Friday, 9 December 2016

A little butterfly

You may recall my blog post on the famous Ring Nebula (M57), in which I explained that this nebula's not a ring at all but a cylinder that we coincidentally see face-on. Today I'd like to present a very similar planetary nebula, but one that we see edge-on: M76 or the "Little Dumbbell", due to its resemblance to the much bigger and brighter Dumbbell nebula (M27). Personally I prefer to call it the little butterfly for obvious reasons.

On my sketch you can clearly see the bright central cylinder or the body of our butterfly The axis of the cylinder coincides with the rotational axis of our dying central star (which I couldn't identify during my observation). As I've explained before, the star's rotation causes an accumulation of matter along its equator, making it more difficult for the hot gas to escape from there. That's why many planetary nebulae have an elongated inner structure, or in extreme cases such as this become cylinders. 

The butterfly's wings on the other hand were formed much earlier, probably when our star was still a red giant that was running out of fuel and became unstable. It began to expand, cooled down, contracted under its own gravity, heated up and expanded again, blowing out large quantities of gas in the process. A good example of a star in this phase, ableit ten times more massive, is Betelgeuse. Eventually the star collapsed, expelling what remained of its atmosphere into space (the central cylinder). 

It's one of Messier's faintest objects but still surprisingly easy to observe, also in small telescopes (which may not reveal the "wings"). To my personal taste it's also one of the most beautiful planetary nebulae and every autumn night when I'm out under the stars I pay it a visit. It lies some 2.500 lightyears away from us and is headed towards us at 19km/s.

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