NEAT SOUTHERN PLANETARIES : 04
NGC 5844 in Triangulum Australe

This region of the sky is an interesting, yet often neglected area by amateur observers. It contains two enigmatic planetaries — one of which has the most bizarre history. I have divided the surrounding fields into two main areas of sky. The first area contains various pretty pairs, a dark nebula and several marvellous asterisms. Another second selected area is near an interesting globular and several more pairs.

NGC 5844 / He2-119 / Sa2-115 / Wray 16-168 / ESO 99-1 / PK 317-5.1 / PN G317.1-05.7 (15107-6440) is part of a small arc of stars in the western portion of Triangulum Australe. According to most of the modern computer software that uses the Hubble Guide Star Catalogue, there is on right on top of the PNe is the bright 7.6 magnitude bluish star (GSC 9029:2732) that is not at this position. (See Footnotes 1 & 2.)

NGC 5844 at position 15h 10m 42.0s -64° 41′ 00″ is listed as non-existent, with the PNe He2-119 positioned at 15h 10m 39.9s -64° 40′ 19″. Differences here are merely 52.8 arcsec — so likely that the two objects are really the same. So let’s assume it is!

To find this planetary, it is best to use the star Triangulum Australis in the middle of the base of the ‘southern triangle’ and the star η Cir / Eta Circini (15048-6402). Draw an imaginary line between the two stars. Divide this line by quarters, and move towards ε TrA from η. Cir by a whole quarter. This centres directly on the planetary, where the two 8th magnitude stars appear within the same field.

NGC 5844’s brightness is 12.1v and 13.2p magnitude and the diameter subtends an angle of 53 arcsec. AOST2 states that this object is “…70×50 arcsec“, which is partly wrong as even the photographic image of the inner shell never exceeds 55 arcsec.

Observational Descriptions

Dreyer’s original NGC description is; “Pb, pl, r, vgvlbm” — Pretty Bright, Pretty Large, Round, Very Gradual and Very Little Brighter in Middle. This rather odd description for this object, as it does not exactly match its telescopic appearance. It then was subsequently deleted from the revised NGC catalogue and no reason is given for this deletion. However, this difference has meant that most Star Atlases and Catalogues — including Sky Atlas 2000.0, Sky Catalogue 2000.0, and in Uranometria 2000.0 [Map 452], etc.; each have not listed this fairly bright and interesting planetary.

During 1997, at Bowen Mountain, in the Blue Mountains 60 kilmetres west of Sydney, all observers were surprised at the brightness of this object, especially as it lies close to the rear-end of Triangulum Australe and Beta Centauri. Using the 45cm. (18-inch) telescope there on 23rd August 1997, both Mick McCullagh and Don Whiteman took sometime to find this object among the brilliantly starry field of Triangulum Australe. When it was found, it was clearly obvious. (Even some local visitors looked at this object, but as first timers I suppose they did not understand the implications of what they were seeing. However, I did give an explanation the general nature of planetaries and their evolutions.)

As an ovoid shaped object it is just visible in 15cm., best found using an O-III filter with medium power. In AOST2, David Frew states that it can be easily found by blinking across the field an O-III filter. This is true for 10cm or 15cm telescope, but any larger aperture can immediately detect its nebulosity. Using an O-III filter, the brightness is fairly even across the entire disk.

I could also see a slight indentation in the northern edge of the planetary. Using 30cm, the appearance of this indent becomes more pronounced, revealing an uneven edge, and this turns into a “bay” in telescopes exceeding 40cm. Larger telescopes should also start to see some structure in the planetary — perhaps more obvious using an O-III or Hβ. filter.

Also observers at Bowen, including Les Sara, thought that the appearance seemed mottled — and even more so with the O-III. Some faint bipolar features were also noted, with some difficulty, by each of them.

The PNN is invisible in all amateur telescopes and with the stated 16.6 magnitude — this is understandable.

Technical Data on NGC 5844

Little appears in the literature is written about this object until the beginning of the 1990s. During this time any observational data available remained scant, but in the last few years, there has been several studies into this planetary.

Older distances often gave around 2.9kpc. (1996), but this has that has been recently determined by Stanghellini, Shaw & Villaver (2008) as 1372 parsecs.

Comment : The corresponding lack of data is a reflection of the poor quality of information on the object’s catalogued position, perhaps more so with the rejection in the RNGC. Again, the northerners have actually ‘butchered’ this most wonderful object. Yet, if it were in the northern skies, we probably would never hear the end of it! I do recommend that southern observers have a look at this object.

If you do so, please correspond with me. If we can get enough information, we could punish those northerners by telling them in no uncertain terms what they are really missing!

References.

Stanghellini, L., Shaw, R.A., Villaver, E., “The Magellanic cloud calibration of the Galactic planetary nebula distance scale.”, AJ., 689, 1942 (2008)

Footnote 1 : Identifying Problems With NGC 5844

The Guide Star Catalogue, designed to be used with the Hubble Space Telescope, is notorious for having errors with non-existent stars (c.20%) and misidentifications of known NGC and IC objects. Most of these have been ‘sifted out’ by observers like Brian Skiff, amateurs and professional astronomers by noting differences when observing objects. Fortunately, most happen to be faint objects.

This 7.6v magnitude bluish star (GSC 9029:2732) stated in the catalogue proved to be a difficult object to explain.

In the GSC positions, according to the CDS listing of the catalogued position is 15h 10m 41m 01s −64° 40′ 23.0″. The source of the data is a taken with the UK Schmidt on March 1976 (Epoch 1976.256) using III-J emulsion film with an GC395 filter producing the derived magnitude was 7.61±0.45. Visually, no star appears at this position nor in several other photographs of this region. The STsI image also shows no star. No star in this position is stated in the SAO or any of the preceding catalogues. Also the positional data of the centre of the PNe He2-119 and this close 7.61 magnitude star is 1.1″N and 5″E, and well within the NGC 5844 boundary.

No asteroids were near this position during 1976 and any known variable stars. It is not in the New Suspect Variable (NSV) listing and no known nova is expressed for this object. It is obvious that a problem exists with this star.

Some twenty-three references were found during an Internet search with SIMBAD. Strangely, there is no reference to the PNN, nor of this star. Nearest star is the 12.51±0.22 magnitude, GSC 9029-1442, whose stated position is 9″W and 43″S. I could not visually identify this star either!

At first, it was compelling to think that this star is a nova or variable associated with the planetary, but after some checking (and extensive personal searching) a final reply ended the debate. This was received via e-mail on Saturday 30 Aug 1997 from Gareth V. Williams the Associate Director of the IAU Minor Planet Centre at Harvard University, Cambridge, MA;

“The “star” GSC 9029-2732 is actually an artefact of the plate measuring process and is simply a measurement of the ‘center’ of the planetary nebula, NGC 5844. Normally measurements of non-stellar objects would be so flagged in GSC, but this object appears to have escaped that classification. I note that NGC 5844 also appears in the USNO A1.0 catalogue, a list that is supposed to contain only stellar objects.”

This apparent chaos seem likely because of the presumed non-existence of NGC 5844. If the catalogues are wrong, then observers will tend not to place it in their observing programmes. Although this proved not to be an ‘historical nova’, its final value at least will correct updates of catalogue listings. It proves that for all the technical marvels of modern astronomy mistakes still occur.

Interestingly, there remains one puzzling question that still cannot be explained; How could the magnitude of this ‘star’ be measured at 7.6 magnitude if the PNe is 12.1v and 13.2p magnitude?

Footnote 2 : Something is Seriously Wrong With Space

(Title taken from Chapter 31 of Arthur C. Clark’s:
The Lost Worlds of 2001)

This field is strange to about 5° surrounding NGC 5844. Firstly, the SAO catalogue (1966) has some stars placed in this region, while others, and sometimes brighter stars are not listed. For example, surrounding the region of NGC 5844 some other 9.5 magnitude stars are listed. Using my own versions of Atlas Australis and Uranometria 2000.0, I had some trouble identifying the actual field. Between the declinations between −64° and −65°, the SAO is both inaccurate and poorly surveyed.

A reason for this might be the original data in the production of catalogues in this region where the problem seems to have started with the beginning of the Astrographic Catalogue in the 1880s. Declinations to be surveyed were delegated to various sites. Those north of declination −64° were delegated to the Cape Observatory in South Africa. South of −64° this was allocated to both Sydney Observatory and Melbourne Observatory. The Cape Catalogue was used to update the positions of stars to −64°. Later of the 50,000 stars, some 15% of stars were rejected because no data on their proper motions were given. In essence, stellar positions in the declination range of −52° to −64° have a few stars simply not listed. When the Astrographic Catalogue was eventually completed in 1936 — these rejected stars did not make there way in amateur atlases from the 1950 onwards.

Note: Some additional data used in the SAO is also taken from Vol.20 of the Ann. Cape. Obs. Catalogue by Jackson and Stoy. (1950.0)

South of -64°, the details become very confusing. It does seem that some procedural policies in producing the southern parts of the catalogue were different than at the Cape Observatory. Firstly, the magnitude limit is not as extensive from -64° to the pole. Secondly, it seems that the first stars, and therefore the oldest positions, are in the region of declinations −64° to −66°. Data for these stars in the declination range were original made using the Third Melbourne General Catalogue, being reduced without Proper Motion to the Equinox 1900.0, eventually published by Ellery and Baracchi in 1917. Again, many stars without known proper motions were not eventually listed in the SAO.

What this means is that many earlier atlases before this time are relatively poor when identifying stars. If you own an old computer program, such as Megastar 4, Voyager 4 or RedShift 5.0, you will notice the sudden drop off in stars in the declination range of −64° to −66°. (Make sure it is SAO orientated data and not based on the GSC or Guide Star Catalogue.)

Amateurs identifying fields in this region should be cautious — especially object below about 7th magnitude. This includes the southern constellations of Pavo, Triangulum Australe, Circinus, southern Crux, northern Musca, southern Carina and northern Volans. Soon, the general introduction of the Hipparcos and Tycho catalogues to atlases will not have this problem with identification. This will reduce the uncertainties in this area to magnitude of about 11.5, but at the moment, no deeper.

In regards NGC 5844, I am unsure if it has played a part to the misidentification of this object. To me, NGC 5844 is still very likely He2-119 / Sa2-115 / PK317-5.1.

Last Update : 04th November 2011

NEAT SOUTHERN PLANETARIES : 04 NGC 5844 in Triangulum Australe