Tuesday 20 June 2017

Aquila's double cluster (part 1)

Stars are born together in huge clouds of gas, sometimes by the hundreds such as in the Orion nebula. These young and hot stars burn fast and live a wild life, like adolescents, emitting violent radiation which not only illuminates the gas cloud from which they were born, but makes it emit light as well. Eventually the compound radiation from all these young stars will blow the nebula to pieces and the gas disperses into space, leaving only a cluster of stars. Scientists estimate that only 10% of the original gas cloud condenses into stars and that the rest blows away into space, possibly to coagulate into a nebula again when there's enough matter around to let gravity do its work. 

This little gem, denominated NGC6756, is a cluster of young stars in the constellation of Aquila, the eagle, not more than 8 million years old. It lies 5,000 lightyears away and so it isn't the brightest of objects. Actually, the brighter foreground stars, the brightest of which is of magnitude 11,9,  don't belong to the cluster at all but are dwelling much closer to us. The stars of this cluster are therefore quite faint, from mag. 14 down to mag. 18, and are difficult to resolve in smaller telescopes, also because the cluster's still fairly compact. Yet, this makes this particular cluster such an interesting object because even if it doesn't look stellar in a small instrument, it will appear as an attractive nebulous patch. With the binoscope however I had no trouble at all identifying all 40 or so members.

The most interesting thing about this little cluster is that it's one half of a rare double cluster. Although we've identified well over 1,100 of these star clusters in our Milky Way, only a handful of them are double, meaning that two separate clusters share the same origin. The most famous of those if of course the Double Cluster in Perseus, but also NGC1807-1817 in Taurus are two clusters born from the same cloud. As with most clusters, the stars of NGC6756 will eventually drift apart under the pull of our galaxy and most of them will lead solitary lives.


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