admitting that their survey is incomplete as they inspected no stars below magnitude 19.1. The Sun would be magnitude 18.2 at that distance.

  The distance to open clusters is also frequently a matter of some contention. Archinal & Hynes confidently quote 478pc (1559ly). The European Southern Observatory, in their release of a colourful image of the cluster, quote 1300ly. The authors of the above-mentioned Astronomy & Astrophysics paper independently determined a distance of about 484pc (1578ly). These values are all broadly in agreement.

  The age of the cluster has been determined by several authorities to be 300±50 million years.

  All the stars in an open cluster form at the same time (on a cosmological scale). Therefore, they make excellent laboratories for studies of stellar evolution. Being relatively close and having a rich population, NGC 3532 is an exceptionally fine example. The stars are all the same age, yet we see a wide variety of types. There are Sun-like stars, red giants, white dwarfs, everything in-between and almost certainly many red dwarfs, though these would be exceptionally faint.

  The difference is caused by the initial mass of the stars. The more massive a star, the more quickly it will evolve.

  Visually, NGC 3532 is stunning. I have not seen it through a telescope, though I have seen it through binoculars from the pristine skies of the Australian Outback. To the naked eye, it appears as a bright smudge, stretching to the west of 3.9 magnitude V382 Carinae. Binoculars show a large cloud of stars, several of which stand out as orange in colour. The whole cluster sparkles. This area is fabulous in binoculars. Open clusters abound and less than 3° away is the fantastic η Carinae nebula, which shows its huge V-shaped dark lanes even in 10x50 binoculars.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  NGC 3532	11h 05’ 45”	-58° 43’ 41”	Open cluster	3.0

  References:

  1. Astronomy & Astrophysics, Volume 622, February 2019

• Messier 67 in Cancer

  February 2023 - Nebula and Cluster of the Month

  It has to be admitted that February is a very poor month for nebulae and clusters. The Milky Way has passed its winter zenith, and the faint galaxy fields that precede the richer hunting grounds of Leo dominate the midnight sky in the middle of the month.

  Culminating this month is the ancient constellation of Cancer, the Crab. From a naked-eye point of view, it’s a fairly unremarkable area of sky. Dark skies show a wonky ‘K’ shape made of fourth-magnitude stars. A dark sky will also show one of the few deep-sky objects known to the ancients, the open cluster we now call M44.

  Charles Messier catalogued another open cluster in this constellation, number 67 in his list of nebulae and star clusters published in the Almanac Connaissance des Temps (‘Knowledge of the Time’), first including M67 in the list of 68 objects published in 1783.

  

  Messier was not the first to see M67, nor the first to publish its presence. That honour goes to Johann Gottfried Koehler (1745-1801), a contemporary of Messier who discovered 20 objects, including M59, M60 and M 67. Although it isn’t clear exactly when he first discovered the object, it must have been between 1772 and 1779. He published his discoveries in the Astronomisches Jahrbuch in 1780. His description reads A fairly discernible nebula of oblong shape near alpha Cancri. It would seem that Koehler’s telescope wasn’t up to resolving the cluster into stars.

  Messier observed the cluster on 6th April 1780, writing A cluster of small stars with nebulosity below the southern claw of the Crab. Messier’s telescope, then, was slightly (but only slightly) superior to Koehler’s.

  William Herschel observed it in 1783. He wrote A very beautiful and pretty much compressed cluster of stars, easily to be seen by any good telescope and in which I have observed above 200 stars at once in the field of view of my great telescope with a power of 157.

  Curiously, in a later note, penned in 1809, he wrote A cluster of very small stars. There seems to be a faint milky nebulosity among them.

  M67 lies at a distance of about (measurements vary) 800 – 900pc (say 2,700 ly). It is well-populated with stars. Again, estimates vary, but Archinal & Hynes give a very precise 324. It covers a diameter of about 25’ on the sky, though the ‘obvious’ members cover only about 12’. The Trumpler classification is given usually as II2r or II3r (detached from the background, little central condensation, moderate (or wide) range in the brightness of the individual stars, rich). Older catalogues occasionally class its richness as medium, instead of rich. These can be safely ignored. The late Kenneth Glyn Jones (a former president of the Webb Society) in Messier’s Nebulae and Star Clusters (1968) writes M. 67 is known to contain 500 stars between mag. 10 and mag. 16 and a very large number of stars which are fainter still.

  M67 lies well above the plane of the galaxy, which has preserved the cluster during a lifetime that would see clusters that live within the disruption of the galactic plane long evaporated. It is not the oldest open cluster known, but it is one of them. It’s the closest old cluster to us and as such has been extensively studied. Many of the stars within it are solar mass stars about the same age as the Sun, making it an ideal laboratory for the study of Sun-like stars. Over 100 stars of this type have been discovered in M67. About 15% of those stars are relatively quiescent, showing far less magnetic activity than our Sun, whilst 30% show activity significantly greater than that of the Sun at maximum.

  A survey of 20 solar-like stars within the cluster determined the spin rate of each to be approximately 26 days, very comparable with the Sun’s 25.4 days.

  Three exoplanets have been found in M67, around different stars, one of which is of solar type. All these planets are hot Jupiters.

  Visually, M67 is a very pleasing object. It is easy to find, being just 1.5° due west of α Cancri. With a total integrated magnitude of 6.9, it stands out well in binoculars. In a small telescope the cluster is compressed and spangly, with many stars of around 10th magnitude. Larger apertures will reveal a wealth of fainter stars, down to around 15th magnitude. The overall shape, I find, is reminiscent of a goblet or a fruit bowl. The main part of the bowl is represented by a reverse ‘C’ shape which opens up to the west and in a small telescope appears almost devoid of stars, with the exception of a delicate little triple star. In larger apertures this shape is rather obscured by a number of fainter stars. Chains and curves of stars abound here, making M67 a very satisfying cluster to observe.

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  M 67	08h 51’ 19”	+11° 49’ 17”	Open cluster	6.9

• NGC 2245 and Herschel 1 in Monoceros

  January 2023 - Nebula and Cluster of the Month

  I would like to start by wishing you a very happy New Year. 2022 was pretty rubbish, and while it might seem optimistic to hope that 2023 will be any better, this is at least the time of year for optimism.

  The sky in January continues to be dominated by the bright Winter constellations. Culminating at midnight throughout most of the month, the dim constellation of Monoceros, the Unicorn, will be our focus for this month. If you’re not sure exactly where Monoceros is, it’s to the left of Orion, roughly the area between Sirius and Procyon.

  Monoceros, although not visually stunning, is packed with interesting Milky Way objects. We’re going to be looking at two of the smaller objects this month, although technically, our second object is marginally over the border in Canis Minor.

  Firstly, a reflection nebula, NGC 2245, situated in the northwest corner of the constellation, close to the borders with Orion to the west and Gemini to the north, 9½° ENE of Betelgeuse. It lies in a field wreathed in faint nebulosity punctuated with small bright sections like itself. It was discovered by William Herschel on 16 January 1784. He described it as Pretty bright. Much like a star with an electrical brush. He placed it in his catalogue of planetary nebulae as number 3.

  

  Although it must be made absolutely clear that Herschel was the first person to use the term ‘planetary nebula’, and so would be entitled to use the term to describe whatever he wanted, he ascribed no physical properties to it, beyond stars with burs, with milky chevelure, with short rays, remarkable shapes, etc. It was simply a useful tag for certain objects that looked similar to each other. This idea of a ‘brushed star’ is typical.

  The term ‘planetary nebula’ is now defined much more scientifically. Of the 79 objects in Herschel’s class IV – planetary nebulae, only 20 fall within the modern definition.

  NGC 2245 is in actuality a reflection nebula. It is small, 5’x4’. At its heart lies a 10.4(var) magnitude star. The star is variable and bears the designation V699 Mon. It is an Orion-type variable of spectral type A or B and varies between magnitudes 10.3 and 10.8.

  The region more-or-less defined by the three bright nebulae NGC 2245, NGC 2247 and IC 446 contains the Monoceros R1 association, a region of young stellar objects (YSOs). Supersonic outflows from YSOs collide with the interstellar material and produce shocked excitation zones known as Herbig-Haro (HH) objects. Mon R1 contains at least 30 YSOs, and many HH objects, 20 of which were discovered by a survey conducted with the 1m Schmidt telescope at Byurakan Observatory in 2020.1

  I observed NGC 2245 with my trusty 12” (300mm) reflector in December 2015. My observation is reproduced below.

  

  The description reads ‘A small, bright nebula around a tenth-magnitude star, stretching away from it like a short cometary tail.’ In fact, NGC 2245 is a representative of a type of reflection nebula called ‘cometary nebulae’ because of their resemblance to comets.

  Our next object is a little-known open cluster bearing the unusual designation of Herschel 1. I first came across this object when scanning the Catalogue of Optically Visible Open Clusters. OVOC 578 is listed with the alternative designation of Herschel 1, but no other.

  

  Initially, I thought that this object must have been discovered by an astronomer (maybe an amateur) whose name just happened to be Herschel. On digging a little deeper, I found that the discoverer was in fact John Herschel (the famous one). He discovered it in 1827 but for some reason, it didn’t make it into his own catalogues. He first alluded to its existence in a very brief mention in a paper published in 1871. Such scant fanfare meant that the object was subsequently never picked up for inclusion in the NGC.

  In Archinal & Hynes, it is listed as ADS 6866 cl, which simply means a cluster associated with double star ADS 6866 (also known as Σ1141). This double star (actually a triple) was discovered by Herschel at the same time as he first saw the cluster. Receiving the same neglect as the cluster, it was rediscovered and subsequently catalogued by Struve, hence his credit as its discoverer.

  The upshot of all this is that here is a little open cluster that should have been in the NGC but very few people have ever heard of.

  I have looked at the proper motions available to me in this area. At least seven stars across an area of about 13’ square have very similar motions, a strong indicator of physical association. At the centre of this is a small group of five brighter stars, four of which (including the bright double star) share a common proper motion. There is a high likelihood, therefore, that this is a real cluster.

  Intrigued by this object, I observed it in January 2017 with my 12” reflector. Although I made no drawing, I did make a written observation of it. I found a tight little group of nine or ten stars dominated by a fairly bright double star, which I later found to be Σ1141.

  Whilst very few people have heard of this little open cluster, even fewer have actually seen it. Go on, join them!

  Patrick Maloney

  Object	RA	Dec	Type	Magnitude
  NGC 2245	06h 32’ 41”	+10° 09’ 24”	Reflection nebula	-
  Herschel 1	07h 47’ 02”	+00° 01’ 00”	Open cluster	Br * = 8.4

  References:

  1. New Herbig-Haro objects and outflows in the Mon R1 association, T. A. Movsessian, T Yu Magakian, S. N. Dodonov. Mon. Not. R. Astron. Soc. 500 (2), 2440-2450 (2021)