Friday, 20 January 2017

Birds leaving the nest

Stars are usually born together. They're brought forth by giant hydrogen clouds that contract under their own gravity into dense spheres; so dense that they light up and start nuclear fusion. The finest example of this in our sky is of course the Orion Nebula, where we can see a whole cluster of stars that's being born right now. The hundreds of young stars are so hot that their radiation causes the gas cloud to ionise and make it emit light as well. Eventually, the gas cloud will be spent. It will cool down and dissipate into space whereas the cluster of young stars will slowly fall apart under the gravitational force of our galaxy and the individual stars will go their own way. The famous Double Cluster in Perseus is a fine example of a group of young stars that has just emerged from a nebula which is now gone. There may still be gas and dust filaments of the Double Cluster's parent cloud flying around, but they're no longer visible to the visual observer and would need long-exposure photographs to reveal. 

But let's fast forward many millions of years. On my sketch you see a lovely little star cluster, scientifically denominated IC1985, which lies some 1.400 lightyears distant. This cluster's a lot older than the Double Cluster or the Pleiades and most of it's stars have already reached adulthood. Strangely enough, in this case we can still see some of the original nebulosity from which the stars were born a billion years ago. I'm speculating now but perhaps the cloud wasn't dense enough to form more stars, yet it was too important to dissipate into space. Indeed, the cluster doesn't appear very rich, does it? So I assume that star formation broke off in an early phase. The nebula has cooled down now, as have the stars in the cluster, and it doesn't emit light anymore. It only reflects the light from the stars within it, or rather, from the stars that're now flying away from it. Yes, this cluster's falling apart and every single one of these stars will become like our own Sun. Planets will already have formed around them and perhaps we could already find the first traces of life there?

There's one oddball on the sketch though, and that's the very bright star somewhat further away. This star's named Atik (or omicron Persei) and it's easily visible to the naked eye, even in light-polluted skies. This star's a blue giant with a mass twenty times our Sun's and it's 80.000 times as bright. Actually it's a complex system of two massive stars that are very close and are evolving around each other so rapidly that both stars have become ellipsoids, rather than spheres. It appears that also this strange couple of giant stars originated from the same cloud as the others, but that it's taking off with greater speed than the rest. Being so large, they will consume their hydrogen much faster and therefore have much shorter lives. Analysis suggests that one of the two has already run out of hydrogen and will soon evolve into a helium-fusing red giant, becoming even hundreds of times bigger.

In the future, the stars of this cluster will lead solitary lives, apart from those that are gravitationally bound such as Atik, but also quite a few other stars as you can see here. They will become double or even multiple stars, which are very common in the universe. Perhaps there are even more double or multiple stars out there than single ones. Also Alpha Centauri, the closest star to our Solar System, is in fact a triple star which... originated from the same cradle as the Sun. Our gas cloud's now long gone, but it's a nice thought that there are still many sister stars of our Sun flying around out there. 


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