M13 (NGC6205)

1. The history
2. To find M13
3. To observe M13
4. The stars, and more

Edmond Halley discovered M13 in 1714; the first description of this object appears in his Memoir of 1715, "Of Nebulae or Lucid Spots among the fixt Stars", that was the first list to be compiled which consisted of nebulae alone as distinguished from nebulous object included in star catalogues. Halley describes M13 as "...a little Patch, but it shews it self to the naked Eye, when the Sky is serene and the Moon absent.".
Messier, in 1764, described it as "A nebula which I am sure contains no star. Round and brilliant; centre brighter than the edges...reported in the English Celestial Atlas".
William Herschel, with his great reflector, resolved M13 into "a most beautiful cluster of stars exceedingly compressed in the middle and very rich."

M13 lies about one-third the distance from h to z Herculis. Exploring this region with a pair of binoculars, you have no difficulty in locating the object: under a clear, very dark sky, it may be een with the naked eye, as a little cloudy star of the 6th magnitude.

Small telescopes show M13 as a bright rond nebula about 10' in diameter; the size is more than doubled in the long exposure photographic plates. A 100-150 mm. telescope begins to resolve into hundreds of small stars, and in a 300 mm. the globular is an unforgettable sight.

The photograph of M13 give an impression of an incredibly density of this cluster, and suggest that the stars are virtually in contact. But this is an illusion, due to the great distance and the merging of the numberless star images. In fact, the central region of the globular covers an area about 100 light years across, i. e. a million cubic light years in volume; if a million stars populate this space, the density is no greater than about one star per cubic light year.
In his "Celestial Handbook", Burnham explains this fact with an interesting example: he construct an imaginary scale model of the cluster, where the stars are represented by a million grains of sand, distributed throughout a spherical volume same 300 miles in diameter. Each grain would be 0.03 inch in size, and separated from the next nearest grain by a distance...of three miles!

The distance of M13 has been extimated as about 25.000 a.l., so its angular size of 23' corresponds to about 100 l. y. The age is calculated as 14 billion years.
Herschel estimated that M13 contained some 14,000 stars. According to Burnham, when he writed his "Celestial Handbook", more than 30,000 stars were counted, down the 21st magnitude, in a survey at Mt. Wilson. Obiously, it's impossible to count the stars in the rich central core: in this region, the innumerable star images merge into a great glowing mass; but it seems certain that the total population exceeds a million stars.
According to Kenneth Glyn Jones, M13, like M3, contains a young, blue star of spectral type B2: its membership of the cluster has been confirmed by radial velocity measurements. In consideration of the age of the globular cluster, this fact seems inexplicable: probably, the gravitational field of M13 accidentally captured the star.
M13 contains only a few variable stars, 15 according Kenneth Glyn Jones; other globulars, i. e. M3 or M15, contain more than a hundred of them. The reason for this difference is not clear.