Two distinctive star clusters follow the Moon as it descends the western sky this evening. The dipper-shaped Pleiades cluster is close to the upper right of the Moon, with the V-shaped Hyades cluster farther to the upper left of the Moon.
Each cluster is a great family of stars — hundreds of them packed in a region that spans a few dozen light-years. The stars in each cluster share a common birth — the collapse of a giant cloud of interstellar gas and dust. And today, each cluster moves around the galaxy as a group, bound by the mutual gravity of its stars.
The Hyades is the closer of the two, at a distance of about 150 light-years. It includes all the bright stars in the V except orange Aldebaran, at the top left point of the V, which is only about half as far. The Pleiades is about three times farther than the Hyades, and includes all the stars that make up its tiny dipper.
Clusters like the Hyades and Pleiades are great scientific laboratories. All the stars in a cluster are the same age and the same distance, and they formed from the same mix of ingredients. The main difference is mass — some of the stars are small and lightweight, others are big and heavy. Seeing how these different weight classes have evolved helps astronomers develop better models of how all stars age.
Again, look for these two beautiful star clusters keeping company with the Moon as they drop down the western sky this evening.
Script by Damond Benningfield, Copyright 2011
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