This is my second favourite PN because it is bright, quick and easy to find near Crux. Another is it extraordinary light blue or greenish colour against the starry field. Its colour reminds me of either NGC 3195 in Chamaeleon or NGC 3132 in Vela, though neither of these is not as prominent as NGC3918. I have on many occasions used it for a typical example of a planetary nebula for my Astronomy classes and during observation sessions.

At first glance using low power the tiny ‘even-light’ circular disk reveals no structural details. In appearance, it reminds me of the planet Uranus - however it is not as green. Higher powers will, with averted vision, reveal a small faint circular hole 1"-1.5"sec.arc in diameter. The total size is 8" to 10"sec.arc. David Frew also states, in the same reference as above, "...it makes a nice contrast when using high powers." Larger apertures reveals the very blue colour, but no structure - even using an O III filter. It is classed as a type 2b in the 1934 Vorontov and Velyaminov classification indicating a uniform distribution of brightness. A small knot or bulge, about PA 250^O is suspected towards the edge of the planetary, but I have never seen it. For apertures above 40cm. the planetary takes on an oval shape. The oval angled at PA ~300^O. Photographically the size has been measured to 19"sec.arc. with an overall photographic magnitude of 8.4.

The planetary nebula nucleus (PNN) is doubtfully listed as a 10.7 magnitude star. Because of the planetary’s overall brightness the PNN is invisible to telescopes below about 60cm. Yet other catalogues state that the central star is more like 15.6. According to the studies made by Clegg, Harrington, Barlow and Walsh (AJ 314, 551-571 (1987)) the V magnitude of the central star is 14.6. To quote this paper in how difficultly seeing the central star is; "It is of high excitation and has a high surface brightness such that the central star is not seen." In the images of Robin Ciardullo using the HST shows more structural detail, including the central star. This is reproduced with permission by Robin Ciardullo of Penn State University, and a colourised version can be seen from web site http:/www.astro.psu.edu/users/rbc/NGC3198.html. This image extends to about 25"sec.arc. in diameter, with some prominent wisps of nebulosity protruding at PA 330^O. These ansae appears symmetrical but mirrored. According to David Malin of the AAT this image is too small to usefully enhance the full extent of the outer nebulosity by unsharped masking. He was kind enough to look at the IIIa-J plates from the UK Schmidt, suggesting the nebulosity does indeed extent further at this PA. It is interesting to note that IIIa-J plates are selected because it is within the range of the O III - the most prominent line of planetaries. As David states "It reminds me of the bits of nebulosity around NGC3242. (Vela)" [See Page xx.] The UKST copy shows the nebulosity extending further - perhaps some 30"-40"sec.arc. It is larger only because the image is deeper.

PNN is some 6 900 times more luminous than the Sun, with a surface temperature at some 140 000^OK. This temperature was determined by measuring the ultra-violet Helium (He II) line (or Hydrogen (HII) that produces the so-called Zanstra temperature. This value is useful, as it can by theoretical means, enable the calculation of the true atmospheric temperature of the nebula. The Zanstra temperature here is some 117 000^OK. This is a very hot temperature for the nebulosity - suggesting that the nebulosity is very young. Measuring this, and the expansion velocity of the nebula suggests that the planetary be a mere 3 000 years old. Clegg, Harrington, Barlow and Walsh study is highly detailed on the object - using ultra-violet, visible and radiometric observations. From these we have learned the chemical composition of the nebulosity.

The distance has been calculated to be about 800 parsecs or 2 600 light years. Pottasch (1983) based on the value of the amount of interstellar extinction and calculated the distance as 1300pc. AOST2 states about 1000 pc. The most recent by Clegg, Harrington, Barlow and Walsh (1987) states a distance of 1 500 pc. Ciardullo and Bond have yet to publish their distance estimates. Total mass of the nebulae is estimated to be 0.08 solar masses.

The Surrounding Field of NGC 3918.

The surrounding fields to about 5^O seems rather devoid of bright interesting objects. However, on closer inspection, a number of interesting objects exist for amateurs.

I 892 (11552-5659) is the closest is the pair. Lying 22'min.arc. north-east, it is the central star of the three recommended earlier to find the target planetary. Discovered by Innes in 1911, the 5.7 and 11.0 stars are widely separated by 57"sec.arc. The fainter 11th magnitude star is an equally bright pair, separated by 5.2"sec.arc. Known as I 892b, it forms a direct line at PA 122^O with the distant third component. Burnham’s states that the PA has slightly decrease since discovery - yet I can find no evidence for this statement. The primary appears white in colour, while the others are too faint to tell. Since its discovery the I 892a pair has slowly increased at rate of 1.1"sec.arc per decade while the position angle has changed very little. It is likely this is an optical triple.

HD 114 (11551-5605) lies 1.2^O NE, near the top star of the three used to find NGC 3918. These two yellow 7th magnitude stars (7.3/7.7) are separated by 3.8"sec.arc at PA 169^O Since the discovery by E.S.Holden in 1887, the pair has slowly widened while the PA has decreased by some 40^O. As yet it is uncertain that this pair is physically associated, but if so, it is likely a very long period.

h.4460 (11392-5744), 1.7^O S.E.E. of NGC 3198, is another pair. The magnitudes are 7.7 and 8.9 with a separation of 8.6"sec.arc. at PA 176^O. Little has changed in the positions since the first measures by H.C.Russell on the 9th June, 1874 using the 7.25" Refractor at Sydney Observatory. The only spurious observation, of the eleven known measures, was made by Hargrave at Sydney Observatory using the same instrument, on the 6th June, 1879. He measured the separation as 8.0"sec.arc. Russell described both stars as white, but I see them as bluish. The pair is in an elegant starry field and is worthy of a glance.

NGC 3960/ Mel 108/ Cr 250 / C1148-554/ Bennett 48 (11506-5541) is an open star cluster directly north by exactly 1.5^O of NGC 3918. In size of the 107-odd stars, of which 45-odd in the core are visible in moderate apertures. The core covers about 6.0'min.arc. while the cluster’s total diameter is more like 18'min.arc. In physical size the stars in cluster occupy and area of 4.0 parsecs. It is a relatively bright cluster with many blue stars with a Trumpler classification is I 2 m (Highly concentrated, medium rich), as first determined by Ruprecht in 1966. Having a total magnitude of 8.8, it is easily visible in a 10cm. telescope. Distance is estimated at 1700 pc. with an age c.1.07 billion years, similar to the age of the Praesepe (M44) in Cancer.

NGC 3882 (11462-5624) is the SB spiral galaxy, west by 1.8^O, and perhaps a little south of NGC 3960. This galaxy is just visible in a 30cm, but is difficult without averted vision. Measured at photographic magnitude 13.3, this galaxy appears slightly elongated, along PA 65^O-245^O. In apparent size it is stated as 2.3'x1.3' - close to the size seen in the telescope. In a 30cm. I could not discern any structure, although there is a hint of brighter core and some detail in the southern portions of the galaxy. (I have not seen this structure myself) The visual poorness of this galaxy is understandable as it is immersed within the Milky Way.

In Sky Atlas 2000.0 is listed as a reflection nebula, adding to my seventy known errors for southern objects. However, this is perhaps too critical, as the data taken directly from the NGC and RNGC Catalogues. It is likely that because NGC 3882 is embedded in the Milky Way, that Tirion, along with everyone else, just suspected it had to be a nebula. There is a 8.0 magnitude ‘A0' star that lies on the eastern portion of the galaxy - the presumed progenitor of the nebula. (Vehrenberg-Blank’s ‘Handbook of the Constellations’ states the wrong NGC number (NGC3822), the wrong magnitude at 8.2 and an incorrect distance of 400 pc.!)

To complete the survey of this area, there are two non-NGC planetaries that are worthy of a look for largish apertures. These are SA2-75 and SA2-77.

SA2-75 (WRAY16-91, HE2-66, PK290+7.1) (11287-5256) lies 5.3^O NW of NGC 3918 or 1.8^O NE of Pi Centauri. At magnitude 11.6p this planetary is surprisingly bright. The diameter is some 16.0"sec.arc. The type is classed as ‘3+4' - Irregular Disk with a Ring structure. It is recommended to use an O-III filter to find it. A 20cm. or 25cm. is required to see this object with it’s nebulosity. A 10.5cm. could possibly see it with an O-III. The western side of the nebulosity is slightly brighter.