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When a typical star cluster is born, it’s a tightly packed family — hundreds of stars that were born from the same big cloud of gas and dust, all jammed into a fairly small volume of space. As the cluster orbits the center of the galaxy, though, the stars tend to head their separate ways. The cluster is pulled apart by the gravity of the rest of the galaxy, and pushed apart by interactions between members of the cluster itself. So within a billion years or so, most clusters fall apart.
A rare exception is Ruprecht 147, a cluster in Sagittarius. The constellation is in the south this evening, with its brightest stars forming a teapot. Ruprecht 147 is to the upper left of the teapot’s handle, although you need a telescope to see it.
The cluster was discovered in 1830, but until recently it was largely ignored. A few years ago, though, astronomers began studying the cluster in detail. To their surprise, they found that its stars are roughly two-and-a-half billion years old — much older than the typical cluster.
That’s important because clusters are great stellar laboratories. Since the stars in a cluster were all born at the same time, from the same mixture of ingredients, studying how they look today reveals important details about how all stars evolve. So Ruprecht 147 is a rare cluster that provides a look at stars that are well into middle age — a special insight into the lives of the stars.
More about Sagittarius tomorrow.
Script by Damond Benningfield, Copyright 2014