NGC 5189/ IC 4274/ PK 307 -3.1/ Gum 47/ RCW 76) (13335-6559) is a peculiar planetary nebula in the northeast corner constellation of Musca that is commonly named the 'Spiral Planetary'. It lies close to the eastern boarder of Musca, near the constellations of Circinus and Centaurus, some 2.5^o SWW of the double star Theta ( ) Muscae and 1.7^o SSW of m Centauri. E.J.Hartung in 'Astronomical Objects for Southern Telescopes : 1' (AOST1) states that is '...a remarkable gaseous nebula', but at the time it was still thought to be a bright nebula and not a planetary. This confusion still occurs in the most modern atlases. Burnham's Celestial Handbook Vol.2 (BCH2) also lists it as a bright emission nebulae - likely taken from the original N.G.C. Catalogue published in 1888 by John L.E. Dreyer. However, the description is not exactly the same as Dreyer’s original NGC "! B, pl, pE" (Bright, pretty large, planetary emission) The Revised NGC states it as ‘B, PL, CE, BMCURVEDAXIS, 4* INV’ - Bright, pretty large, compressed and extended, four stars involved; but gives no modern extension. Sky Atlas 2000.0 still has it listed as an emission nebula, while the Sky Catalogue Vol.2 has it listed correctly as a planetary! (It is worthy to comment that Sky Catalogue 2000.0 and Sky Atlas 2000.0 has many mistakes for southern objects. Observers should double-check their observing programmes when using Sky Atlas or Sky Catalogue, or using the most recent source about telescopic objects. [Does anyone know of a current definitive list of these errors???]

NGC 5189 was observed by Sir John Herschel in 1835, describing it as a 'very strange object'. Hartung (AOST1) describes John Herschel as the first to discover it, but it was really Dunlop several years earlier. Burnham also makes a more critical error. He states this object is Dunlop ‘ 252' in Vol.2. pg.1216. This is incorrect, 252 is the multiple star Alpha Crucis!

In 1889-99, Williamina P. Fleming made observations in search of planetaries by their spectra, by a photographic method instigated by the Englishman Edward Pickering. Fleming had a reputation as a brilliant astronomical ‘computer’ who had originally been involved in stellar spectral classification. In her astronomical career, she discovered 59 nebulae, more than 310 variable stars and discovered ten novae. Later she produced a Wolf-Rayet catalogue of 108 objects in 1912. (Harvard Obs. Ann. 56, 165 (1912) ‘Stars Having Peculiar Spectra. Type V: Spectrum O.’) [Now that is my kind of observer!] Unfortunately, before the age of ‘political correctness’, Society said women could not become astronomers, the reasoning being the presumed ‘frail’ nature of the female anatomy and to be constantly in the cold was thought to be dangerous. Most women therefore did become involved in astronomy except in the more menial computation or reduction of astronomical observation. During Fleming’s time, universities in fact did not allow women to do science courses at all, so she never reached her full potential. Curiously, her own description in 1898 of NGC 5189 describes it as ‘Planetary, stellar’, yet the stated celestial coordinates are totally correct. (Harvard Obs. Circ. 9 ‘Stars Having Peculiar Spectra’) Although it was already in the NGC, her observation essentially rediscovered the nebula - subsequently listing it as IC 4274. This observation was eventually transposed to the ‘Second Index Catalogue of Nebulae and Clusters of Stars; Containing Objects Found Between 1895 and 1907.’; Memoirs of the RAS Vol. LIX- Part II (1908). Seeing how this could be seen as a stellar object is difficult, even by yesterday’s observational standards. It is obviously nebulous to even a small spectroscope. Perhaps the photographic plate of the field for this object was not exactly brilliant.

Physical Descriptions.

The planetary is contained in a brilliant starry field, and can be found, as Andrew Murrell says in ‘Universe’ August 1995 Vol. 42,8 ‘Objects of the Month’;

"It is best located by finding the two bright stars of about 4.5 mag on the eastern edges of the Coal Sack. Once your finder is centered on these stars you will notice a line of (three) 5th magnitude stars leading away to the southeast. The planetary lies about 20 arc minutes south on top of a 6.5 magnitude star."

This last star is HD117651 / SAO252361 at magnitude 6.4.

In physical size, the photographic image extends some 185"x 130" sec.arc., visually about 150"x 100"sec.arc. Telescopically, some have described it as a boxlike object, but it appears more like a cometary globule. (Maybe it should be named the ‘Boxed Cometary Planetary Nebula’.) Some have described it as a 'Little-Dumbbell', similar in appearance to the famous planetary in the constellation of Vulpecula. The terminology is an unfortunate comparison because M76 in the northern constellation of Perseus is also known as the 'Little-Dumbbell'. It has been more recently it has been named the 'Spiral Planetary' because it resembles a spiral galaxy. (Looking at Figure 1, this structure is obvious.)

‘Astronomical Objects for Southern Telescopes 2' (AOST2) pg.293-4., (Note: NGC 5189 does not appear in the AOST2's index!) has David Frew describe its features as;

"There is a knot of blueish light east, from which a bright curved bar passes axially west..."

The bluish light of this spot is obvious in a Celestron 8 and even perhaps a 15cm. In fact, I think all the visible nebulosity has a ‘greenish-blue’ tinge. However, I do have trouble with a 20cm seeing the curved bar as Frew mentions. A 30cm. starts to reveal the spiral structure. A 40cm., or larger, apparently shows details within the ‘spiral structure’, though I have not seen this myself. Andrew Murrell states that the shape poses some challenge. He also says that the nebulosity is best viewed with high power and a UHC filter to enhance the contrast. I do not have one, so I have had to take his word for it. The OIII filter and the Hydrogen Beta filter partly improves the internal structures quite well. AOST2 states that the ‘spiral structure’ reveals this easily. David Frew might like to elaborate on this?

The total visual magnitude is measured as 10.0, while the photographic magnitude is about 10.5. It is classed by the Vorimtosov and Velyaminov 1934 classification as type ‘V’ or ‘5' - an irregular form, similar in shape to emission nebulae.

Within the planetary are four stars. The brightest star is not the central Planetary Nebula Nucleus or PNN, is a 11.1 ‘B’ magnitude (HD 117694) B9 spectral class star to the southeast but within the nebulosity. More recent data, from the Michigan Spectral Survey (1975), gives a visual magnitude of 10.5 with a spectral type of A0 IV. In Burnham’s Celestial Handbook Vol. 2 pg.1216 this star is stated as 10th., which was likely taken from Antonin Becvar’s ‘Atlas of the Heavens Vol.2’. Two other fainter stars within its area can be seen to the northwest. David Frew says in a ‘Universe’ article in 1984 that there are three stars are in its centre, so I presume he used less a 20cm. in poor conditions.

The fourth star is the PNN, and is a faint 14th magnitude star that lies closer, but slightly SE of the nebulosity’s centre. I suspect it is brighter. c.13.3-13.5. For me, this ‘14th’ magnitude star is too obvious on a dark, clear night. (Some references that I have read stated the PNN as a 12.9 magnitude ‘O’ spectral type IV sub-giant. This is clearly wrong.) The PNN is obviously not in the dead centre of the nebulosity, unlike most planetaries. The central star in the literature is stated to be photographic magnitude 14.1 and visually between 14.8 and 14.9, [Note the discrepancy of approximately a magnitude], (HD 117622, He 2-94) identified by its [WC7-8], O IV spectral type. He 2-94, with a surface temperature ~100 000^OK and produces 500 times more energy than the Sun. Some suspect He 2-94 maybe a close binary - and this may explain the weird structure. Those with the larger Dob’s might like to check it out, and perhaps submit an observation.

Estimated distance is 1 000 pc., though more recent observations make it perhaps as close as 800 pc. Sky Catalogue 2000.0 does not give a distance, though David Frew (AOST2) has taken the lesser value of 800 pc. Difficulty exists in estimating the distance of NGC 5189 because of the high interstellar absorption in the region.

Modern Observations.

Studies since 1982 has shown that the apparent chaotic nature of this object really shows a considerable degree of symmetry. The inner material from the central star is travelling away at 23 kms^-1 some 80 degrees perpendicular to the line-of sight. The entire mass of the nebulosity has been estimated to be about 0.45 Solar Masses by radiometric observations. Like so many other planetaries - strong evidence suggests the central star is really a close binary system.

Phillips and Reay (A&A 117..33P (1983) states that; ‘Evidence...(suggests)...the presence of ansae in the planetary nebula NGC 5189..’ [Ansae (singular Ansa) are ‘handles’, whose usage in astronomy started with Cassini when he was referring to Saturn. Ansae are common in planetaries.] It is suggested that the phenomenon is due to multiple ejection of material from the PNN. Shape and brightness variations are thought to be caused by the PNN being a close binary. The binary influences the flow of the nebulosity away from the nebulae causing variation in torques, which precesses the material into spirals, as in NGC 5189, or multiple ansa within the nebulosity. Understanding of this mechanism has lead to the production of computer generated planetaries. (See S&T ‘July’ 1996 pg.42) In NGC 5189 the evidence is with the two ansae of ‘spiral arms’ radiate at position angle 40^O through 320^O. This also manifests itself with the so-called point-symmetric property where an almost exact inverse copy or counterpart is seen between each ‘arm’. This property is also seen in the HST image of NGC 5307 (13511-5112) in nearby Centaurus.

In this text,the images of both NGC 5307 and NGC 5189 use the Planetary Camera of the Hubble Space Telescope taken by Robin Ciardullo and Howard Bond of Penn State University.  Their programme was designed to find the distance of planetaries or detect the PNN nucleus. So far they have observed more than 100 to date. Most images were exposed for approximately five minutes. One minute in a V filter (V for visual) and four minutes using the I filter in the near-infrared. The final image is a false-colour image combining the exposures. Reproductions of the colour image are given as blue for the visual wavelengths, and those that are brighter are coloured red for the infrared portion. Image scale is about 40 square seconds of arc. The team that produced the observations has stated that they will be writing a decent article for S&T later this year. These have been reproduced in the ‘Universe’ with their permission. (Note: After I have written this, Astronomy 25, 5 pg.60-65 ‘Inside a Star’s Cocoon’ by Karen Southwell has published these colour images in the May issue. The article is reasonably good, however there is no discussion on the southern objects.)

[Note: In the next installment of this article on NGC 3918 will show other images taken by this team.]

MyCn18/ The Etched Hourglass Nebula./ PK 307-4.1, VV66, SA2-96) (13381-6723), it lies 1.5^O SSE of NGC 5189, and surprisingly because of its proximity to NGC 5189, is the best example of multiple ansae. This object is in described in AOST2 (pg.294) and appears in Sky Catalogue 2000.0 Vol.2, although it is not placed in the maps in Sky Atlas 2000.0. It is a really tough object to find in small to medium apertures, even with an O III filter. Visually, the central nebulosity extents about 4"sec.arc., though in moderate apertures, it is more like 2.5"sec.arc. AOST2 states it is magnitude 10.5 ‘star’. Most of the available literature tends to 13.0, with a ‘B’ mag = 11.8. Photographically, the central part of the nebula is magnitude 12.2, reaching about 25"sec.arc. in size. The central star has a ‘B’ magnitude at 13.7, while the visual magnitude is stated as 14.4. This star is He 2-95 (LS3169), named in the catalogue "Observation of Southern Planetary Nebulae." by K.G.Heintz (AJ.Sup.14,125 (1967)) Spectra first suggests that it is a Wolf-Rayet type WR-Of but it has provisionally been changed to a [WC9] with a surface temperature of 110 000^OK. It is unlikely that an amateur observer can see this star, though a large Dob could probably see it. It is not placed in the Vorimtosov and Velyaminov classification, but it is likely a type ‘3a+4' or ‘3a’. The distance is estimated to be about 2.4 kPc or 8 600 ltys. (A magnificent photograph taken by the Hubble Space Telescope’s Planetary camera appeared in S&T ‘July’ 1996 pg.40 and later in Astronomy ‘May’ 1997.

The Surrounding Field of NGC 5189.

Within the field has a circlet of stars, some 6'min.arc. in diameter, encircling NGC 5189. A small telescope will only see a few stars. Moderate apertures will see a semicircle of stars east and south of the planetary, with 30cm. or larger, that completes the circle.

Two Pairs: Just outside the planetary's outer boundary, are two uncatalogued but distinctly wide 10th and 11th magnitude pairs west and north of the central of the planetary by c.5' and 8'min.arc., respectfully. The magnitude of the most-distant pair, ‘Pair 1', is 10.7/12.0 47"sec.arc. PA 55^O. ‘Pair 2' is 11.1/12.0 51"sec.arc PA 351^O.

NSV 6296/ HD 119078, LSS 3169, PK 309 / He 2-99; is the suspected variable that lays some 5'min.arc. directly west of NGC 5189. First discovered by C.B.Stephenson and N.Sanduleak in 1971, the magnitude of this star is about visual magnitude 13.6, with B=14.6. The variations are suspected to be between 1 and 1.2 magnitudes below this magnitude. Its variable type and period are not presently known, but it is one of the rare Wolf-Rayet star, showing strong carbon lines. It is only one of 12 known [WC9] spectral class stars. It is unusual because some WR’s in planetaries are variable. This star has no association with a planetary nebula, though some theorists have suspected to be associated with NGC 5189. Further debate about this star as being a true galactic WR because it is appears evolutionary different. In 1982 it was deleted from the ‘Sixth Catalogue of Galactic Wolf-Rayets’ (1981). By classification, all suspect WR’s have the brackets around the spectral type. In this star, the blue spectral lines of O IV (Oxygen IV has three electrons stripped from the atomic nuclei.) at 381.1nm. and 383.4nm. are very strong, unusual for a WC9 star. It is only one of those rare times when nature has placed two of these unusual stars within 5' min.arc.! (Some 2.5 degrees away is another brighter Wolf-Rayet, Theta Muscae.) The ejected gas from this star is not optically visible!

BN Muscae is another variable that lies 10'min.arc west and north (c.PA 45^O) of NGC 5189. It is a known irregular-type variable with little observational data and varies between magnitude 13.4 and 14.1. It was discovered photographically in 1960's. The AAVSO states that this object is a peculiar variable, but like so many variable stars, little data has been collected on this star. Spectra has yet to be ascertained.

LDS 444 / HD 117007 (132911-660441) is a dainty yellow pair that was discovered by J.J Luyten in his ‘Bruce Proper Motion Survey’ in a subsequent catalogue made between 1939 and 1963. It is located almost due west of NGC 5189, south following, some 28'min.arc. away and is the most prominent nearby field object. LDS444 is just visible in a 7.5cm., though a 10cm. would be much better, and I could resolve the pair cleanly in the C8 at 60X magnification at Bowen, but it looked best at around 150X. Both stars are nearly equal in magnitude, namely 8.6 and 8.9, though I estimate the difference is only a mere 0.1 of a magnitude with the measured separation of 2.9"sec.arc., and the combined magnitude is 9.1 (B=9.6). Spectral classification is F7V and G, measured in 1975. As yet, no motion has been observed with the system. Proper motions, as mention in the Notes in the ‘Washington Double Star Catalogue-1996’ states that LDS 444B (CPD-65 2344.) has a motion B-134 +007 (In units of "sec.arc. per century) As the ‘A’ system has a similar motion, the pair is likely physically associated but the period must be very long.

The nearby star fields are fascinating.