Nebula and Cluster of the Month Archive 2021
In this series of articles we draw your attention to Nebulae, Clusters and other Galactic objects that are particularly worthly of an observer's time.
NGC 6633 in Ophiuchus and NGC 6742 in Draco
July 2021 - Nebula and Cluster of the Month
Culminating at an altitude of 43° in mid-Britain at midnight on 16th July, NGC 6633 is well placed for observing throughout July. This open cluster was initially discovered by de Chaseaux in around 1745 and then independently discovered by Caroline Herschel in 1783. It was observed by her brother William on 3rd November 1786. William described it as
A cluster of considerably scattered little stars, placing it his class VIII (coarsely scattered clusters of stars) as no. 72.
Whilst this description may be accurate, there is much more that can be said for this cluster. Firstly, and to some perhaps most importantly, it is pleasing to the eye. With low to medium magnification, it shows as an elongated grouping of bright stars, stretching more-or-less north-east to south-west. The diameter of the cluster (or more accurately, the length of the cluster) is given as 20’ by Archinal & Hynes and as 27’ by the Deep Sky Field Guide to Uranometria. My own rough estimate was 30’ when I made my observation.
The number of stars in the cluster is given as 30 by Uranometria and The Night Sky Observer’s Guide, 38 by Wikipedia and, rather enthusiastically, as 159 by Archinal & Hynes. My own count of stars that looked like they could be members was about 50. It just depends on what you count. The cluster contains at least seven catalogued double stars.
The Trumpler classification (given by Archinal & Hynes) is III2m, meaning that the cluster is detached from its background, has no discernible concentration, is composed of both bright and faint stars, and that it is moderately rich. The definition of ‘moderately rich’ is 50 – 100 stars, which is at odds with A&H’s own listing of 159 stars, and also with the much lower estimates given in other sources.
This month’s nebula is a somewhat more challenging object, but is well placed in July for northern observers, culminating at 85° at midnight on the 24th July in mid-Britain.
NGC 6742 lies in Draco; whose head lies close to the zenith on Summer nights. It is a planetary nebula with a low surface brightness, so may require some teasing to pull out from the summer sky.
It was discovered, unsurprisingly, by William Herschel on 8th July 1787. He placed it in his class III (very faint nebulae) as no. 742. He described it as
very faint, stellar, verified 300 [magnification], a description that survived unchanged into the NGC. The description of this object as ‘stellar’ is odd, as it is 30” in diameter. It is small, certainly, but quite clearly not stellar.
It is also certainly faint, usually being ascribed a visual magnitude of 13.4 (though considerably fainter photographically). It has a fairly smooth disc with hints of annularity. It is almost, but not quite circular and in images shows a clear green colour.
Visually, I find it difficult, though not impossible, through my 12” (300 mm) under moderately poor skies. It is faint in an unfiltered view, but clearly visible most of the time. The addition of an OIII filter very much improves the view. With the filter in place, I can see a small, smooth disc, which just occasionally hints at a slight elongation. The central star is a very difficult object at magnitude 19.4.
Object RA Dec Type Magnitude NGC 6633 18h 27m 16s +06° 31’ 58” Open cluster 4.6 NGC 6742 18h 59m 20s +48° 27’ 55” Planetary nebula 13.4
NGC 6171 in Ophiuchus and NGC 6210 in Hercules
June 2021 - Nebula and Cluster of the Month
NGC 6171, the globular cluster commonly referred to as M107, was discovered by Pierre Méchain in April, 1782, and independently by William Herschel on 12th May 1793. Herschel described it as
A very beautiful, extremely compressed cluster of stars. Extremely rich, 5 or 6’ in diameter, gradually more compressed towards the centre.
Note that the description given in James Mullaney’s book The Herschel Objects and How to Observe Them is the description of the objects from John Herschel’s General Catalogue of 1864, and is not William Herschel’s description.
M107 does not appear in Charles Messier’s lists of nebulous objects published in the Connaissance des Temps of the French Academy, but letters between Messier and Méchain make it clear that this object had been seen by both men. Messier had intended to add further entries to his catalogue, but never did.
Work by Helen Sawyer Hogg in the 1940s led to the inclusion of the objects now known as M105, M106 and M107 in the canonical list, and few today would argue that it is not a ‘true’ Messier object.
As a visual object from light-polluted sites, M107 presents some difficulties. Its apparent magnitude of 8.1 would suggest an easy object, but it’s relatively low in the sky (maximum altitude from mid-Britain is 23°). Also mitigating against it is its concentration. I have mentioned before that the most important measure of the visibility or otherwise of a globular cluster is its concentration class. Generally, the more concentrated a cluster, then the more visible it is.
Globular cluster concentrations range from I (most concentrated) to XII (least concentrated). M107’s class is X, ‘loose’.
From my light-polluted site, I only saw the object on my fourth attempt to find it with my 12” (300 mm) Newtonian reflector. On all previous occasions, I had positively identified the field, so I knew that I was looking in the right place. It simply couldn’t be seen.
Then, finally, on 9th June 2015 I spotted it. The fact that I could see it at all was a testament to the better than usual sky conditions on that night. The cluster appeared pretty large, though it should be pointed out that although its diameter is listed as 13’ in catalogues, its visual diameter is closer to 5’. It still appeared pretty faint to me, though it could be held with direct vision. It looked quite speckly, indicating that it consists of (just) unresolved stars. The brightest star in the cluster is magnitude 13.
When I was first making my way through the Messier Catalogue in the 1970s and 80s, there was much debate about which was the most difficult of them. M76 and M98 were often touted, following the lead of Kenneth Glyn Jones in his Messier’s Nebulae and Star Clusters. However, KGJ only listed the Messier objects up to and including M104.
M76 and M98 are both easy objects in my 12”. Barring those which are denied to me through their low declination, I would put M107 as my personal ‘most difficult’ Messier object.
By contrast, this month’s nebula – NGC 6210 is a nice, easy object. Located 30° higher, and almost due north of M107, NGC 6210 sits in Hercules, about a third of the way between β and δ Herculis and shines at a magnitude of 8.8. It is thus nominally fainter than M107, but its brightness is compacted into an object less than 20” in diameter. It was discovered in 1825 by the great double-star observer, F.G.W. Struve.
It lies about 6,500 light-years away and has a true diameter of a little over half a light-year. The central star, HD 151121 is magnitude 12, but has been suspected of being variable. When I made my observation of the planetary nebula, I didn’t notice the central star. In Visual Observations of Planetary Nebulae, Kent Wallace also notes that the central star was not seen.
Some observers have noted a distinct green colour to this object, but to me it always appears bright blue. Using my 12” reflector, I noted that it was very, very bright at x83 and that the OIII filter added nothing to the view. The nebula appears fairly homogeneous but the north side is brighter than the south, where there is some sort of darkness detectable. The Hubble image shown above (where north is up, as opposed to my observation below) shows clearly what I was seeing. The disc is slightly elongated, and outer nebulosity is visible, especially at the extremities of the nebula.
Two quite different objects, then, for this month, representing opposite ends of the spectrum of nebulae and clusters.
Object RA Dec Type Magnitude NGC 6171, M107 16h 32m 32s -13° 03’ 09” Globular cluster 8.1 NGC 6210 16h 44m 29s +23° 48’ 02” Planetary nebula 8.8
NGC 5634 in Virgo and NGC 6058 in Hercules
May 2021 - Nebula and Cluster of the Month
The sky continues to be dominated by galaxies throughout May. If you want a small break from the extragalactic wonders, here are a couple of objects from within our own Galaxy that you may find interesting.
Our cluster this month is a remote globular, located about halfway between the galaxy fields of Virgo and Antares in Scorpius, almost due south of Arcturus.
It was discovered, like most of the objects in this column, by William Herschel. He first sighted it on 5th March 1785 and described it as
very bright, considerably large, irregularly formed, very gradually brighter towards the middle.He placed it in his category I, bright nebulae, as no 70. In 1888 it entered the NGC as no 5634.
The fact that he placed the object in his ‘bright nebulae’ category rather than under one of his three ‘clusters’ categories suggests, like his description, that he was unable to resolve any stars. I can find no direct information about the magnitude of the brightest stars in the globular, but there is a rough correlation between the overall magnitude of a globular cluster and the magnitude of its brightest stars. For a cluster as bright as NGC 5634 (mag 9.4), I would expect the brightest stars to be between magnitudes 14 and 15. As it reaches a maximum altitude of about 30° from Britain, this would probably explain why Herschel could not detect any stars.
NGC 5634 is one of a small number of globular clusters that have probably been stripped from dwarf galaxies captured by the Milky Way. More work is needed, but it seems likely that it originally resided in either the Sagittarius Dwarf Spheroidal Galaxy or the Sagittarius Dwarf Elliptical Galaxy. It lies at a distance of over 82,000 light-years, about four times the distance to M13, for example.
It is a class IV globular cluster, indicating that it has a moderately rich concentration towards the centre. The higher the concentration, the easier a globular is to see visually, and a class IV object should present little difficulty.
I observed it with a 12” (300mm) Newtonian reflector in 2014, before the imposition of the blue LED streetlights that completely wiped out my sky. I noted that the object appeared bright, small and compact. No resolution was achieved, even at x375, though the object did appear distinctly mottled.
For this month’s nebula, we’re moving north to Hercules, to find the planetary nebula NGC 6058, another Herschel discovery. He discovered it on 18th March 1787, when he catalogued it as the 637th entry in his category III, very faint nebulae. He described it as
very faint, extremely small. 300 [power] showed two very small stars with nebulosity.
NGC 6058 lies about 10,000 light-years away and has a true diameter of a little less than half a light-year.
It is not especially bright, shining at magnitude 12.9. Kent Wallace, in Visual Observations of Planetary Nebulae reports that using an 8” (200mm) reflector, he needed an OIII filter to identify the object.
I observed this planetary nebula under a June twilit sky and was surprised at how bright it seemed, even against the twilight. At x83 I found it to be round and distinctly non-stellar. I noted that it was sharply brighter in the middle, but I didn’t think I had seen the central star. It would appear that I was mistaken, as the central star is bright (magnitude 13.6) and there is nothing else at the centre of the nebula! I found that use of the OIII filter noticeably increased the apparent diameter of the object.
two very small starsare the central star and a 15.9 magnitude star on the northern edge of the nebulosity.
Object RA Dec Type Magnitude NGC 5634 14h 29m 37s -05° 58’ 33” Globular cluster 9.4 NGC 6058 16h 04m 27s +40° 41’ 05” Planetary nebula 12.9
The Large and the Small
April 2021 - Nebula and Cluster of the Month
Clusters and nebulae are still fairly thin on the ground – or in the sky – during April. This month, I’m going to present two objects to you, one I expect that you’ll be very familiar with – though I’m going to ask you to look at it again and do something – and one which you may not be so familiar with.
First to the familiar. Even with the massive domination of the sky by galaxies this month, one of the sky’s finest showpieces is also on show.
On 3rd May 1764, Charles Messier, using a small, poorly-made telescope, discovered an object in Canes Venatici, close to the border with Boötes. He described it as follows:
Nebula without star, centre brilliant, gradually fading away; round. In a dark sky, visible in a telescope of one foot [focal length].
This object turned out to be the first in his famous list that he actually discovered himself. Known today as M3, it is a spectacular globular cluster, shining at a magnitude variously quoted as 5.9 or 6.3. It is easily visible in binoculars, even on a poor night.
The globular cluster lies at a distance of 32,000 light-years from us, and sits high above the galactic plane, relatively isolated. M3 is a very heavily-studied globular cluster, instrumental as it has been in the study of cluster ages and stellar evolution. It contains more known variable stars than any other globular – 274 at the latest count. Its true diameter is close to 200 light-years.
The reason that I’m bringing this very obvious object to your attention is because I’d like you to do something for me.
On 5th March 1978, I made an observation of M3 with an 18” (450mm) Newtonian reflector which shows the cluster to have four long, triangular arms, spread out equally, which made the cluster look like a four-armed starfish. The observation lay as a more-or-less ignored curiosity for many years.
Then on the early morning of 24th March 2014, after a long night of galaxy observing, I turned my 12” (300mm) Newtonian to M3, something I like to do as a treat after all that straining after tiny, 13th magnitude blobs. As I looked at the breathtaking ball of stars, I was immediately struck by my view’s similarity to that of my observation of 36 years before (I said the observation was ignored, not forgotten!).
I looked again at M3 on 18th April 2014, when the cross-shape appeared less obvious. Comparing notes with the previous observation, I noticed that in March, I’d been using a 4mm eyepiece, whereas in April I was using a 10mm eyepiece.
To settle my mind, I took another look on the 12th June of that year. Initially I used the 10mm eyepiece but then switched to the 4mm. The difference was dramatic, with the cross-shape standing out clearly.
The magnification I get from my 10mm eyepiece on that telescope is x150, the 4mm gives x375. Back in 1978, the 18” telescope was being used with a 12mm eyepiece, giving a magnification of x212.
I’d like to ask you to have a look at M3 in the coming months. If your telescope is large enough to support it, try looking at magnifications in excess of x200. Can you see the fat, long, triangular arms that I can?
Below is a plot of the brightest stars in the cluster. I think it bears out my observations. What do you think? Just click on my name at the bottom of this article and tell me what you’ve seen.
On, then, to our second object, one you may not be so familiar with. This month’s nebula is IC 3568, a surprisingly bright planetary nebula in Camelopardalis, just 7.5° from the North Celestial Pole.
It was missed for the NGC, but discovered by Robert Aitken (he of double star fame) and included in the Second Index Catalogue of 1908, where it is described as
Planetary or nebula, star magnitude 9.5; star of magnitude 13 preceding by 15”, which is unhelpful.
It is mistakenly included in the Uppsala General Catalogue of Galaxies as UGC 7731, which also gained it an entry in the Principal Galaxy Catalogue (PGC 41662) and in the Morphological Catalogue of Galaxies (MCG +16-06-000).
It is about 20” in diameter and shines at magnitude 10.6. The central star is easily visible at magnitude 11.3.
It lies about 4500 light-years from us, implying a true diameter of a little less than half a light-year.
It’s a lovely little object. My observation reads
Very bright, pretty small and quite round. There is a small, very bright centre surrounded by a bright disc and a fainter outer disc. It stands high magnification well. The OIII filter makes little difference.
Object RA Dec Type Magnitude M3 13h 42m 12s +28° 23’ Globular cluster 5.9 IC 3568 12h 32m 54s +82° 33’ 50” Planetary nebula 10.6
A Remnant and a Ghostly Nebula
March 2021 - Nebula and Cluster of the Month
With the coming of Spring, the night side of the Earth begins to look away from the Milky Way and the deep-sky objects that lie within it. The time of the galaxies is arriving. There are consequently very few nebulae and clusters available to us. Open clusters are particularly scarce this month, but I can bring you details of one.
NGC 3231 lies in Ursa Major, about 6.4° north-west of α UMa. It was discovered in 1832 by John Herschel, who described it as
A cluster of 20 stars more or less, 10, 11 and 12m, scattered over a space 10’ diam. A star 7 m s.
The seventh magnitude star south of the cluster is 7.9 magnitude SAO 15184 (HD 90318). Placing this star south of the cluster rather than south-west indicates that Herschel considered the cluster rather larger than modern images suggest. His diameter of 10' confirms this. Modern estimates are closer to 4'. As with any cluster, though, it depends on which stars you count. To my eye, the concentration of brighter stars to the east looks like the cluster, the fainter stars further west look more like background objects. Without proper spectroscopic and proper motion studies, though, it’s quite impossible to say what is and what isn’t a member.
The stars that I can find proper motions for all seem to be travelling in different directions, suggesting that NGC 3231 is no more than an asterism. Still, it's a nice little object to come across in the cluster-starved skies of March.
For the adventurous deep-sky observer with good skies and plenty of aperture, the very dim galaxy UGC 5671 lies about 10' to the west of NGC 3231.
For this month’s nebula, we’re travelling much further south, to Hydra, and to a considerably more impressive object. NGC 3242 is a magnitude 7.7 planetary nebula, big and bright but for northern observers blighted slightly by a fairly low declination of -18°. Fortunately, the object is bright enough to be visible even from poor skies.
NGC 3242 was, predictably, discovered by William Herschel on 7th February 1785, who described it as
Beautiful, brilliant, planetary disk ill defined, but uniformly bright, the light of the colour of Jupiter. 40” diameter.
One of the principal attributes of this object is its distinct blue colour, which makes William’s description of it as being
the colour of Jupiterrather odd. Kent Wallace, in Visual Observations of Planetary Nebulae, describes its colour twice, once as pale green and once as green.
William’s son, John Herschel, in his magnum opus Cape Observations, makes the following comment: “A most remarkable peculiarity of the planetary nebula, … h. 3248 [NGC 3242], but which cannot be represented in an engraving, is its very decided though pale blue colour, which is noticed in three out of the four observations recorded in the sweeps. This and the beautiful planetary nebula h. 3365, [NGC 3918 in Centaurus] in which the blue colour is much more striking and intense, are the only objects of that colour in the heavens so situated as to admit of no suspicion of contrast with a red star influencing the eye. It is true that in the latter instance a considerably bright red star is near, and may be brought into the same field of view, and that is its presence greatly enhances the tint of the nebula. But the star is remote enough to be easily excluded, and the nebula does not cease thereby to appear of a fine blue colour.”
Reading this, I’m interested to note that neither of my observations of this object note any colour. This may have been due to the use of an OIII filter.
Its flattened disc and apparent diameter of about 40” present a view very similar to a dim Jupiter, which led William Nobel in 1887 to coin the name ‘The Ghost of Jupiter’ for this object.
This is a wonderful object in almost any sized telescope. My observation from 2014 reads
Very, very bright. The object responds well to the OIII filter. At x150 the nebula is clearly slightly elongated and there are hints of annularity. At x450 a darker centre is seen, with a brighter patch on the south side of the nebula. A thin outer area of pale nebulosity can also be seen. Amazing.
The magnitude of the central star is quoted in the Deep-Sky Field Guide to Uranometria as 12.1, and comparable magnitudes are given elsewhere. I have never seen it, and I know several other observers who have failed to see it, even with larger telescopes than mine. This magnitude must surely be wrong.
Distance measurements for planetary nebulae are always a bit uncertain, but NGC 3242 seems to be about 1400 light-years distant, giving it a true diameter of around half a light-year.
Object RA Dec Type Magnitude NGC 3231 10h 28m 24s +66° 42’ 38” Asterism? Br st =10.7 NGC 3432 10h 25m 44s -18° 44’ 38” Planetary nebula 7.7
An Asterism and a Planetary Nebula
February 2021 - Nebula and Cluster of the Month
For this feature, I try to restrict myself to objects that culminate within about an hour of midnight (UT) during the month in question. Sometimes, as this month, this puts serious restrictions on what is visible. February is a poor month for open clusters. The NGC lists only six clusters north of -30° declination within these parameters.
Three of these objects are in Cancer and two are very famous, M44 (or Praesepe) and M67. The third object lies close to M67 and is, I’m afraid, a bit of an imposter.
It lies 34’ south-west of the centre of M67 and was first observed (as ever) by William Herschel on 15th March 1784. He entered it as number 10 in his category VIII (coarsely scattered clusters of stars). His description reads
A cluster of very coarse scattered stars. Not rich.
His son, John Herschel, put four observations of it in his 1833 catalogue, where it appears as no. 528. His descriptions show that he was uncertain of the reality of the cluster. The first two observations were made in 1828 and read
The chief star 9 or 10 magnitude of a place rich in starsand
An insignificant cluster. No other near.
The first observation simply defines the location he gives for the entry, but it’s interesting that he calls it 'a place rich in stars' rather than 'a cluster'. The second observation sounds like he’s thinking 'Am I looking in the right place? There’s no other cluster nearby, so maybe I am.'
The other two observations, from 1830, both use the word 'cluster', but are not enthusiastic:
A very coarse and poor cluster…and
A poor cluster of 4 or 5 large and a few scattered small stars.
In 1888, the object made its way into the NGC as no. 2678, bearing the uninspiring description
Cluster, very little compressed, poor.
John Herschel’s final description is an accurate summary of what is seen in the eyepiece. If it wasn’t for the presence of the 4 or 5 bright stars, there is nothing here that would draw the eye.
Most authorities now list NGC 2678 as an asterism, but there are still several (particularly online) that list it as an open cluster.
One of the defining features of a true open cluster is that its member stars are all travelling more-or-less in the same direction and at the same speed. They are, after all, gravitationally linked to each other. Here, for example, is an image of nearby M67 with lines added to indicate the stars’ proper motions over the next 100,000 years.
The stars are clearly all travelling in pretty much the same direction and at the same speed. Those whose proper motion vectors are significantly different are not, therefore, members of the cluster.
Now let’s have a look at NGC 2678 in an image to the same scale.
As you can see, the bright stars have almost no commonality of direction or speed, certainly not enough to identify this 'place rich in stars' as an open cluster. It’s an asterism.
Turning to a more real object, our nebula for this month culminates at midnight on 31st January. From the centre of the British Isles, it only attains an altitude of 20° and thus may be problematic for observers in light-polluted areas (which is most of us, I know).
NGC 2610 is a planetary nebula in Hydra. Its declination of -16°, its faintness (around 13th magnitude) and the fact that it lies in a rather barren patch of sky (the nearest bright(ish) star is the 4.9 magnitude 9 Hya, 2° to its east) make it a less than popular target for visual observers in Britain. It is, however, worth seeking out if you can.
Data on this planetary nebula is difficult to come by. If we assume a true diameter of about one light-year (an average for planetary nebulae) then it must lie at a distance of around 3,500 light-years.
I offer two of my observations here to illustrate the difference latitude and aperture can make. Both were made in dark-sky sites and both were made at x150 with a 20’ field. The first was made with a 16” (400mm) Newtonian reflector from a latitude of 24°N.
Bright and easy. Initially, this object was perceived as a disc, but closer observation revealed a darker centre, giving the object a slightly elongated annular appearance.The object was at an altitude of 49° at the time of this observation.
The second observation was made with a 12” (300mm) Newtonian from a latitude of 55°N.
Pretty difficult, even with the OIII filter in place. The low altitude of 19° didn't help. Quite large and circular. No structure seen. A brighter point was seen at the centre (but only without the OIII filter). This is unlikely to have been the central star which shines at magnitude 15.9.
The more northerly observation is reproduced below.
I’d be interested to hear of your experiences of NGC 2610. How easy or difficult is it?
Object RA Dec Type Magnitude NGC 2678 08h 51m 08s +11° 15’ 20” Asterism Br st =8.5 NGC 2610 08h 34m 19s -16° 13’ 06” Planetary nebula about 13
Four Open Clusters and a Variable Nebula
January 2021 - Nebula and Cluster of the Month
I am writing this on the day that the first people in the UK are receiving the COVID-19 vaccine, so it is with some optimism that I wish you a very Happy New Year!
To break in the New Year, I’m going to look at an interesting chain of open clusters in Gemini and an always-fascinating nebula in Monoceros.
In about 1746, the Swiss mathematician and astronomer Philippe de Chéseaux wrote a letter to his grandfather in which he gave a list of 20
truly nebulous stars. This list was read out at a meeting of the French Academy of Sciences in August 1746 but was never published. Number 12 on this list was described as being
above the northern feet of Gemini, which although vague by modern standards is almost certainly the first known mention of the open cluster later to be immortalised as Messier 35.
Charles Messier made his observation of the cluster on 30th August 1764, when he described it as
A cluster of very small stars near the left foot of Castor; a little distance from the stars mu and eta of that constellation.
M35, or NGC 2168, is a real showcase open cluster. Its integrated magnitude is 5.1, making it an easy binocular object. It spans a diameter of 25’ and contains, according to Archinal & Hynes, 434 stars, the brightest of which is magnitude 8.0. Twenty of these stars are brighter than magnitude 10, giving a magnificent view on low power. Most of the stars are white, being of spectral classes B and A, but two of the brightest stars are of class G and K.
My most recent observation of the cluster reads
A very large and bright cluster of mostly moderately bright stars, splashed all over a low power field. Very rich. Two of the brightest stars mark the beginning and end of a long, slightly curving chain of stars that is to me the defining feature of this lovely cluster. Hundreds of stars.
Located 25’ south-west of M35, and in the same low-power field, lies our second cluster. NGC 2158 is a huge contrast to its more famous neighbour.
It was discovered by William Herschel on 16 November 1784. He described it as
A very rich cluster of very compressed and extremely small stars. 4 or 5’ diameter. A miniature of [Messier 35]…He entered it as no. 17 in his class VI – Very compressed and rich clusters of stars. This was the class in which he placed eight of his globular cluster discoveries and indeed NGC 2158 looks much like a faint, loose globular. It contains around 1000 stars and has an overall magnitude (according to Archinal & Hynes) of 8.6, although my own experience of this cluster suggests that this is overly optimistic. There are 20 stars brighter than magnitude 14, and on a good night, 2158 looks like a little pin cushion, with tiny twinkles amongst the background of unresolved, fainter stars.
In a small telescope or on low power, it appears as a small, oval nebulous patch attached to a star of magnitude 10.6. Despite appearances, NGC 2158 is much richer than M35, though it lies 5 or 6 times further away, at around 13000 light-years. It is one of the most distant open clusters which can be seen relatively easily with a modest telescope.
Swing your telescope a further 35’ west of NGC 2158 and you will come across a third open cluster. IC 2157 is a small and relatively unimpressive cluster and again shows marked differences to the previous clusters. It was passed over unrecognised by William Herschel and had to wait until 1899 to be recognised by Thomas Espin. Discovered too late to be included in the NGC, it finally appeared in the Second Index Catalogue in 1908. The description in the IC is stark;
It is actually much easier to see than NGC 2158, with its brightest stars being of magnitudes 10 to 12. My observation reads
A small, compact cluster of faint stars, shaped like a bow tie. Although small and faint, it is clearly visible in the 25mm eyepiece (x60). 4' or 5' diameter.
Finally, a slightly controversial one. Just 6’ north of IC 2157 lies another little group of stars which looks very much like it. Again discovered by Espin in 1899, this group is usually considered an asterism and is plotted on very few atlases. It bears the catalogue number IC 2156. I wrote this when I saw it:
In the same x150 field as IC 2157. Very similar; small, moderately compressed. Eleven stars counted. Interesting to see two such similar objects in the same field, one a real cluster and the other an asterism. I can't tell the difference just by looking.
For our nebula this month, we’re going to look at that ceaselessly fascinating object, NGC 2261, better known as ‘Hubble’s Variable Nebula’.
It hardly needs saying, but it was discovered by William Herschel on Boxing Day, 1783. He placed it in his fourth class (planetary nebulae) as no. 2, describing it as
Considerably bright. Fan-shaped, about 2’ long from the centre.
This object has the unique distinction of being the subject of the first photograph taken with the new 200” telescope on Mount Palomar on 26 January 1949, by Edwin Hubble himself. The nebula is famous, as its name implies, for being somewhat variable in nature. Images strung together to make animations show dark blobs moving across the nebula, from the bright point at the tip (R Monocerotis) up towards the sides and ends of the 'fan'. What we are seeing here is a shadow-play. The star which lights the nebula is the variable star R Mon, classed as a T Tauri. T Tauri stars are very young, pre-main sequence stars that have not yet begun hydrogen fusion. They are of solar mass and it is believed that the sun was probably a T Tau star in its infancy. They are associated with dense molecular clouds, and it is these clouds, spinning around the star which cast the shadows on the nebula. The prototype star, T Tau itself, is also associated with a variable nebula, NGC 1555 or 'Hind’s Variable Nebula'.
The shadow-play visible in animations takes place on the timescale of days or weeks, and generally, these variations are too subtle to be followed visually, but differences can be seen visually over longer periods. In 2013, I noted that the brightest portion of the nebula was a bright streak down its centre. In an observation made in 2001, I wrote that the eastern edge of the nebula was the brightest section.
NGC 2261 is not a difficult object to observe and should be visible in a 6” (150mm) telescope under good conditions without too much of a problem. The fan shape is immediately obvious. If you look at this object, take a careful note of its appearance – or better still, make a drawing. Then go back to it next year and compare your view. It is very dramatic to observe for yourself a deep-sky object that actually changes its appearance over such short timescales.
Object RA Dec Type Magnitude IC 2157 06h 04m 50s +24° 03’ 18” Open cluster about 10 IC 2156 06h 04m 53s +24° 09’ 37” Asterism about 10 NGC 2158 06h 07m 26s +24° 05’ 40” Open cluster 8.9 (?) M 35 06h 09m 03s +24° 21’ 16” Open cluster 5.1 NGC 2261 06h 39m 10s +08° 44’ 51” Reflection nebula about 11