Saturday, 3 September 2016

A slightly blue snowball

At first sight, all planetary nebulae look more or less the same. They look like fuzzy little disks and nothing much else. But appearances may deceive as I shall explain in this post. I can't stress enough how important it is to observe properly. When you get the chance to look through a telescope, please, relax and take your time. Don't feel pressured; other people will wait. Let your eye (or eyes in the case of a binoscope) adjust to the image. It can take minutes before the really interesting details appear and it's exactly the detail that makes every planetary nebula unique.

Let's talk about NGC7662, for example, or in human language: the Blue Snowball. It's one of the brightest and most easily visible planetaries on the northern hemisphere and you can already spot it with a small telescope. The Snowball has the typical three-layer structure of a fairly young nebula, in scientific terms a phase II. In the first phase, the dying star ejects its atmosphere but the remaining central star's still too cool to ionise the gas bubble and hence make it glow. Phase II is the so-called compression phase. Gravity compresses the star so much that its temperature rises to 100 million °C! The gas bubble's heated up to 10.000°C and even 25.000°C nearer to its centre. Extreme stellar winds blow up the bubble, creating a cavity in the nebula's centre and different layers of gas around it. Imagine that you're ploughing snow with a shovel, pushing it in front of you. You'll notice that the snow will also form different "waves", the largest of which against the shovel, a smaller one in front of the first and an even smaller one in front of the second. This is exactly what we're seeing here: A very bright and thick internal bubble, a less dense outer bubble and a very faint halo around it. In the third phase, the central star reaches its maximum temperature and the density contrast between the inner and outer shells is the highest. Eventually we reach phase IV. The central star begins to cool down and the nebula's violent expansion process slows down. The inner shell catches up with the outer and the clear structures that you can observe in a phase II or III nebula fade. Eventually the nebula will dissolve into space and the central star will extinguish.

But there's more. The winds generated by the incredibly hot central star are certainly not uniform and at times sudden bursts may appear. This is exactly what we observe in the Blue Snowball, where the inner shell is ruptured at opposite sides by such a burst of high-energy particles. 

Finally, these nebulae feed the universe with heavier and complex elements which were formed in the star before it died. The Blue Snowball, for example, contains a large amount of iron in its outer shell. Perhaps one day these elements will form the planets around a newborn star?

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