Tuesday, 11 October 2016

Suppose that a gigantic star nears the end of its life...

It'll explode as a supernova, right? Yes, it probably (but not necessarily) will. But let's talk about what happens just before the inevitable end. I've already written a lot about planetary nebulae, shells of gas ejected by ordinary stars that run out of hydrogen and become critically unstable. Eventually the remaining, extremely hot core, will cool down as a white dwarf and extinguish while the ever-expanding bubble of gas - the planetary nebula - will dissipate into space. Also our Sun will suffer the same fate. 

Supermassive stars, on the other hand, like to go with a bang. We're talking about stars which are ten to even forty times as massive as our Sun. By the time they run out of hydrogen and become red supergiants, they start fusing nitrogen or heavier elements in their core. They expand so much that they don't have a real surface anymore. Imagine an ocean full of gigantic tidal waves... gas blown up by the unstable core and then pulled back by gravity... One such example is Betelgeuse. This giant star's arrived at a point where it starts shedding its atmosphere into space and at a fast rate too. 

Now let's fast forward many thousands of years. Our giant star's blown vast quantities of gas into space. A planetary nebula on steroids, as it were. These stars, however, are so massive that they're able to regain some stability after having got rid of their outer atmosphere. They contract and become extremely hot with surface temperatures reaching 200.000°C (against 5.500°C for our Sun!). Stellar winds from the revived star that's now fuelled by the heavier elements in its core generate a tremendous shock wave. You've probably already seen footage of a nuclear explosion where the shockwave blows everything in its surroundings to smithereens. Well, this is worse. About a gazillion times worse. The stellar winds from a "Wolf-Rayet" star can reach velocities of 3.000km... per second! They quickly catch up with the previously expelled gas bubble and cause such havoc as you can see on my sketch (note the bright central star at the centre of the nebulosity). In the end, the "Wolf-Rayet" star will collapse and explode as a supernova.

Such "Wolf-Rayet" stars are quite rare because they can only come from the most massive of stars, which are also rare because they don't form very often and have exceptionally short lives. It's estimated that there are only some 500 "Wolf-Rayets" in our Milky Way, 206 in M33 and only 154 in M31. Although the Andromeda Galaxy's bigger than ours, it doesn't contain as many supermassive stars because it has a much lower metal content.

The object of my sketch is called the "Crescent Nebula", or NGC6888 in scientific terms. It's a fairly difficult object at the heart of the constellation of Cygnus and needs quite a bit of telescope to be admired fully. But my binoscope clearly showed me the many ripples caused by the violent radiation of the central star. It lies 5.000 lightyears away from us.

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