What is the difference between a globular cluster and an open cluster?
Globular clusters are collections of 100,000 to several million stars which orbit the Milky Way galaxy outside the plane of the galaxy. They contain very old stars in excess of 8 billion years. Open clusters are much smaller collections of dozens to perhaps tens of thousands of stars that orbit the galaxy within the plane of the Milky Way. These clusters can be as young as a few million years, or in the case of NGC 188, as old as some of the youngest globular clusters.
