Q: How do astronomers get dates? A: Ask a lot of heavenly bodies. Go ahead and ogle NGC 6791:
Whoa. That's an open cluster, a collection of thousands of stars that are (in general) gravitationally bound to one another. In reality, over millions of years, the stars interact with each gravitationally, and a lot of the stars get flung out of the cluster, becoming loners. But a large fraction of the stars stick around, aging and eventually dying while still in the cluster. They're like city dwellers who never feel the need to leave town. In this newly released Hubble image, you can see thousands of stars in just this one small patch of the cluster. You can see far more distant background galaxies, too (I love that kind of stuff). But how old are these clusters, and the stars in them? Those are good questions, and important ones. The ...
