M-13
M13, also called the `Great globular cluster in Hercules', is large and bright enough to be seen with the naked eye on good nights. As one of the most prominent and best known globulars of the Northern celestial hemisphere, it is about 12-15' in diameter, rather easily resolved across the center, and is seen to be somewhat ragged in appearance. There are long strings of stars curving away from the center, and a curious propeller shaped area at its southwest edge that seems almost devoid of stars.
Edmond Halley discovered it in 1714, and noted that `it shows itself to the naked eye when the sky is serene and the Moon absent.'
At its distance of 22,200 light years, its angular diameter of 23' corresponds to 150 light years. It contains several 100,000 stars; Timothy Ferris in his book Galaxies even says "more than a million". Towards its center, stars are about 500 times more concentrated than in the 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.