NGC 3699 / PK 277+7.1 /SA2-74/WRAY 16-90/ He2-65 (11279-5957) is the most northern of the two planetaries NGC 3699 and SA2-76. (See Below) Together , that I name "The Double Planetaries of Centaurus", lies in the eastern portion of Centaurus next to Crux. Both planetaries are separated by some 0.19^O or 11.1'min.arc., make an interesting pair for apertures above a 20cm.. The photographic magnitude is 11.0 and subtends a diameter of 45.0"sec.arc. This is a strange looking object - and you would be forgiven to mistake it for emission nebula. Using Sky Catalogue 2000.0 or Sky Atlas 2000.0 it is still printed as an emission nebula! Interestingly, AOST1 also states it as a emission nebula. From what I can ascertain, NGC 3699 was first recognised as a true planetary in about 1978. (A photograph in AOST1 can be seen in Plate 10, that was taken by Westerland and Heintz in 1969. A description and this photograph can be seen in the AJ.Suppl. 14,154-169.)

Contained within a starry field, the planetary telescopically extends some 60"sec.arc. NGC 3699 is just visible in a 10.5cm., though a 20cm. improves its appearance. Dividing the image into two portions shows a dark rift, slightly south of centre, at position angle 80^O-200^O . This can be seen in a 20cm using an OIII filter, but visually can be seen in telescopes >25cm. It reminds me of a smaller version of the irregular galaxy NGC 5128 (Centaurus A). In AOST2 the appearance is said to appear mottled, but this effect I could be just see using a 30cm. The edges of the planetary appears irregular and ‘wavy’.

NGC 3699 is considered a Type 1 bi-polar planetary, with extreme filamentary structure. Structural details of these types are thought to be caused by extreme shocks and instabilities. In theory it is produced by high velocity dense shell(s), catching up and taking over the circumstellar envelope previously ejected. In turn, this produces the chaotic and bizarre appearance of the nebula. All progenitors of this type are suspected to have a mass between 3 and 5 solar masses, and have surface temperature c. +140 000^O. In essence, they are the hottest PNN’s known.

Another example of this type is NGC 2899, near the southern boarder of Carina, some 1.5^O SW of the 2.8 magnitude star Kappa Velorum. In appearance, it has a somewhat chaotic Butterfly shape. At magnitude 12.2 It can be found using a 15cm, though it is easily visible in 10cm. by simply switching on and off the OIII field across the eyepiece. The apparent diameter is 70"sec.arc, though photographically it is more likely 95"sec.arc.

The distance of this planetary at present is not known. Having a magnitude of 16.5, the PNN is invisible in all amateur telescopes - so this is understandable.

SA2-76 / PK 292+1.2 /WRAY 16-92 / He2-67 (11288-6007) lies towards the direction SE of NGC 3699. Such planetaries are a ‘dime a dozen’ in the sky. (It is extraordinary that other descriptions of NGC 3699 never mentions this particular planetary!) At magnitude 13.2 it is just visible in a 20cm. Like most of these planetaries, an OIII filter is a godsend. It appears stellar-like, c. 5"sec.arc., and jumps out at you while flickering the OIII filter over the visible field. To find it, it is recommended to use a power of about 80X , with a wide-angle eyepiece of at least 0.4^O to 0.5^O degrees in apparent diameter. Place NGC 3699 at the very top south-western edge of the field, this planetary then become obvious and close to the centre of the field.

In appearance, all that is seen is a smallish grey disk, though I suspect that one of the large Dobsonians could see some colour. In a 30cm., I could see no apparent structure, except for a glimpse of an uneven edge.

The PNN of this object is at 15.8 and could be glimpsed in >40cm., though I have never seen it. Although not physically related to NGC 3699, it has the same Type 1 structure as it’s neighbour. The distance of this galactic planetary is likely to be many kiloparsec.

The nearby fields (to about 3^O) are starry - but contain few interesting objects for the amateur.

NGC 5307 / PK 312+10.1 (1351.0-5112) (Centaurus) can be found lying 0.8^O east and 0.1^O slightly south, of the 4.7 magnitude star M Centauri. (2.3^O NE of Epsilon ( ) Centauri and next to the Globular Star Cluster; NGC 5286). A telescope as small as 10cm. will identify it with some certainty, although David Frew (UNIVERSE June 1986 pg.14) states that he thinks it could be seen in a good 75cm. The photographic magnitude of the nebula is measured at 12.1, but is brighter in visual magnitude.

Telescopically, this small but slightly oval-shaped planetary can be observed as a 12th magnitude bluish 'star', that on closer inspection appears as a small disk. AOST2 states that in good seeing condition it appears as hourglass, which I admit I have never seen visually. Faint structure are revealed with high magnification and especially with an OIII filter. Its diameter is c.10"-12"sec.arc., with a high uniformity of brightness across the entire disk. Much larger telescopes do not add to its apparent diameter. The actual PNN is not visible in small to medium apertures, having a visual magnitude of 14.6.

Distance is estimated by Hartung as 2 400 pc. or 7 800 ltys., though the minimum distance upon recent calculations makes it perhaps as close as 1 900 pc. The entire mass of the nebulosity is estimated to be ~0.45 Solar Masses.

Nearby Fields of NGC 5307.

NGC 5286/ 388(13465-5122) is a Globular Star Cluster some 0.8^O SSW of NGC 5307. It was first observed by James Dunlop from Parramatta Observatory in May 1827. Easily found on top of M Centauri (Mag. 7.6), it is an easy object for a 7.5cm., though a 15 cm. in needed to start to reveal the internal stars. Appearance-wise, this is a very condensed - given as Shapley class of Type III, though most of the latest data suggest it is Type V (As in Sky Catalogue 2000.0). The globular subtends and angle of some 10'min.arc, and is slightly ovoid at position angle 170^O.

Both AOST1&2 state that the distance is some 9.0 kpc., though more recent measures suggest that it is perhaps closer to 10 kpc.. Sky Catalogue 2000.0 states the distance is 8.9 kpc. For the larger Dob’s note the 15th magnitude red star within the GSC some two-thirds the way out from the centre of the globular.

M Centauri / HDO 225 (SAO 241157) (13467-5126) is a bright yellow 4.7 magnitude star which is also the double star some 4.5'min.arc. SE. This pair is tough, though it can be seen easily in a 20cm. using high power. Magnitudes are 4.7 and 11.0, separated by about 4" sec.arc at PA 54^O. The colour of the star I observed as yellow and white. The primary is a known spectroscopic binary (Boss 3547) with a period of 437.0 days. Discovered in 1922, the first orbital parameters were determined by Jones in 1927. (Published 1928). Velocities vary by ±5 km.sec^-1, and is a likely candidate^. for future interferometry measures. True separation between the stars is 73 million kilometres - or about half the distance between the Sun and the Earth. Little observational data has been achieved in the last six decades.

From NGC 5307, 1^O north-west, is a trapezium shaped set of 5th to 8th magnitude stars. There is also two additional stars to the north. Using a power of 60X or so, in a small telescope, this asterism makes a very pretty field. The region, to about 1^O, also contains a number of ‘S’ spectral class stars. Spectral analysis of the all stars indicate high interstellar absorption.

NGC 3132 / The Eight-Burst Nebula / PK 272+12.1 (10070-1026) is so named because of the prominent filaments that can be seen radiating around its centre in astronomical photographs. Little needs to be said about this object. In the last twenty years, I found no less than seventeen, (Of which eight are observational descriptions) references to this object in UNIVERSE alone! [The latest, is Andrew Murrell’s article on planetaries in (UNIVERSE June 1997 pg.8), includes a CCD image of this object.] A detailed description of this in likely unnecessary. It would be far too long! The planetary NGC 3132 lies on the boarder of Vela and Antlia, in the northern part of Vela whose nearest star is q Velorum 2.2^O to the northwest.

As a planetary is extremely bright - even for small telescopes in our light polluted city skies. The nebula is slightly blue in colour and definitely oval in shape, covering a size of about 25"-30"sec.arc. Sky Atlas 2000.0 states the size as >47"sec.arc., though telescopically this at maximum is about 30"sec.arc. Astronomical photography covers a size about 85"x53"sec.arc. Hubble observations have it now extended this to over 100"sec.arc. Integrated magnitude of the nebulae is currently estimated to be 8.8 (1990), some 0.4 magnitudes fainter than the 'standard texts', but the photographic magnitude is stated as 8.2.

Unusual among the planetaries is the prominent white 10th magnitude central star, though a telescope greater than 15cm. is required to see it. This star is also a known binary, having a 16th magnitude companion, some 1.6"sec.arc away. Unfortunately, this is beyond the reach of most amateur telescopes. It is thought in the literature that it is this star, and not the 10th magnitude primary that is responsible for illuminating the nebula. Simply, the presumed progenitor does not produce enough ultraviolet to illuminate the gas. The planetary nebula nucleus is a bright example of the Wolf-Rayet class of stars, having a surface temperature around 135 000^O!

The nebulosity comprises of a H/He ratio of 12.7%, and an excess of Nitrogen that is higher than most of the other planetaries known. This suggests that the mass of the star was originally about 2.4 Solar Masses, at the lower limit for stars that can produce planetary nebulae.

In size the circumstellar bubble is estimated to be nearly one light year across, expanding at a relatively slow rate of about 15 kms^-1, and a having a mass estimated to be 0.06 Solar Masses. Hartung (AOST1&2) states that the distance is about 600 parsecs, though the more recent literature places it a bit further away at around 800 parsecs or 2 600 light-years. The planetary is approaching the Sun at a velocity about 20 kms^-1.

In summary, this object is impressive, real impressive!