​                                                             M13 - Hercules Star Cluster

Messier 13 , also known as the Great Globular Cluster in Hercules, is a globular cluster located in Hercules constellation. Messier 13 is one of the brightest and best known globular clusters in the northern sky. The cluster has an apparent magnitude of 5.8 and lies at a distance of 22,200 light years, or 6,800 parsecs, from Earth.  Its designation in the New General Catalogue is NGC 6205.
     

     The Hercules Globular Cluster has an estimated age of 11.65 billion years and contains about 300,000 stars. The estimated mass of the cluster is about half a million solar masses. M13 stretches across 20 arc minutes of the sky, which corresponds to a linear diameter of 145 light years. The cluster can be seen without binoculars in exceptionally good viewing conditions, with clear skies and no light pollution.  To the naked eye, the Hercules Cluster appears as a fuzzy star. Binoculars will reveal a comet-like hazy patch with a clear centre, while 4 to 6-inch telescopes will begin to resolve the stars in the cluster.

Instruments with larger apertures will resolve stars across the entire cluster.

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     There are a couple of curious affects visible at a high magnification. Many of the outer stars seem to be arranged in long arcs weaving their way across the cluster face and the distribution of the bright stars is not even across the surface. This can result in an optical illusion of apparent voids or relatively barren areas interspersed across the cluster. Also visible at medium/high magnifications in telescopes of the order of 300mm (12-inch) or greater, are three dark dust lanes that form a Y shape towards the southeast of the core. This is known as the "propeller", first noticed by Bindon Stoney in the 1850s from Birr Castle in Ireland using the 72-inch reflector, the largest telescope in the world at the time.