As I explained already several times, planetary nebulae are formed when an old star becomes unstable and dies. The fusion process comes to a halt, making the core collapse whereas the outer layers of the star are expelled and form a gaseous bubble. Hence the name "planetary" because these nebulae are usually small and round and therefore somewhat resemble a planet.
The one I'm presenting to you now's denominated IC2165 and can be found in the constellation of Canis Major, the great dog, almost half-way between bright Sirius and Orion's left knee. It appears very small because it's quite distant, being more than 6.000 lightyears away from us, but as with most planetaries it's still quite bright and supports high magnifications well. Yet it was difficult for me to make out any detail in it, although I did see a brighter, elongated inner shell. This inner shell's caused by the collapse of the central star whereas the fainter, large bubble consists of gas that had already been expelled earlier, during the star's last, unstable phase. The inner shell obviously travels a lot faster than the outer, being propelled by the sudden collapse of the star, and is quickly nearing the outer shell where both will merge and eventually dissipate into space together under the strong stellar wind from the collapse. Scientists believe that this nebula has entered a late-intermediate phase due to the proximity of the inner shell to the outer and the much faded central star, which was indeed invisible to me.
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