Monday, 8 February 2016

NGC1807-1817, the little Double Cluster in Taurus

Yes, also the constellation of Taurus has its double cluster. Perhaps it's not as famous as the one in Perseus (NGC869-884 of which I wrote earlier), but it's certainly beautiful in its own right and without any doubt worthy of our attention. These two, which may have originated as a single cluster that later split up, are quite old. Whereas the Perseus double cluster's still very young with its 13 million years, the Taurus' one is actually between 0,8 and 1,2 billion years old. Not quite as old as our Sun, which is now a middle-aged star at 4,5 billion years, but for a cluster this is still quite remarkable. Most clusters dissolve over time under the gravitational infuence from our galaxy and the individual stars each go their own way. This cluster on the other hand has remained surprisingly compact. The ageing process of its stars is also much faster than that of most ordinary stars and some of them have already reached their red giant stage. Stars transform hydrogen into helium through fusion, the energy source which makes them emit so much light and other radiation. But at a certain point they'll run out of hydrogen and start fusing helium into carbon, oxygen or nitrogen. This causes the stars to expand greatly (when this'll happen to our Sun within 5 billion years it'll swallow up Mercury and Venus and the Earth will be scorched by its atmosphere) and cool further down. Young stars are hot and emit blueish light, just like blue flames are much hotter than yellow or red ones. But over time they cool down and change colour, becoming white, green, yellow, orange and eventually red. I could easily distinguish at least two orangy stars in the left cluster (NGC1817). Please don't consider the big orange star in the foreground because it's not part of the clusters at all! It lies at a distance of 343 lightyears whereas the clusters are a whopping 6.400 lightyears away from us, in the opposite direction of the centre of our galaxy.

So through my sketches I've granted you a view of the different stages of star cluster formation, from the newborn baby stars within the Orion Nebula, over the young Perseus double cluster, the Pleiades that are currently breaking up and this old one. 

This little double cluster's easy to find on the border between Taurus and Orion and is within reach of even a small pair of binoculars, although in order to resolve the entire cloud of tiny little stars in the background a somewhat bigger instrument's required. For those who want to leave the beaten track...

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