March 18, 2012

Tonight after sunset look high in the southeast. There you will find the constellation of Gemini. There you will see the pair of stars, Castor and Pollux. Castor is on top and Pollux is on the bottom. Castor is a most interesting object. It is 45 light-years away from Earth, and is the 23rd brightest star in the sky. Castor actually consists of three pairs of stars. Castor A consists of two spectral A type stars nearly identical in luminosity. These two stars orbit each other in only nine days. Castor B is also a pair of stars of spectral type A, however these two stars orbit each other in only three days. Castor A and Castor B can be seen as two separate stars in a small telescope. Castor C is a spectroscopic binary, two faint red stars so close to each other that they orbit each other in less than a day.

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Updated September 4, 2011