Wednesday, 14 June 2017

Very old, but still alive and kicking

Globular clusters are among the oldest entities in the universe. These strange and extremely dense balls of stars are generally older than the galaxies they accompany and some have emerged only just after the Big Bang. With "just" I intend hundreds of millions of years of course. At first sight they all seem alike, but when you take a closer look you'll notice that they're surprisingly different from one another. Take extremely compact M15, for example, and compare it to majestically large and irregular M5. There's NGC2419, so far away that it only marginally feels the gravitational pull of our Milky Way and NGC5466 that's literally torn to bits by it.  There are 150 to 160 of these globulars known to orbit our Milky Way but other galaxies such as giant M87 dominate over 12,000 of them!

And then there are the survivors, the oldest of them all. NGC6426 is, with a distance of almost 70,000 lightyears, quite far away from us. It's so far that you need a sizeable telescope and a good-quality sky in order to resolve some stars in it. Yet, it's a very interesting globular cluster because it's one of the oldest there is. We can tell because this cluster has an unusually low metal content. With "metal" astronomers mean everything heavier than hydrogen or helium. When the universe came to be 13.7 billion years ago, the first chemical element that arose was hydrogen of course, quickly followed by helium which was created through the fusion of hydrogen in the very first stars. But for anything heavier, we had to wait until these first stars ran out of hydrogen and started fusing helium into more complex elements such as carbon, nitrogen and oxygen. This took hundreds of millions to even billions of years. The stars in globular clusters, on the other hand, are extremely slow burners and although very old most of them are still in their hydrogen-fusing phase. The fact that the stars of NGC6426 contain such an extremely low level of "metals" indicates that this cluster was formed very early in the evolution of our Universe.  

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