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An open cluster is exactly that: A relatively loose collection of stars. Stars are normally seperated by a few light years although they can be closer. Eventually, interactions and the slightly different orbits of the stars around the galactic centre will force the open cluster to disperse. Large open clusters will form streams, vast groups of stars sharing a common motion through space but seperated by tens of light years. Our Sun is in the Ursa Major Stream at the moment, but isn't a true member, just passing through. Also at this stage, the most tightly bound members of the former open cluster will still be close together but not close enough to be called an open cluster. This is a moving cluster, most of the stars in Ursa Major are members of the Ursa Major Moving Cluster. This was once the core of the Ursa Major Stream. Alpha Persei is the lead star in the Alpha Persei Moving Cluster, another easy moving cluster. Distant moving clusters can't readily be detected since the proper motions of the stars need to be known. Of the well known clusters, the Hyades and the Pleiades, the Hyades are very nearly a moving cluster, being relatively sparse and well seperated. In a few million years, they will be a moving cluster. Globular clusters aren't found in the plane of the galaxy, but all around it. Many of them are probably the cores of galaxies long since merged into ours, having had all their gas, dust and dark matter stripped from them until only the denser stars remain. |
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