Friday, 22 September 2017

NGC6852: a rare, pulsating planetary

NGC6852 looks like a small, ordinary planetary nebula. It doesn't come as a surprise that it looks so small from our point of view because it lies some 10,000 light-years away from us. Yet, it still appears surprisingly bright in medium to large telescopes and its annular shape was quite evident to me.

Yet, this seemingly plain and uninteresting little nebula hides a very interesting secret. First of all, it's one of the rare planetaries that's hydrogen-deficient. In most planetary nebulae hydrogen is the most abundant element but in this case there doesn't seem to be a lot of it around. But what's even more odd, NGC6852 is one of only 13 known pulsating planetary nebulae. The mechanics behind these pulsations must be sought in its dying central star but are still poorly understood. In this particular case, the nebula exhibits a low-amplitude pulsation with a period of about an hour and a half. These pulsations are irregular and may even change rapidly over time due to the central star's mass loss. In any case, I don't expect these pulsations to be visible in amateur telescopes, but it's a nice trivia.

A much more famous example of such a hydrogen-deficient, pulsating planetary is NGC246, the Skull Nebula, in Cetus (the whale). 

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