M13 The Great Hercules Cluster; Hercules
Astro Systeme Austria N12 f/3.5 astrograph
Total Exposure 2+ Hours; LRGB 40:30:30:30min; unbinned
April 27-28, 2007; Inkom, ID
Comments: The largest globular cluster seen from northern latitudes, the Hercules cluster can been seen with the unaided eye against very darks skies as a very faint, fuzzy "star". Through a modest telescope many of the stars may be resolved. A closer view of the core may be seen here.
At its distance of 25,100 light years, its angular diameter of 20' corresponds to a linear 145 light years - visually, it is perhaps 13' large. 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 by Sandage as 24 billion years and by Arp as 17 billion years around 1960; Arp later (in 1962) revised his value to 14 billion years (taken from Kenneth Glyn Jones) [from SEDS].