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Question:
How much is known about the nearest cluster of stars?



Replies:
According to the text "Astronomy: the Cosmic Journey" by Wm. Hartmann, the closest open cluster is the Ursa Major cluster, centered about 22 parsecs (about 72 light years) away. This cluster contains (among others) most but not all of the stars that make up the Big Dipper. Its total mass is approx- imately 300 solar masses and it is about 200 million years old. It is a fairly typical loose open cluster. There are about 900 open clusters in the Milky Way, concentrated in the plane of the galaxy. Typically they contain 100 to 1000 members. The text says "...most open clusters have prominent young stars or associated clouds of star-spawning gas." Stars generally form in open clusters, but "most such clusters break apart into individual stars within only a few hundred million years because of dynamic forces acting on them." That is why their component stars are young. By contrast, globular clusters "are much more massive, more tightly packed, more symmet- rical, and very old. They typically contain 20,000 to several million stars." The closest is several thousand light-years away.

RC Winther


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