Messier 13M13, also known as the Great Globular Cluster in Hercules, NGC 6205, C 1639+365, GCl 45, MWSC 2445, 2MASX J16414163+3627407, 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 from Earth; right ascension 16h 41m 41.24s, declination +36°27’35.5”. 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 and stretches to a linear diameter of 145 light-years. The brightest star in M13 is V11, a red giant classified as a Cepheid variable. V11 has a visual magnitude of 11.95 and lies approximately 25,100 light-years from Earth. The Hercules Globular Cluster contains an unusually young, B2-type star, designated Barnard 29. The star does not really belong to the cluster but was presumably picked up by M13 on its orbit around the Milky Way. Other stars in the cluster are very old and only have about 5 per cent of the Sun’s iron content as they were formed before the stars in our galaxy created metals. M13 also contains about 15 blue stragglers, old stars that appear younger and bluer than their neighbours. Messier 13 is a class V globular cluster, one with an intermediate concentration of stars toward the centre. It has a densely packed central region, with up to a hundred stars populating a cube only 3 light-years on a side; the density of the stellar population is about a hundred times greater than the density in the neighbourhood of our sun. These stars are so crowded that they can, at times, run into each other and even form a new star. Alpha Centauri, the nearest star system to Earth, is just over 4 light-years away. In other words, stars in the cluster’s core region are about 500 times more concentrated than those in our immediate stellar neighbourhood.
The Hercules Globular Cluster was discovered by the English astronomer Edmond Halley in 1714. Charles Messier catalogued the object on June 1, 1764. When Charles Messier added M13 to his catalogue in 1764, he was convinced that the nebulous object did not contain any stars at all. Because they are so densely packed together, the cluster’s individual stars were not resolved until 1779. German astronomer Johann Elert Bode observed the cluster in 1774 and described it as a “rather vivid nebula.“ William Herschel first observed the cluster on August 22, 1779, and resolved it into stars. The most detailed early description of the Hercules Globular Cluster came from Admiral William Henry Smyth on August 1836.
The Cluster was the target of the Arecibo message, a message beamed from the Arecibo Observatory in Puerto Rico in 1974, which contained information about the human race, Earth’s location and other data. The message was sent from a radio telescope in the direction of M13 as a way of potentially contacting extra-terrestrial civilizations. However, the message will never reach its target. It will arrive at the past position of M13 in about 25,000 years, but the cluster will no longer be there at that point. The cluster is located near NGC 6207, a 12th magnitude galaxy seen edge-on. The galaxy can be seen 28 arc minutes northeast of M13. Another galaxy, IC 4617, lies between the two, to the north-northeast of M13’s centre. Messier 13 lies within the Keystone, a familiar asterism that marks the torso of the celestial Heracles, about one-third of the way from the bright star Vega in Lyra constellation to Arcturus in Boötes. Both these stars are prominent in the spring and summer sky in northern latitudes. Messier 13 is best seen from May to September. It is particularly high in the sky in the summer months. Credits: Messier Objects, NASA, Universe Today, Wikipedia.