Monday 20 February 2017

The helmet of mighty Thor

I've already written about this spectacular nebula in Canis Major here, but it's so beautiful that I couldn't resist dedicating a second blog post to it. NGC2359 or "Thor's Helmet" is another astronomical showpiece that leaves anyone who has the chance of seeing it through a big telescope in awe. As I explained, the cause of this incredibly complex nebula is the star at its centre. It's one of those rare Wolf-Rayet stars, just like the one in the Crescent Nebula. They contain at least twenty times the mass of our Sun and burn their hydrogen much faster. Once they've run out of sufficient fuel to keep the fusion process stable, they start fusing helium into heavier elements such as carbon, nitrogen and oxygen. The consequence is that the star's blown up to gigantic proportions, so big that even Jupiter might be swallowed by it if that star were at the centre of our solar system! The star's surface cools down and turns orangy-red. A famous example of a supermassive star in this phase is Betelgeuse, Orion's left shoulder (from our perspective). The star becomes highly unstable, gravity fighting with the outward energy generated by the nuclear fusion in its core. It expands, cools down, contracts, heats up and expands again... until the critical limit's breached and the star's outer layers are expelled into space. But contrary to normally sized stars, these supergiants will not simply let their atmospheres dissipate into a planetary nebula while their cores slowly extinguish as a white dwarf. Oh no! They're simply too big for that!

By shedding a part of their atmosphere, a supergiant can regain a certain amount of stability. It contracts so much that its surface reaches a temperature of 200.000°C or more and fusion's re-ignited, only this time carbon and oxygen are being fused into heavier elements still, like iron for example. Imagine the power behind all this! Imagine the fierce stellar winds that such an extremely hot star generates! The previously expelled bubble of atmosphere's blown up into space at incredible speed, like a balloon. That's exactly what we can see here at the nebula's centre. 

Soon the central star will collapse under its own gravity and explode as a supernova, which will feed the universe with heavy molecules that are vital for the creation of planets and life.

For the record, the previous sketch of Thor's Helmet was made with my old home-built 18" Dobsonian, whereas this was observed through my 18" binoscope. 

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