Messier 13, also designated NGC 6205, and sometimes called the Great Hercules Cluster, is considered the most spectacular globular cluster in northern skies.
History and Observation
The Great Hercules Cluster 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." Fifty years later it was examined by Charles Messier, who cataloged it in 1764. M 13 is also reported in John Bevis' Celestial Atlas. In 1787, Sir William Herschel pronounced it "a most beautiful cluster of stars, exceedingly compressed in the middle, and very rich."
At magnitude 5.8, M 13 is barely visible to the naked eye on very dark nights. It appears about 1/3 of the distance from Eta to Zeta Herculis, the two western (leading) stars in the Keystone asterism of Hercules. Even small telescopes resolve it into an extensive, magnificent mass of stars, perhaps 13' across visually. Observers note four apparently star-poor regions. The faint, 11th-magnitude galaxy NGC 6207 lies nearby, about 28' to the north east, and is visible in many wide-field photographs of M 13.
Properties and Evolution
One of the reasons M 13 appears so large and bright is that is relatively nearby, about 25,100 light years away. At that distance, its angular diameter of 23' corresponds to 145 light years. The cluster also looks large and bright because it is, intrinsically, large and bright. M13 has an absolute magnitude of -8.7, which corresponds to a luminosity of a quarter million suns.
Messier 13 contains several hundred thousand stars; some sources even quote more than a million. The brightest is the variable star V11, with an apparent magnitude of 11.95. Toward the center of M 13, stars are about 500 times more concentrated than in the solar neighborhood. While the probability of collisions between stars in such a crowded region is negligible, the night sky seen from a planet near the center of of this globular cluster would be filled with thousands of stars brighter than Venus and Sirius!
Unlike open clusters, such as the Pleiades, globular clusters are tightly bound together by gravity, and contain very old, mostly red stars. The age of M 13 has revised to 12 billion years - nearly as old as the Milky Way galaxy itself. Born before the Galaxy's stars had a chance to create metals and distribute them them in its star-forming regions, M 13's iron content relative to hydrogen is just 5% of the Sun's.
Strangely, for such an old cluster, M 13 contains one young blue star: Barnard No. 29, of spectral type B2. The membership of this star is confirmed by radial velocity measurements; apparently it is a captured field star.
Arecibo Message
In 1974, the Arecibo radio telescope transmitted a message toward M 13, intended to communicate the existence of human intelligence to hypothetical extraterrestrials inside the cluster. The reasoning behind the selection of M 13 as the target for the message was that with a higher star density, the message had a greater chance of reaching a planet harboring intelligent life.
But the message will not reach M 13 for another 25,000 years, and when it arrives, M 13 will no longer be where the message was aimed. The Arecibo message was really more of a technological demonstration than a serious attempt to contact alien life - for it could be detected by an identical radio telescope, located inside M 13.