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.
The Pleiades in Taurus
November 2023 - Nebula and Cluster of the Month
November is dominated by the arrival of the brilliant winter constellations. Leading the pack is the beautiful open cluster, the Pleiades. This little shining web of stars is so familiar that deep-sky observers tend to ignore it. It’s too bright, it’s too easy, it won’t fit in the field of view, etc. BORING!
But no, it’s not boring. The Pleiades have been important for pretty much the whole of human history (and prehistory). In ancient Greece, the heliacal rising of the Pleiades (the first time they could be seen in the pre-sunrise eastern sky) signalled the start of the sailing season and also the agricultural year.
The Nebra Sky Disc, discovered in Germany in 1999 and dated to 1600BC shows a cluster of seven stars which is believed to represent the Pleiades.
The Pleiades appear in myths all around the world, and these myths are surprisingly similar. In the familiar Greek legend, the Pleiades were the seven nymph daughters of the Titan Atlas, whose job it was to carry the Earth on his shoulders. With this burden, he was unable to protect his daughters so Zeus (who sired children on more than one of the sisters), turned them into stars to protect them from the depredations of the hunter and woman-chaser, Orion.
Australian Aboriginal legends speak of the Pleiades as seven young girls, fleeing either a hunter (represented again by the constellation Orion) or fleeing several young men (also represented by Orion).
This and remarkably similar myths occur in nearly all cultures across the world. How could Australian Aboriginal legends be so similar to Greek ones? Was the Greek legend imported into Australia some time in the past 200 years? It would seem not. The Australian myth seems to be of very considerable antiquity.
The similarity of so many legends from disparate cultures is so remarkable that Australian astronomers Ray and Barnaby Norris have suggested1 that the legend originates in Africa, before the spread of modern humans across the globe. They suggest that the legend is at least 100,000 years old, making it the oldest of all humanity’s stories.
From an astronomical viewpoint, the Pleiades is one of the closest open clusters to us, being about 400 light-years away. It’s a young cluster, the bright stars all being B-type giants. Although the cluster is best known for its six (or seven) brightest stars, there are in reality about 100 stars visible in a small telescope. This number rises to over 1000 with proper motion and distance studies of the stars in the vicinity. Imagine a roughly conical cloud of ordinary stars (spectral types A to K) with a long chain of brilliant blue stars projecting out of its wide base, like an umbrella or a mushroom. We are lined up with this chain of bright stars, so we see them clumped together. This is what the Pleiades actually look like.
The individual brightest stars all have names, the names given to the seven sisters and their parents in Greek mythology.
Ten brightest stars in the Pleiades Star Name Magnitude Spectral Type η Tau Alcyone 2.8 B5 16 Tau Celaeno 5.5 B5 17 Tau Electra 3.7 B5 19 Tau Taygeta 4.3 B5 20 Tau Maia 3.9 B5 21 Tau Asterope (Sterope I) 5.8 B8 22 Tau Sterope II 6.4 B9 23 Tau Merope 4.2 B5 27 Tau Atlas (the father) 3.6 B8 28 Tau Pleione (the mother) 5.0 B8
21 and 22 Tau are separated by only 2’, and can’t really be distinguished from each other by the naked eye. Combined, they form the star called Asterope, but individually they have been named Sterope I and Sterope II.
Through a telescope (other than a wide-field instrument), the Pleiades can be a little underwhelming, being too large to fit in the field of view. At best you can get two or three of the bright stars in. In large binoculars or a wide-field telescope, however, they are breathtaking.
The Pleiades are also famous for the bright blue nebulosity in which they are wreathed. This is caused by light from the brilliant blue stars reflecting off dust particles. It was once thought that the nebulosity represented the remnants of the material from which the cluster was formed, but what we’re actually seeing here is a dusty region of the interstellar medium that the Pleiades are currently passing through. It is unrelated to the cluster.
The brightest part of this nebula is around the star 23 Tauri (Merope). It bears the designation NGC 1435 and given the right conditions is not difficult to see, even with a small telescope. The sky must be dark and this is the biggest stumbling block to seeing the nebula.
I have never seen the nebulosity from a suburban site, despite much looking. However, I was amazed by just how clear it was from a dark-sky site. The NELM was 6.0 (5 Aurigae at an altitude of 60°). I was using my 12” (300mm) Newtonian reflector with a 25mm eyepiece which gave a magnification of x60 and a field of view of 50’. I used no filters.
NGC 1435 was visible as a quite distinct fan-shaped nebula with Merope at its apex. Although nowhere hard to see, it was clearest at its edges. It fades away imperceptibly to the west. It does not look blue!
If you are fortunate enough to be in a dark-sky location with your telescope and the Pleiades are visible, do have a go at seeing the nebulosity. If the nebula around Merope is clear, scan over the whole cluster. You may get to see some of the fainter wisps.
The Pleiades can be enjoyed, therefore, with everything from your naked eye up to a large telescope.
It is a magical thing simply to look up at the Pleiades in wonder and to feel a distant, tenuous but real connection to our long-distant ancestors as they stared in awe into the dark African skies and, not understanding what they saw, first began to tell stories.
Object RA Dec Type Magnitude NGC 1435 03h 46m 20s +23° 57’ 00” Reflection nebula within the Pleiades
- Advancing Cultural Astronomy, Springer 2020.
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 663 in Cassiopeia
October 2023 - Nebula and Cluster of the Month
During October, the galactic equator passes very nearly overhead, bringing with it a wealth of open clusters. Nowhere is this more evident than in Cassiopeia. The galactic equator passes just north of the ‘wonky W’ until it cuts the asterism roughly halfway between the stars δ and ε Cassiopeiae.
The sky is full of open clusters great and small in this region and proves a rich hunting ground for the cluster-seeker. One of the largest and brightest of the objects in this region is NGC 663, located about 2,200 parsecs (7,200 light-years) away from us, in the Milky Way’s Perseus arm. It is probably part of the Cassiopeia OB8 association. Other open clusters in the region within the association are NGCs 654 & 659 and M103.
It was discovered, of course, by William Herschel, who first saw it on 21 September 1788. He described it as
A beautiful cluster of pretty large stars near 15’ diameter. Considerably rich.He placed it in his class VI ‘Very compressed and rich clusters of stars’ as number 31.
As is typical, the Trumpler type for this object varies with the source. The Deep-Sky Field Guide to Uranometria 2000.0 (once its text is converted to a Trumpler type) gives II3r, meaning that the cluster is well detached but has little central condensation, that the magnitudes of the member stars are widely ranged and that the cluster is rich (i.e., over 100 stars). The number of member stars is given as 80 (contradicting their own definition) and the magnitude of the brightest star is given as 8.42.
Archinal & Hynes in Star Clusters also give a type of II3r, but suggest 108 member stars, the brightest of which is, they say, magnitude 9.0. The Night Sky Observer’s Guide (Volume 1) gives a somewhat stunted Trumpler type of III2, detached with no concentration, only a moderate range of brightnesses.
All I can suggest is that you go out and look for yourselves.
The cluster is young, with the dominant bright stars mostly being B-type. There are a large number of Be-type stars amongst them, that is, B-type stars whose spectrum displays emission lines. The prototype Be-star is γ Cassiopeiae, which lies 6° west of the cluster. There are currently (as of 2015) 42 emission-line candidates within the cluster1, though it is thought that not all of these may be giant B-type stars.
NGC 663 is clearly a young cluster, with an age calculated at 20—25 million years2.
From a visual perspective, NGC 663 is a very pleasing cluster, being large and bright and immediately obvious. I observed it with my 12” (300mm) Newtonian reflector and found it to be bright, large and rich. About 100 stars were seen splashed liberally across the 20' field. The brighter stars are in pairs and form the outer edge of the cluster. The centre is filled with a tight multitude of fainter stars which at first sight under low power may look like a void. The brighter stars are magnitude 8.5—10, the fainter easily down to 14 and probably beyond. It appeared slightly elongated north-west—south-east. The field was so rich that on this occasion I made no attempt to draw the object.
Object RA Dec Type Magnitude NGC 663 01h 46m 17s +61° 12’ 51” Open cluster 7.1
- Searching for Be Stars in the Open Cluster NGC 663, The Astrophysical Journal, Vol 149, No 2 (2015)
- Stellar contents of two young open clusters: NGC 663 and 654, MNRAS Vol 354 Issue 4, April 2005
NGC 7538 in Cepheus
September 2023 - Nebula and Cluster of the Month
We’ll only be looking at a single object this month. It’s a little-known gem that rides high in the September sky, being circumpolar from latitudes north of N39°.
NGC 7538 lies in the constellation of Cassiopeia, about 1.3° west of the splashy open cluster M52, and close to the border with Cepheus. It was first seen by William Herschel on the night of 3rd November 1787. He wrote
Pretty bright middle. Two considerable stars involved in nebulosity, 2’ long, 1½’ broad. He placed it in his second class of objects, ‘faint nebulae’ as no. 706.
NGC 7538 lies at a distance of a little over 9,000 light-years, in the Perseus arm of the Milky Way. It is an active star-forming HII region and has made news by harbouring what is believed to be the largest protostar yet observed.
Within the nebulosity lie three infra-red sources, labelled IRS 1, 2 and 3. Studies of 1.3mm continuum emission have revealed nine sources, labelled MM1–MM9. By far the strongest, MM1 is roughly coincident with IRS 1. Within MM1 lies a still-forming O-type giant protostar, with a mass of approximately 25 suns.1
Other condensations have been detected within the infra-red sources, leading astronomers to describe the NGC 7538 star-forming region as ‘a massive cluster in the making.’2
It is fascinating to think that in just a few million years, a new, brilliant open cluster will emerge from the formative gas and dust that is NGC 7538.
From the point of view of the visual observer, NGC 7538 is not spectacular, but it is a remarkable little object and well worth hunting down. Herschel considered it ‘faint’, and without any help from filters it is, but a good UHC filter or an OIII filter will improve your chances of seeing it enormously.
The sources do not give a magnitude for the object (nor, in fact, for any nebula). I don’t know why, but for what it’s worth, I would estimate the magnitude to be somewhere between 11 and 12.
Immediately visible are two equal-brightness (magnitude 11.6) stars, separated by 30” and orientated NNE-SSW. With the filter in place, the nebulosity should be comfortably visible. Data sources, such as the Deep-Sky Field Guide to Uranometria, give the size of the visible nebulosity as 9’ x 6’. Perhaps unsurprisingly, I saw less than this from my light-polluted site. I estimated a size of about 4’ x 3’. The nebulosity appeared elongated and oval in shape, sharing the same orientation as the two bright stars.
I found that it was clearly seen using both a UHC filter and an OIII filter. I suspect that from a site not much superior to mine, it could well be visible without any filters in place.
Object RA Dec Type Magnitude NGC 7538 23h 13m 31s +61° 30’ 09” HII star-forming region
NGC 6910 and IC 5117
August 2023 - Nebula and Cluster of the Month
The Milky Way continues to hold sway throughout August. With astronomical darkness lengthening to an hour and three-quarters for mid-Britain by the end of the month and Cassiopeia, Cygnus and Aquila arching above our heads, there is plenty to look at.
Riding high this month is Cygnus, one of the north’s most crowded constellations. We’ll look at two examples this month, an open cluster and a planetary nebula.
NGC 6910 lies 30’ north and slightly east of second-magnitude gamma Cygni, the heart of the swan. It was discovered by William Herschel on 17 October 1786. He described it as
A small cluster of compressed scattered stars of various sizes. Extended like a forming one.
I’m not sure what ‘Extended like a forming one’ means. A forming open cluster? Maybe. If so, it would appear a prescient comment, as NGC 6910 is indeed a young cluster, being formed mainly of early-type stars of spectral classes O and B.
The Trumpler classification is given by Archinal & Hynes as I3m n, indicating that it is a well-detached cluster with strong central condensation, it has a wide range of magnitudes amongst its members and contains 50–100 stars. Specifically, they state it has 66 member stars. The detached ‘n’ refers to nebulosity which may be associated. This is the huge swathe of nebulosity that surrounds gamma Cygni.
The Wikipedia entry for this object gives a Trumpler classification of I2p, meaning that it has only a moderate range of brightnesses and is poor in stars (fewer than 50). The entry then goes on to describe
125 stars, probable members of the cluster, are located within the central part of the cluster, and 280 probable members are located within the angular radius of the cluster.This would make it a rich cluster, not a poor one. I suppose with reference material, you get what you pay for.
Most sources give an angular diameter for the cluster of around 10’, though looking at proper motions of stars in the field, I can see a number of stars that appear to be sharing a common proper motion over a diameter of at least 30’.
Archinal & Hynes give the magnitude of the brightest star as 9.61. There are, however, two very prominent stars in – or at least in the line of sight of – the cluster, lying at the northwest and southeast edges of the central condensation. The northwest star, HD 194241, is of spectral type K2 and is magnitude 7.4. It has proper motion virtually opposite to that of the member stars. It cannot be a member.
The southeast star, on the other hand, has a spectral type of B0 and a proper motion quite comparable with the member stars. At magnitude 7.1, my rough and ready reckoning makes this the brightest member star. There are several other brightish stars across a good half-degree of sky here that at least superficially seem to share a common proper motion. Maybe this cluster is bigger than current estimates suggest.
I made an observation, with a 12” Newtonian reflector at a magnification of x150 and a field size of 20’. I found the cluster to be well detached and moderately compressed. It comprises two 7th magnitude stars, several 10th-magnitude and many fainter. It has a triangular form and the most prominent feature is a knot of 10th-magnitude stars. I found both the 7th-magnitude stars appeared orange, despite one being a B-type. The density of interstellar dust and gas between us and NGC 6910 reduces its brightness by about one magnitude and does lend a yellowish hue to these blue-white stars.
Our second object for August, IC 5117, is a tiny planetary nebula lying one degree south of Rho Cygni. It was discovered photographically by Williamina Fleming in 1905 and made its debut to the world in the Second Index Catalogue in 1908.
It is classed as a ‘compact’ planetary nebula, and is only about 1” in diameter, making it potentially challenging to spot as anything other than a star. The IC description is terse;
Planetary, stellar, but as with the other well-known compact planetary, IC 4997 (see this column for September 2020), there’s more to it than that.
The nebula shines at magnitude 11.5, and spotting a stellar object of that magnitude in a crowded Milky Way field may seem somewhat daunting. However, an OIII filter will make the identification relatively painless. A UHC filter will also work, but not as dramatically. The central star is magnitude 17.2, so that’s not going to trouble you.
I made an observation of it in June 2014 and found it pretty bright and easy, though stellar at all magnifications. It lies about 20” from a magnitude 9.9 star and it is clearly fainter than this star. The OIII filter brightens the object so that it appears brighter than the ‘companion star’. It was identified this way. Even at x450, it remained stellar.
Object RA Dec Type Magnitude NGC 6910 20h 23m 12s +40° 46’ 48” Open cluster 7.4 IC 5117 21h 32m 31s +44° 35’ 53” Planetary nebula 11.5
NGC 6885 and NGC 6781
July 2023 - Nebula and Cluster of the Month
The Summer Miky Way is on full show this month and by the end of July, astronomical darkness will have returned for most of us.
This month, then, we’re going to look at two fine objects – a cluster and a nebula, both easily visible during the brief darkness of the summer night.
Our first object is a planetary nebula, NGC 6781. It lies in Aquila, 4° northwest of 3.4 magnitude δ Aquilae, or if you prefer, 8.3° southwest of first-magnitude Altair.
It was discovered, unsurprisingly, by William Herschel on the night of 30 July 1787. He placed it in his third category – very faint nebulae (but never let that put you off) – as number 743. His description reads
considerably faint, irregularly round, resolvable, 3 or 4’ diameter. Clearly, Herschel was still under the impression that all nebulae could be resolved into stars at this stage. More unnervingly, he regularly attested to the resolvability of these objects.
Despite Herschel putting it in his ‘very faint’ class, NGC 6781 shines at a quite comfortable magnitude 11.4. It is large, though Herschel’s estimate is somewhat exaggerated. It’s about 2.5’ in diameter.
Images show a nebula that looks like a ring, which has burst on its north edge, spilling matter out a small distance to the north. In fact, we’re probably looking down the long axis of a barrel-shaped nebula, much as we are with M57 in Lyra.
I observed NGC 6781 with a 12” (300mm) Newtonian in 2014. I found it easily visible, even without the help of an OIII filter. My description reads
A very odd-shaped nebula. The southern end is brighter with a short, almost cometary tail to the north. There is a brighter section to the southeast. Fairly large and visible without the OIII filter at x83. The OIII filter adds some detail.
I make it a rule never to look at an image of an object before I observe it, so ensuring that what I draw is what I see, not coloured by any expectations or preconceptions. The fact that I saw the ‘burst’ in the ring, which I described as a ‘...short, almost cometary tail’, shows that this feature is visible to the visual observer, despite looking faint in images.
Our second object is a cluster in Vulpecula, NGC 6885. Again discovered by William Herschel, this time on 9 September 1784. He described his find as
A cluster of coarse, scattered stars. Not rich.He entered it into his catalogue as 20 H.VIII. On the following night, he recorded another cluster, 12m of RA and 15’ of declination away. He entered it as 22 H.VIII and described this one as
A cluster of coarse, scattered stars.
This second object entered the NGC as NGC 6882. The only problem is that at the location of the second cluster, there is nothing to be found. The coordinates of 22 H.VIII (NGC 6882) are right on the edge of the previously discovered cluster 20 H.VIII (NGC 6885). There can be little doubt that this was an accidental reobservation of the cluster he discovered the night before.
To further complicate matters, Per Collinder in his 1931 listing of clusters for his thesis ‘On Structural Properties of Open Clusters and Their Spatial Distribution’ gave additional catalogue numbers to NGC 6885 and NGC 6882 (NGC 6885 = Cr 417, NGC 6882 = Cr 416), leading some to believe that there are up to four open clusters in this area.
‘The Collinder Catalogue of Open Star Clusters: An Observer’s Checklist’1, edited by Thomas Watson, in the notes for these two entries says
Cr. 416 & 417: Possibly only a part of NGC 6885.
Some observers have noted that there may (with a squint, I think) be two different concentrations of stars here. There is a more concentrated area of fainter stars, mostly within the larger concentration of brighter stars, that some authors have identified as Cr 416. The Lund catalogue says they are all in the same group. Trumpler disagrees, placing them at different distances.
My feeling is that Herschel made a mistake on 10 September 1784, and all the disagreement stems from that. I have studied the region in much detail at the eyepiece and to my eyes, there is only one cluster here.
It is a large, loose cluster centred on 5.9 magnitude 20 Vulpeculae, which is unlikely to be a member of the cluster as it is so much brighter than all the other stars. I counted about 50 stars, formed into a triangle about 15–20’ long on each side. In the centre of the triangle is a near-void, in which 20 Vul lies. The stars are generally 9th–13th magnitude.
Object RA Dec Type Magnitude NGC 6781 19h 18m 28s +06° 32’ 25” Planetary nebula 11.4 NGC 6543 20h 11m 55s +26° 29’ 14” Open cluster 8.1
- The Collinder Catalog (updated) on Cloudy Nights (accessed 24 June 2023)
NGC 6229 and NGC 6543
June 2023 - Nebula and Cluster of the Month
The northern Milky Way begins to swing back into view during June. This month also provides the brightest and shortest night-times of the northern year. For those of us plagued by light pollution, this makes little difference, as we observe through a permanent, artificial twilight anyway.
We’re going to be looking at a couple of objects that are far more accessible to northern observers than those featured in recent columns. On offer this month are a small globular cluster in Hercules, NGC 6229, and a bright planetary nebula in Draco, NGC 6543.
NGC 6229 lies in northern Hercules, about halfway between the stars 42 & 52 Herculis. It shines at magnitude 9.4 and can be seen fairly easily, even against a twilit sky, with only modest equipment. A dark sky will show it to 10x50 binoculars.
It was discovered by William Herschel on 12th May 1787. He described it as
Very bright. Round. 4’ diameter. Almost equally bright with a faint, resolvable margin.
This was at the time when Herschel believed that all nebulae would ultimately succumb to resolution into stars, including planetary nebulae. He placed NGC 6229 in his fourth category, Planetary Nebulae, as number 50. It wasn’t until 1790 that an observation of NGC 1514 began to turn his thinking towards the fact that some nebulae might not be made up of stars at all. At this point in his career, this object looked like what he called ‘planetary nebulae’, so into that category it went.
We now know that NGC 6229 is actually a globular cluster. It lies at a distance of almost 32,000 parsecs (105,000 light-years) which is about five times further away from us than its near-neighbour in Hercules, M13. Like an increasing number of globular clusters, it is suspected of being the core of a dwarf spheroidal galaxy previously consumed by the Milky Way.
The globular cluster is about 1.6’ across. Its diameter is often quoted at around 4’, but this is inclusive of all the faintest outer stragglers that visual observers have no chance of seeing. If you go out to look at it, look at it carefully and see if you can understand why Herschel thought it was a planetary nebula, always bearing in mind that there was no scientific definition in play at the time. I think it’s quite understandable. The brightest star in the cluster is magnitude 15.5, so unless you have a pretty large telescope at your disposal, you’re not going to resolve it.
I include my own observation here, made with a 12” (300mm) Newtonian reflector. I described it as follows:
Bright, round and with some mottling but otherwise unresolved at x150. Gradually brighter towards the centre. x450 gives a hint of resolution and shows the surface brightness of the globular to be irregular. The centre may be slightly elongated.
Our second object for this month lies further north, in Draco. NGC 6543 is a bright planetary nebula, magnitude 8.9, so again should punch through that annoying twilight.
It’s another Herschel discovery. He found it on the night of 15th February 1786, writing:
A planetary nebula. Very bright. Has a disc of about 35” diameter but very ill-defined edge. With long attention a very bright, well defined round centre becomes visible.
Although distances to planetary nebulae are notoriously vague and uncertain, most authorities seem to agree on a distance of 3,300 light-years for NGC 6543.
It is a very complex nebula with folds and convolutions whose origin is far from clear. Two possible theories are that the central star has a strong, twisting magnetic field or that there is an unseen companion star warping the nebula.
Clearly visible in deep images is a large, outer region of very faint nebulosity, about 6’ across, which may be detected visually under optimal conditions. The outer nebulosity has a chunky appearance around its edge, and two of the brighter knots have earned themselves their own catalogue entries.
The largest and brightest was first seen by E. E. Barnard on 24th April 1900. It was catalogued in the Second Index Catalogue as IC 4677, where its description is
very small, faint. With a large telescope, very dark skies and an OIII filter, it can be seen as an elongated streak 1.5’ west of NGC 6543.
A second, much fainter blob also has its own catalogue entry. This blob is 2.5’ south of the centre of NGC 6543 and is incorrectly listed as a galaxy in the PGC/LEDA catalogue as PGC 2688853.
More visible than either of these is the 13.6 magnitude galaxy NGC 6552, 9’ to the east of the planetary nebula.
I have observed NGC 6543 on a number of occasions and I have included an observation from 2017, which gives a good impression of what can be seen under less-than-ideal circumstances.
Many observers see green in this nebula. To me, it always appears blue.
I described the object on that night as
Very bright, large and oval. More or less uniform in brightness, but slightly darker at the edge. There is a very faint outer envelope. To me, it has a bluish colour. The OIII filter makes little difference. Lovely.
Object RA Dec Type Magnitude NGC 6229 16h 46m 59s +47° 31’ 42” Globular cluster 9.4 NGC 6543 17h 58m 33s +66° 38’ 04” Planetary nebula 8.9
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’).
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
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.
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
- 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...12which 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.
Object RA Dec Type Magnitude NGC 3532 11h 05’ 45” -58° 43’ 41” Open cluster 3.0
- 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.
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!
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
- 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)