Thursday, 31 August 2017

NGC6638: old or not?

It is generally accepted that globular clusters are among the oldest entities in the Universe and that the stars of which they're composed are among the first that lit up after the Big Bang. The reason for this is that the stars in globular clusters have an unusually low metal content. In astronomical language, this means that they don't contain many chemical elements heavier than hydrogen. The first element that formed in the early Universe was hydrogen of course. Gigantic hydrogen clouds then contracted and gave birth to the first stars, which began to fuse the hydrogen in their cores into helium. When they ran out of hydrogen, they started fusing helium into oxygen, carbon and other heavier elements, which in turn got dispersed into the Universe when these stars eventually died.

In the hippie-age, people liked to believe that we humans are made of stardust and, as surprising as this may sound today, that statement is actually correct. No, I haven't been smoking anything weird lately! The calcium in our bones, the iron in our blood and the magnesium in our brains were all created in the cores of massive stars that exploded and fed the Universe with these heavy elements. 

Now let's get back to our globular clusters and their overall lack of anything heavier than hydrogen and helium. Some of them appear to have a metal content much higher than average, although still much lover than the average metal content of a galaxy. This may be because the clusters in question are relatively young, but recent scientific studies also focus on the effect that young, big straggler stars may have on the population of a globular. It may happen that such stragglers are being hurled out of our galaxy and get caught by the gravitational pull of a nearby globular. Such stars are in general metal-rich and disperse these elements in the globular through collisions or matter transfer from one star to the other. As you know, the stellar density in a globular is unusually high and they may contain hundreds of thousands of stars in a volume with a radius of merely a few tens of light-years. Stellar interference and even collisions must therefore happen regularly. 

One such metal-rich globular is NGC6638 in Sagittarius. It's not a very spectacular one from our Earthly perspective. Actually it's quite faint because it's not very large and yet hovers at a distance of some 26,000 light-years. Even with my binoscope at 507x I could hardly resolve any star in it. The interesting thing about this particular cluster is that it contains a lot of metals, for a globular that is, and therefore the exact determination of its age is not easy because it's impossible to tell if this is due to a relatively young age (stars formed when there were already a lot of heavier elements around) or if it absorbed a lot of young straggler stars in its lifetime.

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