M 13

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, who noted that `it shows itself to the naked eye when the sky is serene and the Moon absent.' 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' corresponds to 170 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. The age of M13 has been determined as 12 to 24 billion years.

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.

Observers note 4 apparently star-poor regions in M13 (e.g., Mallas). Suggestions of them can be noted in some photos.

More images of M13
Nearby, about 40 arc minutes north-east, is the faint (mag 11) galaxy NGC 6207, visible in many large- and medium-size-field photographs of M13.

Right ascension	16 : 39.9 (hours : minutes)
Declination	+36 : 33 (degrees : minutes)
Distance	25.0 (light-years*10^3)
Visual magnitude	5.7