Messier 13 (also known as NGC 6205, The Great Globular Cluster in Hercules and the Hercules Globular Cluster) is a globular (star) cluster of about 145 light-years in diameter, located some 25,100 light-years away in our Milky Way galaxy, in the constellation Hercules. It is one of the most prominent and best known globulars of the Northern hemisphere and contains over 300,000 stars, the brightest of which is the variable cepheid star V2. Its total luminosity is 300 thousand times the Sun.
Globular clusters are gravitationally bound concentrations of ancient stars (up to a million such stars) which form a nearly spherical system. They orbit the galactic center along highly elliptical paths, and on average one revolution takes 300 million years. Recent estimates indicate that about 150 globulars exist in the Milky Way (the Andromeda Galaxy has been estimated to contain approximately 500 globular clusters).
Messier 13 is composed of some of the oldest stars in the Universe. They are estimated to be about 14 billion years old. Born before the Mily Way’s stars had a chance to create metals and distribute them in star-forming regions, Messier 13’s iron content relative to hydrogen is just 5 percent that of the Sun.
The density of stars near the core of Messier 13 is so great (about 500 times more concentrated than in the solar neighbourhood) that the visible-light and near-infrared images all become a bright central blur.
When the Hubble Space Telescope was pointed towards Messier 13 they found 15 blue straggler stars (unusually hot and bright stars) and 10 other possibles. Blue stragglers are so named because they seemingly lag behind in the aging process, appearing younger than the population from which they formed. Real blue stragglers should exhibit signs of lithium, carbon and oxygen depletion, which are chemical signatures of past mass transfer from less massive and older stars. These stars appear to be bluer and so younger than most other, apparently older, stars.
Messier 13 was selected in 1974 as target for one of the first radio messages addressed to possible extra-terrestrial intelligent races, and sent by the big radio telescope of the Arecibo Observatory. The reason was that M13 was a large collection of stars with a high star density, so, the chances of a life harboring planet with intelligent life forms, were believed to be higher. However, more recent studies suggest that planets are very rare in the dense environments of globular clusters.
Image Credit: Mike van den Berg