The Blue Planetary

NGC 3918 in Centaurus

Image exposure:
34 minutes
Object size:
10 arc sec
Image date:

Generally known as the Blue Planetary, this planetary nebula is sometimes also called the “The Southerner” due to its declination of 57º South.

Presenting in amateur telescopes as a plain blue circular shaped object, its appearance is reminiscent of the planet Neptune but I managed to extract just a slight hint of mottled detail in this image of the 8th magnitude planetary nebula – or is that just my wishful imagination?

The characteristic blue colour is due to helium in the nebula being ionised by ultraviolet light emitted from the newly formed white dwarf.

Despite a luminosity equivalent to 7,000 Suns, the central white dwarf is hard to detect, because of the high brightness of the nebula surrounding it – and according to Kanipe and Webb in volume 5 of their epic Annals of the Deep Sky, the 15.7 magnitude central star cannot be detected at optical wavelengths. Well, discovering that was a big relief to me, because I couldn’t capture it either!


NGC 3918 is about 4,900 light years away and the diameter of the outward moving nebula is between 0.3 and 0.5 of a light year. For comparison, the one-way light time between Earth and the most distant spacecraft ever launched by humanity, Voyager 1, is less than 22 light hours away at the time of writing. It’s been travelling for nearly 45 years.

The nebula is between 1,500 and 3,000 years old, so it is a relatively young planetary nebula.


Telescope:Meade LX-90 200mm Schmidt-Cassegrain
(deforked); 2000 mm f/l @ f/10.
Optics:Astronomik light pollution filter.
Mount & Guiding:SkyWatcher EQ6-R Pro mount.
Imaging camera:ZWO ASI 071 MC cooled.

Images © Roger Powell

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        1. I have found this object somewhat infuriating. My past attempts at it were poor but I have had success with getting detail from other planetary nebulae.

          Looking at the complex Hubble image had inspired me to try for the detail evaded me in the past – but alas not.

          Webb is clearly going to be brilliant for planetary nebulae.


  1. I have a spreadsheet that tracks Pioneer 10 (18 light-hrs away as of today) based on an estimated speed of 26,900mph and the current date. Of course, that’s assuming it’s not been destroyed by something.

    I don’t match This Site’s data (they have it at 19 light-hrs), but I don’t make as many assumptions as they do.

    I stopped looking at the spreadsheet once Voyager surpassed Pioneer 10 as far as the most distant human-made object.

    Liked by 1 person

    1. NASA stopped publishing the current Pioneer 10 distance in 2003 when the mission ended – but you probably already knew that, hence the spreadsheet. I suppose that even without communication they should have a pretty accurate idea of its location, even after twenty years.

      The Sky Live indicates 18 hours 19 minutes, which is close to your calculation.

      The Sky Live is a useful website, I used it to determine the coordinates of the Webb Telescope when I imaged it back in February and it was spot on. However it does not seem to be providing that information right now. 😕

      Liked by 1 person

      1. No, I don’t, but why would I? Everyone knows there’s no gravity in space! Didn’t you watch all them videos of people floating around up there, not a gravity care in the void?

        What I’m not taking into account is the motion of the solar system within the galaxy, nor any expansion or contraction of the galaxy. I am, however, taking into account the effect of dark matter and dark energy . . . luckily, for the purpose of my calculations, they cancel each other out.


        1. Just to be clear, I think your spreadsheet is quite valid and I’m not implying that you should have included gravitational deceleration in your calculation, just mentioning that it might eventually affect the calculation over long periods and may be the reason for the discrepancy which you mentioned.

          Even at a distance of 131 AU, where it’s in the realm of the Trans-Neptunian objects (in solar orbits with aphelions which stretch out as far as 2,500 AU), the spacecraft would still be subject to the drag of solar gravitation.


          Liked by 1 person

          1. I did a bit of a search . . . Voyager’s loss of speed due to the gravitational pull of the Sun is .0185 km/s per year, and that rate is slowing.

            I then got to wondering how much the solar wind compensates for the loss due to gravitational pull (if any). I’m now thinking of the solar sails tests . . . could the solar panels act a bit like solar sails?

            I assume it’s in the minuscule range as far as impact on velocity, but now I have to update my spreadsheet . . .


            1. That is a fairly negligible deceleration rate.
              The Voyagers have passed beyond the heliopause, so are no longer subject to solar wind but I don’t think Pioneer 10/11 have reached that stage yet.


              1. Right , but up to then, I wonder how the two effects compared.

                … But can’t find anything that gives me an answer.

                However, I’m ready to put this to rest … Until V’Ger comes back.

                Liked by 1 person

                1. I recalled this conversation we had last year, about the speed of Voyager, when I saw this tweet about the effect of Earth’s rotation on the speed of Voyager relative to Earth:



                  Liked by 1 person

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