NEAT SOUTHERN PLANETARIES : 25a
Surrounding Objects of NGC 2818

NGC 2845 / h.3159 / PGC 26306 / ESO 314-10 (09186-3801) lies 47′SE (PA 136°) from the yellow F6III 4.6 magnitude, K Vel / K Velorum (09158-3725) in northern-western Vela. It is placed some 1.4°SSE (PA 158°) from NGC 2818 within the neighbouring constellation of Pyxis. NGC 2845 is visible in 10.5cm. with care in dark skies, though certainly easy in 20cm. Discovered by John Herschel in 1835 whilst in South Africa, who described it as “vF, S, R, *12 att sf”, this deep-sky object remains an established faint bright spiral galaxy of 13.0v, 13.7b, 12.9B magnitude with the surface brightness (SB) of 14.1 magnitude. Images suggest the size is 2.0′×1.0′ in PA 67°, and visually it appears about 70% this size.

Fig.5. NGC 2845 in Pyxis Image (Aladin) →

Telescopic appearance in 20cm. show just fairly evenly bright nebulosity without much indication of structure. Larger apertures might be able to see more of the outer fainter ring, but little better regarding anything else of the inner structure. Classed in Hubble classification is the fairly atypical Sa-type spiral galaxy, but the most detail and modern Revised Morphological Types (1981) states .SAR0* — suggesting the slightly uncertain non-barred spiral with another fainter outer ring. [Figure 5, in false colour, shows the nature of the galaxy, which has been enhanced to see the extent of its outer regions.]

Ru 74 / OCL 757 (09210-3707) is a small unimportant open star cluster, in the same field some 14′ESE (PA 119°) from NGC 2845 (09186-3801) (above). Ruprecht 74 is also 1.0°SE (PA 132°) from the 4.6v magnitude star, K Vel. The cluster was discovered and published by Ruprecht in 1960, whose size subtends about 2.2′ and contains about twenty stars. The appearance looks like to opposing arcs of faint stars. It classed as the poorly Trumpler classification of 3 1 p -. I had some problems at first identifying the stars in 20cm., and could not really say on visual appearance that the concentration of the components was much different from the general field. This is sometimes the case, as such clusters have been only really identified by wide-field photographic image that shows condensation of an aggregate of stars.

HJ 4166 A-BC / RST 3619 BC (09033-3336) in Pyxis is listed as a triple star in WDS06, and lies only 3°N of NGC 2818a. The main AB visual pair was found by John Herschel in 1836, being disjoined by 13.7 arcsec along position angle 153°, and whose components are 6.7v and 8.6v (7.10V and 7.93V) magnitude. Both stars are white (colourless, more precisely), whose combined spectra finds two A0V main sequence stars. Contained in a fairly poorly populated field, this is a very easy pair even in 7.5cm.

It was Rossiter in 1936 who found the 8th magnitude ‘B’ companion had another 11.8 magnitude ‘C’ star very nearby. This addition component is separated by 0.9 arcsec at PA 74°, whose last measure was made back in 1951.

Unusually, the close companion is the star that is the double, and not the primary as one would expect. These are rare kinds of multiples, because the brighter component usually suggests it holds the greater mass. Also gravitationally speaking, determining any companion just orbiting some single star is problematic. This is generally specially true, as such orbits have been proven to be more unstable, and throws up the additional problems of how such systems form. Proper motions of all three components, however, cast a slightly different story.

David Crump’s only description from all the sources mentioned, previously calls this HJ 4166 as;

“This double consists of a 6.5 magnitude primary which I saw as white and a 7.5 magnitude companion which I saw a white with a hint of yellow. The distance between this pair is 13.7″, so it is within the range of the smallest telescope. This system is really a triple, for the companion is also a double with an 11.5 magnitude companion at a close 0.8″.”

The stars in summary are as follows;

HJ 4166 A is HIP 44442 / HD 77737 / PPM 285989 / SAO 199924
Mag. : 7.087B, 7.093V, A0V+
π=7.55±2.09 mas., d (pc.) = ± d (ly.) = ±
pmRA; -18.51±1.87 ; pmDec; +0.25±1.25 mas.yr^-1.

HJ 4166 BC is unresolved by Hipparcos, and given as single star
HIP 44443 / SAO 199925 (Unlisted in PPM or HD Cats)
Mag. : 7.2B. 8.5V
π=8.10±4.21 mas., d (pc.) = ± d (ly.) = ±
pmRA; -21.24±3.63 ; pmDec.; -0.57±2.67 mas.yr^-1.

Unlisted “Pair” (09336-3332) Another two stars appear in the field 7.8′S, being; HIP 44472 / HD 77788 / PPM 285998 / SAO 199933 (09336-3332) and TYC 7155-168-1 / HD 77808 / GSC 07155-00168 (09337-3333) These are 7.76V and 9.39V, respectively, whose spectral classes are A2IV and A2V. Both stars are separated by 1.28′ or 88 arcsec along PA 146.7° — perhaps a little too far for a recognised pair.

Pyxis Cluster / Pyxis Globular Cluster / C J0908-373 (09080-3713) is an unusual globular star cluster the globular star cluster some 1.7°WSW (PA 249°) from NGC 2818. Although remarkable difficult and faint, it is unexpectedly listed as 12.9v magnitude, and is just 2′N of the Vela / Pyxis border! Amazingly, it was only discovered by Ronald Weinberger in 1995, who was scanning the field using infrared POSS images. Its diameter is about 6 arcsec, whose radial velocity is +34.3±1.9 km.s^-1 away from us. The only visual observation I have seen is by globular observer, Barbara Wilson, that is described in the on-line article “Extreme Halo Globulars”. A summary of this object can be found at the SED’s page “Pyxis Globular Cluster” Distance is given as 39.36 kpc., whose absolute magnitude is -5.73.

Since writing this, I recieved a personal communication from Les Dalrymple of his observations of this elusive globular. This was made on the 11th January 2008, using his 46cm. with a 9mm Nagler and 12mm TII (247×, 185×

“Not 100% certain but I think intermittently, I can see it. Appears just to the W of the mid-point between mag 7 & 8 stars that are aligned N-S and are about 13′ apart. There is also a row of three mag 12 [stars] of similar brightness that are not quite evenly spaced running SE to NW just to the N of the cluster. The spot I can see is just to the S of the mid-most star. Intermittent threshold detection as a round patch of dilute gossamer with no apparent central brightening about 2.5′ diameter and includes 4-5 mags 15.5 — 16 stars sewn over. Looks nothing like a G.C — more like a field irregularity. Very well populated field of mod rich milky way background. 247× the highest magnification allowed by moderately poor seeing.”

Postscript: Don’t Believe Everything You Read!

Planetary NGC 2818a is one of the classic examples, which I whimsically call the “Robert Burnham Principle.” This states;

“The further south a deep-sky object appears, the less reliable is the written information about that object — unless, of course, it really hurts the eyes while you are peering in a telescope, just because of its sheer brilliancy!”

Taking this radical point in hand; the following quotes on this object appear in the literature about NGC 2818a.

a.) Sanford says; “…it is about 40 arcseconds in diameter, faint and located at the western edge of the cluster.“

b.) Harrington writes; “On the west-northwest edge of the cluster [NGC 2818], casting a gray image, lies the dim planetary nebula NGC 2818A. The nebula measures 38 arc seconds across and appears as a hazy, 13th magnitude disk through my 13.1-inch f/4.5 Newtonian on Long Island.”

c.) Ostuno notes that the planetary appears on photographs as a smaller version of M27; “…but in the eyepiece [of a 13-inch reflector at 214×], all I noted was a circular blob of nebulosity.”

d.) The late Walter-Scott Houston said; “…a 10-inch [25cm.] is probably required to see this nebula, [though] I could not see it with a 12-inch [30cm.]”

To me, this is a classic example of NOT listening too much in what has been written of observational astronomy. This is especially if the selected quote is likely from some object-staved northern hemisphere observer — and also if it happens to predate the Dobsonian Era or even the [O-III] era.

The best advice is just to have a look yourself!

Last Update : 25th February 2012

Southern Astronomical Delights © (2012)

For any problems with this Website or Document please e-mail me.