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NEAT PLANETARIES : 8
NGC 2438 in Puppis


NGC 2438 / H IV 39 / h.463 / h.3093 / Sa2-13 / VV′ 69 / ARO 46 / PK 231+04.2 / PN G231.8+04.1 (07418-1444) + NGC 2437 / M46 / H IV 39 / h.464 / C* 0739-147 / OCl 601 (07417-1449) [U274/U275] lies some 14°E of Sirius and one of the best deep-sky objects in Puppis.

Cluster M46 is quite obvious in binoculars, however, the pale blue planetary may only be seen with care in 10.5cm., and on the rare occasions, in 7.5cm. Although open star cluster is fairly run-of the-mill in nature, it is certainly greatly enhances in the minds of visual observers by the fact that it also contains the brightish planetary nebula (PN), NGC 2437. The planetary lies on the north eastern boundary (nf. north-following) of the open cluster, roughly about two-thirds the distance from NGC 2437 central core. It remains a particularly favourite object in our summer skies from our southern hemisphere, as it is very conveniently placed near the zenith in the sky from mid-southern latitudes.


NGC2438

NGC 2437/38 : Open Cluster / Planetary. Image size: 12½′×12½′
Source: ALADIN previewer (Red: AAO/R/DSS2; Blue: SER/I/ DSS2; Green: average. Red/blue image modified by Silvering with reduced saturation.


The larger open cluster, NGC 2437, spans about 25″ across, fitting easily in medium powers in 5cm or 7.5cm telescopes. Total magnitude is a bright 6.1v or 6.33B, making the cluster among the brightest of the medium clusters. NGC 2437 contains about 150-odd visible member stars, and looking just to the north-east, within the clusters boundary, is the approximately 1.0″ (60 arcsec) diameter planetary. Deep images show the nebula is not round as it is by eye, but is slightly oval whose dimensions are about in about 73×68 arcsec, being aligned at about 30° from north (position angle).

NGC 2438s lovely planetary nebula is 10.8v / 10.1b (11.7B?) / 10.2r magnitude, was VV classification is 4+2

In 20cm. telescopes, using medium to high magnification, NGC 2437 looks slightly oval or even annular, shining its obvious nebulous lustre very nicely with its pale and slightly mottled centre. It responds very well with an O-III filter, which is a useful device to also eliminate the many stars of the open cluster and allow the amateur observer to concentrate their attention just on the surface texture of the extended planetary disk.

Such cluster-bound planetaries are not totally unique to open clusters, as another bright example is the Pyxis PNe/cluster combination, NGC 2818/2818a. (NSP 25.)


Historical Interlude


This planetary nebula was discovered by Sir William Herschel on 19th March 1786, and catalogued as H IV 39. The open cluster, without seeing the planetary, was first spotted by Charles Messier in mid-winter on 19th February 1771, who catalogued it as the first object in the second part of the Messier Catalogue. It was later divided in the catalogues as separate objects by John Herschel as h.463 and h.464; and combined as h.3093. It now remains as separate objects as, NGC 2437 (Open Cluster) and NGC 2438 (PNe), within the New General Catalogue (1888).

Charles Messier describes the open cluster as;

M46 Messier Catalogue

Messier Catalogue Extract for M46 From Third Version from 1781, published
in the Connaissance des Temps, objects 1 to 103. (1784).

Text version; Amas de très-petites étoiles, entre la tête du grand Chien & les deux pattes de derrière de la Licorne, déterminé en comparant cet amas à la 2.e étoile du Navire, 6.e grandeur, fuivant Flàmftéed; on ne peut voir ces étoiles quavec une bonne lunette; lamas contient un peu de nébulofité


Best translation for this simply reads;

Cluster of very-small stars, between the head of the big Dog [Canis Major] and two hind paws of the Unicorn [Monoceros], determined by comparing this cluster with the 2[nd] star of the Ship [2 Navis], 6th magnitude, according to Flamsteed ; they can see these stars only with good telescopes; the cluster contains a little nebulosity.

Other Messiers Translations of M46

The commonly accepted version by reads;

A cluster of very small stars, between the head of the great Dog and the two hindlegs of the Unicorn, determined by comparing this cluster with the 2nd star of the Ship [2 Navis], of 6th-magnitude, according to Flamsteed; the stars are not visible except with a good refractor; the cluster contains a bit of nebulosity.

Yet another partially reasonable and well-quoted version appears in Jones (1975), which says;

A cluster of very small stars between the head of Canis Major and the two hind paws of Monoceros, determined by comparing this cluster with star 2 Navis, 6th magnitude, following Flamsteed; the stars cannot be seen except with a good telescope; the cluster contains a little nebulosity.

I continue to be quite appalled with the often gross deliberate distortions of observational descriptions or of foreign translations made by mostly lazy amateur (and sometimes professional) astronomical writers. It seems that most are far too lax to go and examine the actual original source material, and find what is truly said. Many just appear to copy verbatim other works, or worst, just readjust the copied text to make their own words appear as something original”. Such continued reworking means that the quoted translation ends up totally wrong — significantly having implications interpreting what the observer is properly saying. This often perpetuates into the next author who does the same, again making any analysis or guidance almost totally worthless (The science ethic of being honest and truthful goes out the window, especially when it is not attributed nor referenced from the source in which it came.;

For example, the writer Tammy Plotner at Universe Today, has given the distorted translation that does not appear anywhere else. It is quite likely rewritten (but not referenced) from the on-line deep-sky reference source called SEDs, under object / webpage on M46. She writes in her M46 article;

A cluster of very small stars, between the head of the Great Dog and the two hind feet of the Unicorn, [its position] determined by comparing this cluster with the star 2 Navis, of 6th-magnitude, according to Flamsteed; one cannot see these stars but with a good refractor; the cluster contains a bit of nebulosity.

I have underlined here the distortions to highlight the possible misinterpretation in what Messier is truly describing. I spent a few hours investigating what I could find of Messiers quotes by others, and found contradictions everywhere.

Take for example the critical quote by Messier of; lamas contient un peu de nébulofité”. The words here have been translated as either; contains a bit of nebulosity”, contains a small nebulosity”, or contains some nebulosity

Another common mistranslation is deux pattes de derrière, which in English means two hind paws”, where patties in French is directly translated as meaning paws), which is the proper interpretation of these words. Hence, words like hindlegs, feet or even legs, or any variation of these, is not correct.

Also, the best translation from the French is that the cluster contains a little nebulosity.”, which clearly has a quite different implication. Either Messier is saying that the open cluster appear slightly nebulous or is he saying he sees a little nebula?

Some two years later, William Herschel then catalogued both cluster and then the nebula as H IV 39. He first says of the cluster;

A beautiful, very rich, compressed cluster of stars of various magnitudes.

Herschel (1789) then, from an observation made on 19th March 1785 describes;

pB. R. r. [Pretty bright, Round, ]” within the 46th of the Connoiff. des Temps almosft of an equal light throughout 2′ dia. no connection with the clufter, which is free from nebulosity.

John Herschel (1847) later describes;

A very fine PLANETARY NEBULA, oval, uniform in light, and of a very flat appearance; rather faint ; diam[eter] in R.A. = 4′.0 ; has a 15m on it, and one 13m close to its border. The object is excentrically situated in a superb cluster of stars 12…16m (46 Messier).

On another viewing he sees;

A fine, nearly uniform, slightly elliptic PLANETARY NEBULA, 40″ dia[meter]. A 14 m, is excentrically situated in or in it, which is doubtlessly only superimposed and belongs to the cluster Mess 46, in which (somewhat north of the most compressed part) this object is situated. A very uncommon and indeed unique combination, if No.3154 be not the case in point.

William Parsons, the Third Earl of Rosse, in his publication; Objects drawn by Lord Rosse and Johnstone Stoney” (1850), who made a useful sketch (No.12.) at Birr Castle in Ireland on 22nd December 1846, using the speculum 1.8-metre (72-inch) telescope — sometimes named, the Leviathan of Parsonstown. [Incidentally, this book also shows the very famous spiral galaxy sketch of M51 / NGC 5194 (13299+4712) in Canes Venetici. I could not find this specific book or image from it, however, reference to M46 (h.464) appears, including discussions on the appearance, observational methods and the telescope in the on-line Observations on the Nebulae”, Phil.Trans.Roy.Soc. London, 140, 499 (1850). Here contains the words;

The five planetary nebulae we have ascertained to be annular, are as follows: [h.]464 [M46], Plate XXXVIII. Fig 12., has two stars within it;…”

Parsons series of visual observations appear on pg.513-4, which are described as follows;

Fig.12, H. 464 —Annular nebula at the edge of the cluster M. 46. Sketched 22nd December 1848 annular, two stars in it.

January 27, 1849.—A third star suspected in brightest part.

January 29, 1849.— Third star strongly suspected.

February 13, 1849.— Observed nothing further.

March 16, 1849.— Saw but two stars in it


Comment 1: This paper additionally contains several planetary nebulae, including the famous eerie face of M97 / NGC 3587 / Owl Nebula (11148+5501). All serious planetary nebulae observers should acquaint themselves with the text on pages 567/8 of this paper — if only to understand the evolution of early PNe observational descriptions.

Comment 2: According to modern class historical deep-sky book by Webb Society member; Steinickle, W., p.299 (2010), an an unknown star discovered by William Parson, that maybe a possible suspect nova, in which Steinickle states;

A similar case of a supposed nebula is the double star 2 Puppis (6 mag), 50′ east of the planetary nebula NGC 2438 (GC 1565), which belongs to the open cluster M 46. Lord Rosse noted on 28 January 1849 that it was very strongly nebulous.’ But wait, there is more: Another B neb star about 10′ nf the D star.̱ This is 4 Puppis (5 mag).


Admiral William H. Symth, in his famous A Cycle of Celestial Objects” or just Cycle that was originally published in 1840s.

A very delicate double star in a fine cluster, outlying the Galaxy, over Argos poop. A 8½, and B 11, both pale white. A noble though rather loose assemblage of stars from the 8th to the 13th magnitude, more than filling the field, especially in length, with power 93; the most compressed part trending sf. and np. Among the larger stars on the northern verge is an extremely faint planetary nebula, which is 39 H. IV., and 464 of his sons Catalogue. This was discovered by Messier in 1769, who considered it as being rather enveloped in nebulous matter;” this opinion, however, must have arisen from the splendid glow of mass, for judging from his own remark, it is not likely that he perceived the planetary nebula on the north.

Here Smyth is attributing discovery of the planetary nebulae to Charles Messier


Did Messier Discover the Planetary Nebula in M46?


Arguments for Messier discovering the nebula in M46 is of course the quality size of the aperture he was using, and that it was too small to be able to see the faint disk. The other is, the interpretation by the Herschels (and other subsequent observers), who claim the apparent nebulosity he sees is the partially unresolved star cluster that Messier does not seem to describe.

This point I think remains debatable. Messiers failure comes back to his rather inadequate description, who does not describe where exactly the planetary nebula lies among the surrounding cluster stars nor the direction. I have often thought in the back of my mind, the real suspicion that Messier had doubts on what he was seeing. This is especially in the light that his deep-sky catalogue was in the most part hunting for fuzzies that might be mistaken by others as new undiscovered comets. This is why the text is written in the third person, so that when someone read his catalogue, those observers would not fall into the trap of really seeing a deep-sky object rather than comets. Messier could not care much for the niceties of describing the objects in minute detail (unlike the Herschels, for example) but only to avoid these deep-sky objects as uninteresting or unimportant comet traps.” This is likely why the descriptions are so focussed on nearby stars close to the catalogued nebulous (or possibly interpreted as a nebulous) deep-sky object.

Few do seem to seriously take such general ideas into consideration — as nothing written for more than century or more after the event is really questions by todays visual observers. Our astronomical experiences are now often too totally unrelated to the conditions and quality of the older telescopes that people like Messier were once using. Moreover, they knew virtually nothing of the reality of the natures of most deep sky objects, nor of the variety and morphology of their structures or inherent variations. The last comment here is also with real rivalry and nationalism between many observers, who, after decades after their works were usurped and questioned for their shortcoming within the crude descriptions or analysis — whose many vulnerabilities were easy to exploit — especially as no one could discuss with them as they were no longer living. This problem here is a European viewpoint of competition. I.e. Mostly French and England, and in some instance, the Germans, Spanish or Italians (It still happens historically and continues even today!)

This I think is a realistic view of some of the primary issues here in Messier seeing the planetary nebula or not.


Modern Observation Descriptions


There are so many modern observational descriptions for this object it is very hard to pick out the better ones! These ones here below show the evolution and comparison of observational description, especially in light of the earlier observers not knowing if these objects were truly nebulous or unresolved stellar conglomerations. When Englishman William Higgins use the spectroscopy on these objects in 1864, PNe became the nebulous objects we are familiar with today.

a.) Rev. Thomas W. Webb in Celestial Objects for Common Telescopes” (COfCT.), Vol.2, pg.38; Dover Edition (1962); gives he own views on M46 and the PN. Webb places the object in Argo, and rather cryptically says in his own unique but difficult abbreviated style;

2437 (M 46.) VIIh 38.1 S. 14° 38′. Beautiful circular cloud of small stars (H[erschel to 10 mg]) about ½° in diam[eter] : a little p[receding] the group round 4. nearer to it than [NGC] 2422. 2438 a feeble neb[ula] on its N[orth] verge is in La[ssall]s 20-ft. refl[ector] an astonishing and interesting object ; he an E[arl] of Rosse, see it annular ; so Buffham, 9-in[ch] With

b.) According to the late 19th early-20th Century Serbian-Austrian amateur astronomer, Leo Brenner (1855-1926) [Brenner (1902)], who is better known to me for his Venus observations (Aguirre (1995)), and also referenced in Stoyan (2008), suggests this is a;

Splendid object, even for smaller telescopes. 30′ diameter, very rich and bright. Stars of 10m and lower. The cluster contains a planetary or ring-shaped nebula of 3.75&8242;[?] diameter and of considerable brightness.

c.) Heber Curtis (1918a) sees;

a very large, bright, sparse cluster about 25′ in diameter, in which is involved the planetary nebula 2438.

Curtis (1918b) gives the PN is a Class II — Ring Form planetary, whose photographic image is described;

Plate XII. Exposure 3h. The fainter star at centre is perhaps the central star; magn. 16. The nebula is avery irregular, broad, patchy ring 38″ in diameter; it lies in a cluster (NGC 2437). The star at SW lies in a gap in the ring, the others seem to be simply projected on the nebula/ Ring quite faint. Rel. Exp. 50

c.) E.J. Hartung in AOST1 says for Object No.274, in one of the best descriptions in the whole book, That NGC 2437−8 is;

A beautiful open cluster with medium bright stars about 25′ across, rich and broadly concentrated towards the centre. In the Nf. region is a pale bluish planetary nebula 50′across which on closer examination will be seen to be annular with paler centre and single prism image. Several stars are projected upon it for the nebula at 1,600 pc. lies far behind the cluster at 700 pc. Messier discovered the cluster in 1771 but did not see the nebula, which however is plainly visible with 10.5 cm amongst the stars.

d.) In Webb Society Book on Planetary Nebulae Vol.2 by Jones (1979), on pg.64, the visual observations of 2438 are as follows

(16½)[-inch] Annular, N.f. part of ring brightest; ×419 little extended N.p., S.f.; star seen.
(12) Slightly irregular greeny-white patch on the NE edge of open cluster M46.
(8) Fairly large roundish patch with star on E edge; no trace of structure or PA. (6) Circular at LP [low power]; at HP [high power] seems involved in star close S.f.

e.) Canberra Astronomical Society member Albert Brakel, in the Journal Southern Cross, in his series of articles entitled Constellation of the Month — The Ten Best Southern Planetaries (2007), details this 2438 as;

This one is included not so much for its own sake but because of its outstanding setting, in the northeastern part of the open cluster M46 in Puppis. It is pale bluish, easy to see, and 60″ in diameter. Close inspection shows that its outer perimeter is a bit brighter than the rest. The cluster isnt too bad either — rich, about 25′ across, and broadly concentrated towards the centre.

f.) Steve Coe, using 13.1-inch f/5.6 describes;

NGC 2438 Bright, large, elongated 1.2×1 in PA 75, the central star is easy at 100×. Raising the power to 440× brings out several dimmer stars involved in the nebula. Also at the higher power the shape of the planetary is seen to be an incomplete ring, somewhat like a horseshoe. This bright rim is about 270° around and is dimmest on the north side. I have always seen this planetary nebula as light green in color. This is from a great night at Sentinel, 7/10 seeing and 10/10 transparency.

Then Steve at the Ultimate Star Party, McDonald Observatory in October 1995, S=7, T=9, using 25-inch f/5;

NGC 2438 the planetary in Puppis 25′; 12mm; light green, 3 stars, 2 obvious, one tougher, annular, central section light grey, two layers of nebulosity surround the central star.


Technical Data on NGC 2438


Both the open cluster and PNe were earlier estimated to be about 1.8kpc. from us, though many sources consider that the PNe is certainly closer, therefore being superimposed on top of the more distant cluster. Others like Knarchenko, et al. (2005), find the cluster distance as 1.3 kpc., so this is still a real issue of some contention. Adding to the problem is the measured radial velocity for the planetary being +75.4 km.s-1, while the cluster is a more pedestrian +48.09 km.s-1 (Knarchenko, et al. (2005)) Here one would be expecting that cluster star velocities that are gravitationally bound membership would only differ by merely about 5 km.s-1.

A recent distance, according to Stanghellini, Shaw and Villaver (2008) using 6cm. radio emissions and the observed expounding boundary of 35.2 arcsec, find distance as 1215 pc., rounded to 1.2 kpc. or 0.6 kpc. further to us. Some astronomers say the exact opposite, that the planetary is in the distance. So the jury maybe still out!

Perhaps one of the most definitive paper on the association of this PN and the open cluster is Kiss et al. (2008)

Deep images of NGC 2438, I.e. APOD Image taken by Daniel López using the Spanish Observatory telescope (Instituto de Astrofísica de Canarias or IAC) on the Canary Islands, shows a further annular circular ring with the radius about 2 arcmin (120 arcsec), with another, even fainter and more extended bubble or halo, is out to about 4½ to 5 arcmin. The literature says this extends in true distance to about 1.2 parsecs or 4½ light-years — atypical of most PNe. All these structures suggest the the nebula ejection has undergone various traumatic phases as the star evolved from the swollen red giant to its planetary nebula nucleus. One can imagine that three or four distinct phases have occurred within the lat few millennia to the object we now see today. Yet there are other features here to discern. For example, on the northern boundary is an unusual kind of windswept mottling or interaction with interstellar winds that have disturbed the nebulosity symmetry. The cause of this is now under investigation.


Age of NGC 2438


In Stoyan, R., et al. (2008), they claim the planetary; Much younger is the planetary nebula NGC 2438, only about 45,000 years old,…”


Central PNN of NGC 2438


A faint central PNN star discovered by C.M. Anderson in 1934, who estimated the brightness as 17.5v at 479.3nm.. It has been recently found to be brighter at 17.2±0.02v magnitude (Gathier & Pottasch (1988), whose estimated temperature is about 75,000K. Zanstra temperature is a higher value of 131,000K, and lies the very upper range of the accepted PNNe temperatures. The flux at 479.3nm. is a very high 1.26±0.11×1013, which normally for most PNe is in the order of values around 10-16. This suggest this planetary might be only several thousand years old. This PNN is also not exactly central to the symmetry of the nebulosity, being off-centre by about 7 arcsec toward the NNE.

NGC 2438 has been well-studied in the literature for more than a century, but there are still many secrets to unlock here.


References for NGC 2438

  1. Anderson, C.M., The proper motions of thirty-three planetary nebulae.”, Lic.Obs.Bull., 17, 21 (1934)
  2. Aguirre, E.L., Forgotten Legacy of Leo Brenner” section Amateur Astronomers, S&T., p.100-102, August (1995)
  3. Brenner, L., Beobachtungs-Objekte für Amateur Astronomen” Pub. Meier, Leipzig (1902) [In German]
  4. Armsdorfer, B., Kimeswenger, S., Rauch, T., Photo-Ionization Modelling of the Multiple Shell Planetary Nebula NGC 2438”, HvaOB, 26, 49 (2002)
  5. Curtis, H.D., Descriptions of 762 Nebulae and Clusters: Photographed with the Crossley Reflector.”, Pub. Lick.Obs., 13, 11, Part I (1918a).
  6. Curtis, H.D., The Planetary Nebulae.”, Pub. Lick.Obs., 13, 55, Part II (1918b).
  7. Gathier, R., Pottasch, S.R., Magnitudes of central stars of planetary nebulae.”, A&A., 197, 266 (1988)
  8. Herschel,W.[XX.] Catalogue of a Second Thousand of New Nebulae and Clusters of Stars; with a few Introductory Remarks on the Construction of the Heavens.”, Phil.Trans.Roy.Soc. London,, 79, p.212 (1789)
  9. Jones, K.G., Ed., Webb Society Deep-Sky Observers Handbook : Volume 2, Planetary and Gaseous Nebulae”, Pub. Lutterworth Press, New Jersey (1979)
  10. Jones, K.G., The Search for the Nebulae.”, p.67, Pub. Alpha Academic (1975)
  11. Kiss, L.L., et al., AAOmega radial velocities rule out current membership of the planetary nebula NGC 2438 in the open cluster M46”, MNRAS., 391, 399 (2008)
  12. Knarchenko, N.V., et al., Astrophysical parameters of Galactic open clusters.A&A., 438, 1163 (2005)
  13. Parsons, W., Observations on the Nebulae”, Phil.Trans.Roy.Soc. London, 140, 499 (1850)
  14. Pauls, R., Kohoutek, L., Study of the planetary nebula NGC 2438. I. Spectroscopy of the nebula and of some cluster stars”, AN., 317, 413 (1996)
  15. Stanghellini, L., Shaw, R.A., Villaver, E., The Magellanic Cloud Calibration of the Galactic Planetary Nebula Distance Scale.”, AJ., 689, 194 (2008)
  16. Steinickle, W., Observing and Cataloguing Nebulae and Star Clusters : From Herschel to Dreyers New General Catalogue. p.299 (2010)
  17. Stoyan, R., et al., Atlas of the Messier Objects : Highlights of the Deep Sky”, Pub. Cambridge University Press p.193 (2008)



  18. NGC 2437 / M46 / H IV 39 / h.464 / C*0739-147 / OCl 601 (07417-1449) [U274/U275] is the open star cluster.

    Open Cluster Descriptions

    Steve Coe, using 32cm. 13.1-inch f/5.6 describes;

    NGC 2437 (M 46) Very bright, very large, rich, somewhat compressed, I estimated 140 stars by counting 35 stars in the northeast quadrant. The cluster in obvious in the 11×80 finder and can be seen naked eye at most of the observing sites in Arizona.

    Sentinel 13-inch 7/10—11×80 immediately obvious, 4 stars resolved, a smooth cluster, 150× very bright, very large, compressed, round, 116 stars counted, many beautiful chains. Planetary on north edge is obvious, light green with central star.

    RFT 6−inch f/6 Sun Valley 7/10 S+T Both M 46 and M 47 fit in the same field of view with 38mm Giant Erfle EP. Even at this very low power (30×), M 46 shows up like a rich grouping with many pretty faint stars. Using the 14mm EP resolves 68 stars in a very bright, very large, compressed and very rich cluster.


    Surrounding Fields


    M47 / NGC 2422 / ()....

    Observational Descriptions of M47

    Steve Coe writes on the open star cluster as;

    Very bright, very large, not compressed, scattered group with several bright stars of 8th magnitude and more to fill in the background, 44 stars counted at 100×. This nice cluster is an easy, obvious cluster in the 11×80 finder, there are several stars resolved and a lovely orange star, KQ Pup, is about 30 arcmin west of the cluster at low power. Struve 1121 is a double star near the center of M-47, it is a mag 7/7.5 pair, sep 7″, it is easily split at 100×. On an 8/10 night during the Messier Marathon, the 13-inch resolved 85 stars from mags 8 on down. This great cluster shows a bizarre round section in the center with NO stars, strange to see some many cluster stars and none in the center, even on an excellent night.

    RFT 6-inch f/6 very large, pretty rich, not compressed, 24 stars counted with 38mm EP. A wide, dark lane separates the 2 lines of stars. With the 14mm EP 38 stars are counted and William Herschels description of st L & S” for stars large and small” is obvious. This cluster is indeed a mix of stars magnitudes 8 and 9 then a jump to stars of 11th mag and fainter.


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