M13 on 03-07-2008
Globular Cluster M13 (NGC 6205) in Hercules
M13, also called the `Great globular cluster in Hercules', is one of the most prominent and best known globulars of the Northern celestial hemisphere. It was discovered by Edmond Halley in 1714. According to Messier it is also reported in John Bevis' "English" Celestial Atlas.
At its distance of 25,000 light years, its angular diameter of 23 minutes of arc corresponds to an actual diameter of 170 light years. Timothy Ferris in his book "Galaxies" says it contains more than a million stars. Towards its center, stars are about 500 times more concentrated than in our solar neighborhood.
According to Kenneth Glyn Jones, M13 is peculiar in containing one young blue star, Barnard No. 29, of spectral type B2. The membership of this star was confirmed by radial velocity measurement, and is strange for such an old cluster - apparently it is a captured field star.
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