NGC 3918 in Centaurus
| Magnitude: +8.2 | Diameter: 0.4 light years. | Distance: 4,300 light years. | Apparent size: 0.3 arc min |
| Exposure: 20 minutes | Field of View: 18.7 x 12.7 arcmin | Image date: 2021-06-15 |
Also known as “The Southerner”
If I had a much bigger astronomy budget or maybe if I could suddenly become a professional astronomer, then planetary nebulae are the objects which I would choose to study. They consist of gas ejected from dying stars and they are typically somewhat bigger than the Solar System in diameter.
With some notable exceptions, most planetary nebulae appear quite small in an amateur telescope, so they don’t always make the most captivating of images. However, every planetary nebula has evolved differently and will look unique and exquisite in images obtained by the largest professional telescopes.
The Blue Planetary nebula appears quite small at 0.3 arc minutes (one two hundredth of a degree) and my image only picks out the central core of the nebula.
A Previous Image
I’ve imaged it before using a DSLR on an 8″ Schmidt-Cassegrain telescope and the images both look somewhat similar: a small blue disc. This is an old one from 2009:
Meade LX-90 and Canon 300D
It’s actually much more than a blue circle. Its Hubble picture is remarkable and reveals far more of the very faint detail in the nebula.
Location
Location of the Centaurus constellation in the whole sky:
Location of the Blue Planetary Nebula near the Southern Cross:
Sometimes the Astrometry website is off line and the Astrometry images will not appear .
Thanks for reading 🙃
Telescope & Imaging Details
| Telescope: | SkyWatcher Esprit 120 mm apochromatic 3-element refractor; 840 mm f/l @ f/7. |
| Optics: | Field flattener; ZWO Duo-band Hα (656nm) and [OIII] (500nm) filter. |
| Mount & Guiding: | SkyWatcher EQ6-R Pro mount; ZWO ASI120 guide camera. |
| Imaging camera: | ZWO ASI 290 MC |
| Software: | Control: Cartes du Ciel, ASCOM, EQMOD, PHD2. Imaging: SharpCap, Gimp. |
| Observatory: | 34° South. |
Images © Roger Powell
I’m one of the founder members of Macarthur Astronomical Society and current webmaster.
Cosmic Focus 
I think you mean “Its” not “It’s”, although “The” would work almost as well.
It could be the calibration, but your latest photo trends more to green (aqua or turquoise) whereas the older one is blue. And, yes, the Hubble photo has a bit more detail.
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The colour will always vary in astro-images, depending on the camera used, filters employed and the final processing tweaks.
The 2009 image was taken with a standard Canon 300D camera. DLSRs are not renowned for the colour accuracy of astro-images, unless modified by removing the internal IR filter, Some of my astro-friends have done this but it then renders the camera useless for general photography.
https://skiesandscopes.com/astro-modified-dslr/
It’s difficult to tell what a “true” colour is. Look at any deep sky object with the naked eye through a telescope and you will only see black and white.
The offending apostrophe has been banished.
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I vote for calling it the Turquoise Planetary Nebula . . . Blue makes it sound sad.
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Well it does represent the death throes of a previously Sun-like star somewhere inside it, so it’s ok to be sad.
One day, the Sun’s planetary nebula evolution will scorch the Earth. If it survives the event, it will be the Black Planet.
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. . . we should start building vast underground ecosystems now if we are to survive . . .
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I agree: planetary nebulae would be a wonderful thing to study!
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Thanks, JS. The events which precede the gas eruptions are complex and can be made even more so if they occur in close multiple star systems. Some astronomers are lucky enough to spend a lifetime studying what goes on!
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I like nebulae,too.
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We’re all in agreement then!
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Neat. Had never heard of them before.
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Glad you picked up something from this, Stolzy
Regards,
Roger
🙃
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I didn’t realize there were such petite-looking nebulae! It’s quite magical in close-up. Is the apparent size due mainly to distance?
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Planetary nebulae are a different class of object to the massive hydrogen nebulae and really are small in comparison, because they emanate from a single dying star.
The close planetary nebula like the Helix (790 light years away): https://cosmicfocus.wordpress.com/2017/10/02/the-helix-nebula/ appear much larger but the vast majority of them are much further away and so appear tiny, and fascinating but elusive to amateur astronomers like me.
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Ah, interesting! Good to know.
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