NEAT SOUTHERN PLANETARIES : 25
NGC 2818 in Puppis

Next we travel into the mid-southern constellation of Pyxis — the Mariner’s Compasses to meet an interesting planetary nebula that appears to be part of an open star cluster. This is the brilliant and wonderful NGC 2818A, which is well studied and well loved among amateurs and professional astronomers alike. This brilliant combination, however, is not unique. Such arrangements appear in Puppis with NGC 2437 / M46 (07417-1449) [NP 08] & NGC 2438 and within the globular clusters NGC 7078 / M5 (21300-1210) in Pegasus and NGC 6656 / M22 (18364-2354) in Sagittarius, and as we will see, elsewhere. Those interested in the planetary / open cluster in Puppis might like to also read the updated NSP 8.

In the whole Neat Southern Planetaries series, I have been trying to mix it up a little with obscure and some well known PNe. My attempts are to reserve the brilliant objects for various occasions every now and then, and NSP 25 is now one of those times. I should point out that observations of planetaries in the southern Milky Way are probably best between February and August, when the arc of pearly white is nearer the zenith and culminates in the first few hours of darkness. For the rest of the year, the availability of most southern PNe is more restricted, meaning the order in which they appear in this NSP series might be seen as more random than they should be. So while I attempt to introduce more tempting targets nearer the meridian each month, it is not always possible to achieve for everyone’s satisfaction.

Also this time, in our Surrounding Fields NSP 26a, I have chosen one wee little galaxy, another open cluster or two, a newly found globular star cluster; and naturally a nice pair or two!

I do encourage visual observers to hunt down this PNe in the next few months, and either make a written observation — as a Universe submission — or perhaps as updates for some future NSP. (I write the stuff, and would appreciate knowing if I am again “Talking though my Ballarat (nee hat)” or that things are as they are or are as they should be!) For me, correspondence on PNe matters are always welcome!

NGC 2818A / NGC 2818 / Δ564 / h.3154 / Hubble 2 / He2-23 / Sa2-39 / Wray 16-38 / PK 261+8.1 / PN G261.9+8.5 (09160-3638) [U364] seems to have been first discovered by James Dunlop, as he refers to the accompanying cluster also containing a nebula. Both open cluster and planetary nebula are together are located within an obscure patch of sky in the corner of southeastern Pyxis. This fairly easy to observe and find nebulosity is situated on the western edge of the open cluster. Both are sometimes together labelled NGC 2818, but other listed both separately as NGC 2818a and NGC 2818, respectively.

It is best found some 2.8°WSW (PA 255°) from the pale orange 4.5 magnitude star, ε Ant / Epsilon Antliae (09292-3557). I have in the past starhopped from the brighter 3.6 magnitude yellow star, ψ Vel / Psi Velorum (09307-4028), some 4.8°NW (PA 322°) or even used the yellowish 4.0 mag. star β Pyx / Beta Pyxidis (08401-3518) some 7.4°ESE (PA 103°) away. It is near impossible to confuse, as the whole region is scattered with many faint and anonymous galaxies of little interest to us amateurs. The only unusual object near here is the globular star cluster known as the Pyxis Cluster (09080-3713) some 1.7°WSW (PA 249°) from NGC 2818. [See Surrounding Objects.]

Among all known PNe, this is fairly large. Telescopically measures about 40 arcsec, where NGC 2818a increases to 1.9×1.0 arcmin or 114×60 arcsec with most CCD or photographic images. The long axis is aligned at PA 80°, even though most photographed reproductions realign this across the page to make the nebula appear more symmetrical and pleasing to the eye.

Given magnitudes for NGC 2818A are 11.6v or 13.0b, with the V-V classification system as Class 3b — ‘brightness distribution, irregular disk, with traces of ring structure.’ In some ways, it clearly resembles the brighter and slightly more famous outstretched northern sky planetary, NGC 6842 / Dumbbell Nebula / He2-451 / Sh2-95 / PK 065+0.1 / PN G65.9+0.5 (19550+2917) in the constellation of Vulpecula. (13.6p magnitude, 57.0 arcsec across, class 3b, whose central PNN star is 15.9p.) Observers here might just like to try the comparison themselves, as the Dumbbell Nebula is usefully placed in the southern observer’late-spring skies!

Popularity of NGC 2818A has increased with the publication of the Hubble Space Telescope (HST) image in 2008 (Figure 1.) The general structure of the nebulosity is seen in interesting three-coloured image. Figure 2 shows the wider nebula and cluster in the field measuring 18′×18′, being equal to medium to high telescopic magnifications from ground based imaging. Figure 3 shows a false coloured closeup of the nebulosity, obtained by infrared, red and visual imaging. Blue is mostly [NII] and [OIII], with the left and right red edges being mostly from Hα light. The central star is just visible in the central haze, which is unusually surrounded by a darker nebula, more prominent just north of centre as seen in both Figs. 1 and 2.

I would usefully describe NGC 2818 as “per se” (sic) with the colour perse — the archaic term for the colour bluish-grey or steel blue — and this colour certainly is appropriate with another far southern planetary, NGC 3195 / He2-44 / Sa2-57 / PK 296-20.1 / PN G296.6-20.0 (10093-8051) in Chameleon. [NSP 5a.] I have often thought that the odd word “perse” should be used in common planetary nebula terminology as the colour it describes covers about one-third to about half of all PNe.

[Just to confuse this lexicon a bit further for a bit of fun with the nasty “tongue twister” the planetary in M15 was discovered by F.G. “Pease” as Pease 1 in 1928! I.e. Try and quickly say “Perse per se Pease.”!! Not easy!]

Historical Descriptions

James Dunlop discovered this planetary nebula and cluster on 26th May 1826, being listed as Object No.564 or Δ564 in his deep-sky catalogue. Dunlop found very few PNe among the 187 galactic objects that were attributed to be Dunlop discoveries, seemingly favouring or preferring the much larger and more extensive deep-sky objects. His other three PNe discoveries were NGC 5189 (13335-6558) in Musca [NSP 01], NGC 6326 (17208-5145) in Ara and NGC 6563 (18120-3352) in Sagittarius. This lack of planetaries maybe more due to the likely poor quality of his homemade 9-inch speculum reflector than the want of not looking for the many small planetaries that generously dot the entire bright portions of our southern Milky Way.

According to Cozens, G., Walsh, A., Orchiston, W., (2010) (pg.68) states that the reason for this is as follows;

“Instead, to determine which objects he missed, a working magnitude limit was ascertained and a list of bright objects which were missed was compiled. To estimate the working magnitude limit, the magnitudes of identified objects were obtained from a modern catalogue specific to a particular type of object. Open clusters, globular clusters, nebulae and planetary nebulae were found to be unsuitable for finding the working magnitude limit of Dunlop’s telescope. This is because open clusters vary greatly in size and detachment; there is a lack of globular clusters faint enough to test his magnitude limit; only a few diffuse nebulae have known magnitudes; and planetary nebulae are often very small and easily missed.”

They also comment on the possible number of PNe that were available to be seen in Dunlop’s telescope, that;

“Dunlop also catalogued 4 out of 14 (29%) planetary nebulae brighter than magnitude 10.9, and larger than 12′ according to the Strasbourg-ESO catalogue (Acker, et al., 1992).”

Dunlop (1828) describes NGC 2818A after observing it on two separate occasions as;

“A pretty large faint nebula of a round figure, 6′ or 8′ diameter; the nebulosity is faintly diffused to a considerable extent. There is a small nebula in the north preceding side, which is probably a condensation of faint diffused nebulous matter; the large nebula is resolvable into stars with nebula remaining.”

Some twelve years later John Herschel also twice observed NGC 2818 / HJ 3154 in 1838, who usefully describes;

“A very curious object which reminds me strongly of Messier’s 46 [NGC 2437] and IV.39 [NGC 2438]. It is a rich cluster of the VI class; stars 12..14th mag; about 8′ diameter; gpmbM; all but a sort of vacuity, in which is situated a pB, R, neb; 40″ diameter; of a character approaching to planetary, having its edges shading off very rapidly, and being but very little brighter in the middle.”; and;

“pB, R, vgvlbM from the edge, where it fades off very suddenly; being all but a planetary nebula. Situated in a sort of vacancy in the preceding part of a fine rich cluster of stars 11..15th mag, which nearly fills the field. It is a fellow object to Messier’s 46, with its enclosed planetary nebula IV.39. Plate V, fig.8.”

Many wrongly state, like Hynes (1991), that; “NGC 2818 was discovered by John Herschel…” This is clearly Dunlop’s (1828) discovery, as the observation and given position of the cluster is within the stated field. This error probably comes from Acker (1992), who also states John Herschel as discoverer in 1838.

Observational Descriptions

There are so many interesting written descriptions and images on this particular PNe, that it would be impossible to list them all. Likely this huge number is probably down to NGC 2818A having the unusual attractive novelty of being within an open star cluster. Were this not true, perhaps both the cluster and nebulosity would be lower on the interest scale amongst amateurs and professionals alike. Some open descriptions the nebulosity are also slightly different from many other bright PNe, in that it is not exactly uniformly round. This is like Robert Burnham Jr. who notes that it has a “barbell shape.” [See Postscript.]

One of the first descriptions I ever read on this object once featured in David Crump’s “Crump’s Corner” (Universe, 27, 4 [April] 1980)

“This is a sparse cluster containing about thirty stars of magnitude 12. There is also a faint 13th magnitude planetary nebula on the western side of the cluster. Through my 20cm f/10 reflector I could see it best by averted vision. It appeared as the usual circular patch of light…”

Kent Wallace, in his self-published “Planetary Pages — 300+ Planetary Nebulae for an Eight-Inch Telescope” (1997) says of NGC 2818A;

“At 62.5×, can see as a faint disk using averted vision alone. Strong response to the O-III & UHC filters. No response to the Hβ filter. At 100×, the disk shows up better. At 200× the disk shows up best. No central star is visible.“

Les Dalrymple made an interesting observation from Bargo on 18 November 2006. He says, as an introduction;

“Swinging back to the south my mind turned to a PNe I hadn’t observed for approaching 10 years I guess — NGC 2818A in Pyxis that is a bona-fide member of an open cluster (not a mere superimposition like NGC 2438 — see earlier) NGC 2818.”

Les, using his 46cm Dobsonian, with magnifications of 185×, 247×, 317×, (Unfiltered, UHC & OIII), describes;

“This is the first time I’ve seen this PNe in this aperture and it didn’t disappoint — a very interesting and unusual PNe. The brighter areas look like a pair of longish butterfly wings without the butterfly body in the centre — longish opposing mirrored triangles with the long axis in PA 90°. Each long triangle is about 30″ long and 10″ wide at the widest and the N[orth] one is slightly the weaker in brightness and definition. The short sides of the triangle are the best defined but the two long Hypotenuse much less defined on their edges. An area around this is very weakly nebulous out to  60′ diameter. Moderately bright. Good response to OIII and UHC. Some bluish/smoky bluish colouration. Best at 247× with 9mm T1 Nagler.“

An image is here, where he concludes; “I won’t let it escape that log next time! Very, very pretty.“

In Hartung’s AOST2 NGC 2818a is described as;

“…the nebula lies in the western edge of scattered stars about 7′ across, and appears pale grey, not bright, about 40″ in diameter of somewhat uneven light with a single prism image. Together these objects make an interesting field and 15cm. is needed to show them clearly. Both these objects are associated and the distance is about 3200pc.”

Steve Coe, observing with 32cm. (13-inch) f/5.6, notes:

“The planetary nebula is easy at all powers on the Eastern edge of the open cluster. A few dark lanes are seen in the planetary at 165×, it is light green at all powers.“

Deep photographic images look particularly strange and are very similar in shape and surface brightness to two other bipolar planetaries;

1) NGC 2346 (07094-0048) in Monoceros, some 40—SW of δ Mon / Delta Monocerotis (07118-0030), and is more northerly bipolar “3b-type” planetary. [See Neat Planetaries NP08]

2) NGC 6445 / He2-290 / PK 008+3.1 (17493-2001) in northwestern Sagittarius and is in the same field as the globular cluster NGC 6440 (17489-2022). (Separated by 20′ in PA 194&176;) Magnitude is 13.2p over the mean size of 33.0 arcsec, whose V-V classification is 3b+b. The PNN of NGC 6445 is about 19.0 magnitude and therefore invisible to amateur telescopes. [NGC 6445, a personal favourite of mine, will feature in an upcoming NSP nearer the middle of 2012. Promise!]

Relationship of Planetary and Cluster

There seems to remain some open debate regarding the association of the planetary and the cluster. In the wider amateur literature, there seems to be about half agreeing and the other half disagreeing. For example, the only main contradiction between AOST1 and AOST2 on NGC 2818 and NGC 2181a is about the real association of the PNe and cluster, where E.J. Hartung says;

“…although they are unconnected, R for the nebula being given as 3,600 pc and for the cluster as 1,500pc.”

[This last distance in Hartung AOST2 was likely obtained from “ESO Planetary Nebula Catalogue” by Acker (1992).]

Hynes, S.J., (1991) also states;

“Evidence for the association was first provided by Tifft et al. (ref. 1.), based on radial velocity measurements, and the fact of the relationship makes it possible to determine the distance and hence other nebula parameters independently of the problems which normally bedevil such matters. In this case the distance is calculated at 3.5pc.“

Yet the more recent journals now suggest otherwise. For example, Majaess, Turner & Lane (2007) now state;

“The well known spatial coincidence of the planetary nebula NGC 2818 with its surrounding cluster is an example of a case that visually supports an association (Figure 7). Pedreros (1989) determined a distance of d=2300 pc. and a reddening of E(B-V)∼0.18 for the cluster, consistent with the parameters derived for the planetary nebula: d=2660±830 pc. (Zhang 1995), and E(B-V)∼0.28±0.15. Equally encouraging are radial velocities from low dispersion spectra (230Å mm^-1) by Tifft et al.”

The debate, it seems, lies with the measured radial velocities, which they discuss as follows;

“…for the two A-type stars in the cluster that yielded VR=3±20 km.s^-1, compared with VR=8±13 km.s^-1 obtained for the planetary nebula. Such evidence, in conjunction with the general agreement in distance and reddening, has been the basis for the conclusion that the two are associated”

More recent results suggest otherwise. A comprehensive radial velocity study of stars in the cluster field by Mermilliod et al. (2001) yields a cluster radial velocity from 15 red giant members of VR=20.7±0.3 km.s^-1, while the radial velocity of the planetary nebula is established to be VR=−0.9±2.9 km.s^-1 (Durand et al. 1998) and VR=−1±3 km.s^-1 (Meatheringham et al. 1988), consistently smaller than the velocity of the cluster. The greater precision of recent estimates results in a velocity discrepancy of ΔVR=22 km.s^-1 (Mermilliod et al. 2001), implying a spatial coincidence rather than a physical association, as concluded by Mermilliod et al. (2001).

It is recommend that serious amateurs interested in PNe in open galactic clusters or globular clusters might like to read for themselves Majaess, Turner & Lane (2007). Here they give thirteen possible PNe/Clusters, of which seven are now certainly associated. (I have linked the arXiv paper in the list of references.)

Technical Data

There is so much information, and even significant contradictions, throughout the relevant papers I looked at, it is difficult to assess what to present to the reader here. In this case it is best to stick with the latest data. In the general summary, although now somewhat old, is Hynes, S.J., (1991) words;

“The nebula itself has been discovered in detail by Dufour (ref.2.), who described it as having a filamentary structure, consisting of a bright, bipolar “hourglass” orientated north-south and larger fainter “wings” orientated east-west, centred on a faint star. The distance of NGC 2818 indicates that it is exceptionally large, about 2.2×1.0 pc. Taking into account the expansion velocity, an age of 10,000 years is implied for the “hourglass” and 22,000 years for the “wings.” Dufour calculates that the mass of the nebula is 0.6 solar mass and the mass of the progenitor star 2.2 solar masses. Chemically, the nebula is similar to NGC 2440, of helium and nitrogen.”

I can say nothing better! Latest information finds the estimated size of the inner or central nebulosity is about 0.25 pc. or 0.8 ly., but the overall size in much larger than many other PNe observed in the sky.

At radio wavelengths of 5 GHz, the size is 33×55 arcsec, still used to find the 2.0 kpc. distance. Some sources assume a rounded 50 arcsec. Surface brightness of the nebulosity is measured and assumed by the Hβ emissions, is 8.04×10^-9. This is brighter than an average planetary.

Flux of the observed lines in the obtained spectra find strong emissions in Hα, [OIII] and [NII]. This is roughly calculated by making comparison of the [OIII] line to the standard Hβ line (fixed value of 100.0); being in this case, the ratio of 1501:100 or 15:1, being higher than most, and explaining the response to the OIII filter at the telescope. Furthermore, the expansion velocities are also measured by these bright fluorescence or forbidden lines; being 104 km.s^-1 for both the blue-green [OIII] and red [NII]. Some parts of the nebulosity have [OIII] lines with slower expansions of about 42 km.s^-1. Fig. 4 shows the appearance of the nebula through the four main emission lines of Hα, He I, and the forbidden frequencies of [NII] and [OIII]. Differences in nebula structure is obvious, and visual observers using OIII filters should see the PN more elongated than the plain telescopic visual image, which appears more roundish in shape.

Understanding the evolution of these structures of this planetary has been a priority, easily judged by the 224 scientific studies of this object. No doubt future studies will reveal more knowledge of this wonderful object.

Comment : Some risqué wag has even said that the image of NGC 2818a once taken by the AAT (I.e. AOST2; Plate 44 (Not shown here) does truly looks more like two crabs caught in the act of, um, procreation! Already knowing how crabs do this is perhaps even more of a worry!

Distance of NGC 2818A

Acker (1992) has a listing of many of other published distances. Those based on local extinction studies find values between 1.1 to 1.8kpc. (1978 to 1984), while statistical studies of the PN finds 1.4 to 2.2kpc. (1971 to 1984.)

Latest derived distance for NGC 2818a is by Stanghellini, L., Shaw, R.A., Villaver, E., (2008), who find 2.0kpc. (1.998 kpc.) This matches many other quoted values, including the several statistical measures using the so-called Shkolovsky distance of 2.1kpc. However, this is in disagreement with the derived and accepted cluster distance of 3.6 kpc. found first in 1984. Bond (2008), who did the HST imaging and paper for NGC 2818a, suggest the distance is 3.2 kpc. [Readers here can probably now see why PNe distances are so notoriously unreliable — and even with the brightest examples of their kind!]

The PNN of NGC 2818A

Again, Hynes, S.J., (1991) says that;

“The brightness of the central star has been subject to some discussion with a considerable amount of variation in the determinations; …photographic photometry of Kohoutek et al. yields V=18.5 mag., B-V=+0.1 mag., U-B=-1.22 mag.”

Values quoted have been superceded in the last decade or so. Gorny (1997), and some other recent papers, usually states in modern descriptions, that this PNN is 19.45V (AJ., 197, 266 (1988)) Using this, and other observations, the Zanstra temperature is 195,000K from the HI emissions — among the hottest known and typical of what are seen in PNN in bipolar planetary nebulae (BPNe). Absolute magnitude is +7.42 for the PNN, which for a pre-white dwarf the size of just a small Earth-like planet, is most extraordinary. Energies illuminating the nebulosity, are mostly in the ultraviolet.

Mass of the PNN is >0.94M⊙, suggesting the original stellar progenitor weighed between 3M⊙ or 4M⊙. (This seems larger than the surrounding cluster stars, which are now more solar-like in mass. As big stars evolve faster, and die earlier, we can estimate the minimum age of the cluster — if the PN / open cluster are associated — to be about 700 to 900 million years old.

References : NGC 2818A (PNe)

1. Acker, A., et al., “Strasbourg-ESO Catalogue of Galactic Planetary Nebulae. Garching, European Southern Observatory”, Pub. ESO, (1992)
2. Cozens, G., Walsh, A,, Orchiston, W., “James Dunlop’s Historical Catalogue of Southern Nebula and Clusters.”, Journal of Astronomical History and Heritage, 13 (1), 59 (2010).
3. Dufour, R.J., “The Unique Planetary Nebula NGC 2818”, AJ., 287, 341 (1984)*
4. Dunlop, J., “A catalogue of nebulae and clusters of stars in the southern hemisphere, observed at Parramatta in NSW.”, Phil.Trans.Roy.Soc., 118, 113 (1828)
5. Gorny, S.K., “Planetary Nebulae, Morphologies, Central Masses and Nebular Properties.”; A&A., 318, 256 (1997)
6. Kohoutek, L., Roth-Hoeppner, M.L., Laustsen, S., “Study of the planetary nebula NGC 2818. I — Photometry of the central star”, A&A., 162, 232 (1986)
7. Mermilliod, J.-C., et al., “Red giants in open clusters. IX. NGC 2324, 2818, 3960 and 6259.”, A&A., 375, 30 (2001)
8. Majaess, D.J., Turner, D.G., Lane, D.J., “In Search of Possible Associations between Planetary Nebulae and Open Clusters.”, PASP., 119, 134 (2007) [See Section; 2.6. NGC 2818: Planetary Nebula and Cluster (l ∼ 262°)]
9. Pedreros, M., “The open cluster NGC 2818 and its associated planetary nebula.”, AJ., 98, 2146 (1989)
10. Phillips, J.P., Cuesta, L., “The density and shock characteristics of NGC 2818”, A&A.Sup.Ser., 133, 381 (1998)
11. Stanghellini, L., Shaw, R.A., Villaver, E., “The Magellanic Cloud Calibration of the Galactic Planetary Nebula Distance Scale.”, AJ., 689, 194 (2008)
12. Tifft, W.G., Conolly, L.P., Webb, D.F., “NGC 2818, an open cluster containing a planetary nebula.”, MNRAS., 158, 47 (1972)

* I can heartedly recommend the paper Dufour (1984) on NGC 2818a, which has an excellent coverage of NGC 2818a with pertinent images to assist in learning the history of this object beyond this general article.

Immediate Field of NGC 2818a

NGC 2818 / Mel 96 / Cr 206 / Rb 82 (09161-3638) [U364] is listed as the open cluster whose unique distinction is more from the occupying planetary of NGC 2818A. Telescopically, this fairly inconspicuous cluster contains about 30 to 40-odd stars within about 10 arcsec. From the cluster’s perspective, the perse coloured planetary lies towards northwest edge, being roughly 4 arcmin from the apparent centre.

Both the cluster and planetary can be seen in 15cm, but it will appear much better in >20cm. However, various description by observers of the cluster itself are fairly sporadic, mainly because the observers will commonly focus far more on the planetary nebulae instead.

After being placed in the NGC in 1888, when it was classed as the cluster, which the RNGC (1973) gives as “OCL, Pretty Bright, Pretty Large, Round, Very grad. Little Brighter in Middle.” NGC 2818 was then briefly investigated by the open cluster astronomer Melotte in 1915. However, the first study could really be assigned to Trumpler in 1930, who found the distance as 1.68 kpc. — about half the result accepted today. NGC 2818 was first classed under the Trumpler system by Ruprecht (1966) as II 2 m -. (The dash is supposed to indicate cluster nebulosity ‘n’, but it seems that a planetary nebula isn’t worthy enough to designate that!)

It was Tufft (1973) produced the first detail photographic and photometric study of this cluster, including the H-R Diagram with 173 stars, finding the distance as 3.2 kpc. His identification of the bright component stars has become the basis of cluster membership, and presented on reasonably grounds that the planetary was also another associated member.

In another study made by Surendiranath (1990), who surveyed stars down to 21st magnitude. He deduces that the age of the cluster is some 500 million years, and the old disk stars are similar in metallicity to the Sun, with the interstellar extinction E(B-V) of +0.24 magnitudes. Radial velocity of the whole gravitational system averages about +3±20 km.s^-1. Distance appearing in the literature is between 1.61 to 2.64 kpc. ; average of 2.13±0.52 kpc.

A planetary and cluster combination is observationally a very rare phenomena, and only just five or six have been identified with either globular or open clusters. Reasons for this is fairly clear, as all PNe exist in a comparatively short time against the long lives of stars. When compared to the hundreds of millions to several tens of billion of years that it takes stars to evolve, the duration of the stages of the PNe phenomena over 10,000 to 50,000 years is merely a blink of an eye. A further restriction is that PNe progenitors have stars between one and eight solar masses, so that the largest will live and die catastrophically in the first few million years. Also the much more numerous smaller members will not have formed PNe, not having had time to reach the final stages of stellar evolution before turning into white dwarfs. Additionally, most open clusters we observe are much younger than the Sun, and will likely significantly dissipate due to loss of many members. Simply, the chances of catching stars undergoing PNe phenomena is rare — explaining why we see so few of them among the 80,000 known open star clusters.

Selected Descriptions

Steve Coe, observing with 32cm (13-inch) f/5.6, notes:

“[NGC 2818 is] …pretty faint, pretty large, somewhat elongated, 16 stars were counted across a hazy background at 135× on a night I rated 5/10, on a much better night (7/10) I called the cluster pretty bright and could resolve 34 members at 150×.”

I have read few other descriptions, as most tend to focus on the planetary!

References NGC 2818 (OSC)

1. Ruprecht, J., “Classification of open star clusters.”, BAICz., 17, 34 (1966)
2. Surendiranath, R., et al., “CCD photometry in VRI bands of the galactic cluster NGC 2818.”; J.ApAs., 11, 151 (1990)
3. Tifft, W.G., Conolly, L.P., Webb, D.F., “NGC 2818, an open cluster containing a planetary nebula.”, MNRAS., 158, 47 (1972)

Last Update : 25th February 2012

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