I end this year with some more on open star clusters. The page added with this issue discusses how one derives the distance to an open cluster without resorting to measurements of annual parallax, which is the oscillating motion on the sky of cluster stars induced by our orbit around the Sun.
Open clusters provide a wonderful opportunity for studying the structure and evolution of stars. These clusters may contain several hundred to several thousands of stars, all born together with the same chemical composition. Clusters are therefore perfect for testing how stars differ in appearance when they differ only in their masses and their angular momenta. To get the most out of these star collections, however, one needs a good, geometric means of deriving a distance. Annual parallax is the best method for deriving a distance, but this method is limited to stars within a couple of hundred parsecs. Since only 10 open clusters are within 300 parsecs of the Sun, other methods of deriving the distances to clusters were necessary. The classical method is to derive a distance from the stars within the cluster through the Doppler shifts of their spectra and through their proper motions (a star's proper motion is the rate at which the star's position changes on the sky). Another method is to infer the distance from the Doppler shifts, proper motions, and positions on the sky of stars that are members of binary star systems within a cluster. These methods are described in the page added this issue to the web site.
I am taking some time off to celebrate Christmas, so the web site will not be updated until four weeks from today. With luck I'll find the time to add more applets and illustrations to the web site; that's my new year's resolution. I wish all my readers a merry Christmas and a happy new year, and I look forward writing more on astrophysics in 2008.
Next Issue: The next issue of The Astrophysics Spectator is scheduled for January 16.
Distances to Open Star Clusters. Deriving the distance to the stars has always been a challenging problem. Stars bound together in a cluster give astronomers more techniques for determining distance than are available for finding the distance to an individual star within the Galactic plane. In addition to the annual parallaxes of stars within a cluster, the proper motions and spectral red shifts of individual stars and of stars in binary star systems within the cluster provide the means of deriving the distance to an open cluster. Distance can also be estimated by comparing the brightness and color of cluster stars to stellar models. (continue)