The Constellation Hercules
The Butterfly is estimated to be about 100 million years old, M13 is estimated at about 10 BILLION years old, making it so old that it formed before the disk of the Milky Way galaxy formed! While the Butterfly cluster has maybe 100 stars in it total, M13 has no less than 1 million stars. The Butterfly cluster is about 1200 light-years away and about 20 light-years across, M13 is 20,000 light-years away and 160 light-years across.
These numbers are typical, so globular clusters are much, much bigger than open clusters, and they are much, much older - some are nearly as old as the universe!
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Updated 11 November 2011