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NGC 3918 (8,470 of 18,816)

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Blue Planetary

NGC 3918, ESO 170-13, HD 102854, Hen 2-74, PK 294+04 1, PN VV' 109, PN VV 61, PN Sa 2-81, PN G294.6+04.7, Wray 16-101, Blue Planetary, h 3365, GC 2581

RA: 11h 50m 17.73s
Dec: −57° 10′ 56.9″

Con: Centaurus
Ch: MSA:990, U2:428, SA:25

Ref: SIMBAD

(reference key)

Type: planetary nebula

Mag: B=10, V=8.5

Size: ?
PA: ?

Image gallery

Sketches  (1)

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Photos  (1)

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Historical observations

John Herschel (1847) Cape Observations

This planetary nebula was discovered by Sir John Herschel at the Cape of Good Hope with an 18-inch f/13 speculum telescope. He recorded it as "A perfect planetary disc 6 arcsec diameter; quite sharp, with not the least haziness. It is of a most decided independent blue colour when in the field by itself, and with no lamp light and no bright star. About 10' north of it is an orange coloured star 8th mag. When this is brought into view the blue colour of the planetary becomes intense. Shown to my attendant, John Stone, who, on being asked what colour, said at once 'blue.' " His next observation was recorded as "planetary nebula - in light to a star 7th mag; diam. in RA = 1.3 seconds; viewed with all the powers; very beautiful; decidedly blue." On the next occasion he described it as a "planetary nebula, colour a beautiful rich blue, between prussian blue and verditter green. The light is fully = that of a star 8th mag diameter; 2 seconds in RA; 12 arcsec by careful estimation. When kept steadily at rest its outline is sharp and clean, and perhaps a very little elliptic. A feeble lamp light gives it a deep indigo contrasted colour. Ditto if a red star N.p., about 10' distant, be brought into the field with it. My attendant saw it and declared proprio motu that the light has quite a green cast in it. About 90 stars are in the field, none above 11..12 mag, and only one of that magnitude (meaning when the neb. is central). It has none but stars 16th mag near it." Then, on 26 February 1835, after completing the sweep for the night, he viewed it with Maclear: "Blue colour very conspicuous. Has one small companion certain (pos by diagram about 290 ) dist = full diameter, another nearly certain (at about half the distance by diagram, and pos about 135 ) The field is full of stars. Total light of planetary = star of 6th magnitude or 6' m. A very little oval in position about 160 or 165." Some while later, he recorded it as "planetary, diameter 8..10 arcsec, perfectly round and well defined, and of a fine blue colour." His final observation was recorded as "Planetary nebula. Perfectly round, very planetary; colour fine blue; a very little ill defined at the edges; has no 'satellite stars'; very like Uranus, only about half as large again and blue. Diameter in RA = 1.5 seconds."

John Herschel (1835)

"Extract of a letter from Sir John Hershel to Francis Baily, Esq., dated Cape of Good Hope, October 22, 1834", Monthly Notes RAS, 3, 75-77.

"... A brief recapitulation of a few of the more interesting objects and remarks which have fallen under my notice may not be unpleasing to you. ... On the 3d of April I discovered another fine planetary nebula, having a perfectly sharp disc, without the least haziness, of about 6" in diameter. The most remarkable feature about this is its evident blue colour, which needs not the presence of lamp light, or that of any red star, to be very conspicuous, as it appears when the nebula stands alone in a dark field. This also has since been (at my request) observed with the circle at the Royal Observatory."

J. H. Pope (1881)

"The blue Planetary Nebula near the Southern Cross. - This object, No 3365 in Sir John Herschel's Catalogue, is in R.A. 11hrs. 44m., and decl. 56deg 31' S.The color of this strange object is a bright unmistakable blue. This nebula, like other planetary nebulae that have been examined in the Northern Hemisphere, gives a spectrum of one bright line. Possibly, in a large instrument, more lines might be seen. It is, of course, impossible with my apparatus to determine the position of this line, as there are no landmakrs, so to speak, to guide one to a decision. It is most probable, however, that it is one of the hydrogen or of the nitrogen lines, and that this planetary nebula is a spherical mass of one or both these gases in an incandescent state."

Source: "On the Southern Stars and Other Celestial Objects", Science, Vol. 2, No. 40 (Apr. 2, 1881), pp. 151-155.

Published comments

Evans & Thackeray (1950)

A photographic survey of bright southern planetary nebulae. M.N.R.A.S., 110(5), 429-439.

"We find a simple disk with very slight equatorial structure. No central star is visible. Efforts by one of us (ADT) to reveal it, using green filters and F-type plates to cut out the nebulium lines, have met with no success. Diameter 31''. Fig9"

Doig, P. (1925)

Journal BAA, 36(3), Dec, p91

planeb of remarkable blue colour (which is probably partly due to contrast with the 8mag orange star close by.)

James, Andrew (1998+)

"Neat Southern Planetaries - II", by Andrew James

NGC 3918 / The Blue Planetary or The Southerner / PK 294 +4.1, He 2-74, WRAY 16-100 (11505-5711) (Centaurus) lies 18'min.arc. from the eastern boarder of Crux, in a narrow section of hind legs of the Centaur. NGC 3198 is a bright 8th magnitude 'bluish' star, stated by David Frew ('Universe' pg.6. June 1986) "...that it is even visible in finderscopes as a small 'star'." It can be found by moving the telescope west of the bright star of Delta ( ) Crucis by nearly 1.5 degrees. The finder reveals three stars (SAO239443, 239413 and 239392; mags. 5.7, 7.1 and 7.8 stars, respectfully) in a bent line, where the planetary lies directly south of the centre star of the bent line and half way between the 9.4 magnitude star SAO239415. The yellowish K2; 7.6 magnitude star lies some 8.3'min.arc. to the north.

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 250O 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 ~300O. 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 330O. 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 000OK. 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 000OK. 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 5O 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 122O 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 1.2O 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 169O Since the discovery by E.S.Holden in 1887, the pair has slowly widened while the PA has decreased by some 40O. 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), 1.7O 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 176O. 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 . . . [[ see NGC 3960 entry ]]

NGC 3882 (11462-5624) is . . . [[ see NGC 3882 entry ]]

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.

[[ see their individual entries ]]

Sulentic & Tifft (1973)

(Sulentic and Tifft 1973) notes that this is a 8.5 mag planetary nebula.

Modern observations

Walter Scott Houston

Houston, observing with a 6-inch RTF, writes: "NGC 3918 was recognizable as an 8.5 mag planetary nebujla, once its tiny 13 arcsecond disk was ferreted out."

Bahr-Vollrath, Gerd (1992)

Gerd Bahr-Vollrath (Noosa Heads, Queensland, Australia) observing with an 8-inch f/12 SCT, writes in The Webb Society Nebulae and Clusters Section Report No. 10, July 1992: "Quite bright and large irregular disc with fainter envelope; appears 'spikey'."

Hartung, E.J. (1968) Astron.Obj.South.Tel

Hartung writes "it is so bright that even a 2-inch will show the small disk ... this is round, well-defined, vivid pale blue about 10 arcsec across and lies in a fine starry field."

ASV Journal (1971)

ASV Journal Vol 24 No 3 June 1971: "visible in 1-inch 8x finder."

Murrell, Andrew (IAAC)

Observer: Andrew Murrell Your skills: Advanced (many years) Date/time of observation: Location of site: Ilford NSW Australia (Lat , Elev ) Site classification: Rural Sky darkness: Limiting magnitude Seeing: 1-10 Seeing Scale (10 best) Moon presence: None - moon not in sky Instrument: 20" f5 dobsonian Magnification: 160-700 Filter(s): none Object(s): NGC3918 Category: Planetary nebula. Class: Constellation: CENT

Description: NGC3918 is one of the best southern planetaries. It appears as a 30" bright aqua ball of even surface brightness. The colour is one of the strongest that I have ever seen in any astronomical object. No central star is visible, even when listed at 13.24 mag. The planetary has a sharp edge even at high power, and no faint halo was glimpsed about the planetary though it has been photographed. A star of 10th magnitude is positioned 10' northeast? (did not note properly) Of the nebula and was orange in colour giving a nice contrast. A scattering of 15-16th magnitude stars is positioned just southwest of the object. No OIII filter was used as previous observations had shown that it does not improve the view, it just ruins the colour. The planetary is just visible in my 90mm finder as a star. An 8" scope will show the nebula's colour.

Brian Skiff

QBS: m8.5 * 8'.2 NNW.

15cm - whoa! remarkably br, vhisfcbr pn, sl bluish even to me. m8.5, not much

fntr than m8 * N. 295x: nrly circ, nrly uniform but there is a clearly

vis although wk and sm cen dkning 20% total diam, which is 15". no cen *

up to 295x. star plotted next to neb on U2000 is neb itself. BS,

23Feb1990, LCO.

Rui Henriques

1997 July 08

10x50 binocular mounted. 1997-07-08 Dew, no moon. "nothing visible." [Rui Henriques]

Contemporary observations

Auke Slotegraaf

1998 January 24

1998-01-24/25, 6-inch f/8.6 Newtonian, Stellenbosch Rifle Range site. A colourless, perfectly round and smooth disk-shaped planetary. In the field with an 8th mag star north-northwest. The nebula, just fainter than 8th mag, is easy to identify; simply rack the eyepiece out of focus, and then start focusing it. As focus is approached, the disk-shaped images of both objects shrink in size. But at an obvious moment, the star continues to shrink, but the planetary remains disk-shaped. The disk is clear at 72x, and possibly even at 40x.

Interestingly, my photocopy of U428 shows a tiny star touching southeast of the neb.; I looked for but could not see it tonight.

1991

This is a very interesting object; a 10-inch f/5 at 120x shows it as a small, neat planetary nebula, very bright with a smooth disk and sharply terminated edges. It reminded me of a luminous coin. There is a reddish 8th mag star closeby to its north, and when viewing both in the same field, it is not impossible to imagine the nebula as having a bluish cast. Definitely a good object to study at higher powers.

1994 February 17

1994-02-17, Die Boord, 11x80 tripod-mounted, inferior conditions - dew. Clearly seen as an 8th mag star.

1997 March 24

1997-03-24, Monday. Jonkershoek. 11x80 tripod. Full Moon. Thought I had it as a fuzzy star, but its merely the 8th mag star next to it.

Chris Vermeulen

2007 June 09

2007/6/9, 18h20

Sky Conditions: Clear

Quality of Observation: Very Good

Bill Hollenbach's Pad

6" Dobsonian, 10mm Eyepiece

This planetary nebula has a distinct blue tint. It appears as a solid blue disc at high magnification (10mm eyepiece). With lower magnification (25mm eyepiece) it gives the impression of a bluish star with some nebulosity around it. It is not easy to detect, but it clearly is distinguished from the stars in the field of view, especially with a 10mm eyepiece.

Richard Ford

2011 February 6th, Sunday

Location:Koornlandskloof,Sutherland.

Instument:12-inch Dobsonian Reflector Telescope.

Eyepieces:9mm Eyepiece.

7mm Eyepiece.

Sky Conditions:Dark moon and stars magnitude 6 and fainter are barely visible with the naked eye.

Transparency of the Sky:The most clear sky possible.

Seeing:Excellent clean sky,limited star flickering and brilliant objects.

Limiting Magnitude:6.5.

Object Type:Planetary Nebula.

First Impression:This object looks like the larger version of the planet Uranus.

Location:Centaurus.

Time:12:45am.

Chart Number:No.16(Extract taken out of "Atlas of the Night Sky").

Size:9mm Eyepiece:Field Of View:15'/10=1.5'.

7mm Eyepiece:Field Of View:15'/9.5=1.5'.

1.5'+1.5'=3'.

3'/2=1.5'.

Size in Arc Minutes:1.5'.

Ratio:1:2.

Major Axis:1.5'.

1.5'/2=0.7'.

Minor Axis:0.7'.

Planetary Nebula is 1.5'*0.7'.

Brightness:Magnitude 8.4.

Brightness Profile:In the middle of this nebula it grows brighter.

Challenge Rating:Very Easy.

Description

-----------

By observing this planetary nebula it is sharply defined and looks like an out of focus star.It looks like the larger version of the planet Uranus.It has a bluish surface.

Carol Botha

2011 - 01 - 06

Location:Betty's Bay

Time: 20:15

Telescope: 12 Dobsonian f4,9. Eyepiece 15mm. FOV- 36

Sky conditions: Seeing 3/5 (dew)

Actual dimensions:0.2 x 0.2 (Cartes Du Ciel)

Object description:

Planetary nebula in Centaurus

Tiny bright blue disk (appearing just bigger than a very bright star) surrounded by a faint bluish shell.

Quite difficult to spot in the rich star field but definitely a wow when you find it.

To the NE just inside the fov is a string of stars and grouped with, what looks like two dainty doubles, it forms an almost umbrella shape. An arty y-shaped group of stars lies to the SW. All the stars appear to have almost the same brightness. The many "double" stars surrounding this planetary nebula seem to second my wows.

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