The star cluster, M13, is a well known among astronomers for being the most prominent globular cluster in the Northern Hemisphere. It is sometimes called the “Great globular cluster in Hercules,” as it is located in the Hercules constellation. Similar to other globular clusters, M13 is a spherical assortment of stars orbiting a galactic core. The stars are held close to the center because of strong gravitational forces. The strong gravity causes the spherical shape, as stars are bound to the high stellar densities in the core. Globular clusters typically contain hundreds of thousands of old stars. These old stars are low-metal, Population II stars; most of their metallic material was used up long ago in the creation of other stars. No new stars are made in these in M13, and it is assumed that this cluster, like other globular clusters, is among the oldest objects in the galaxy.
The M13 cluster is over 25,000 light-years away from Earth, located in the right “armpit” of the constellation Hercules. It lies about a third of the way down a line drawn from Eta to Zeta Herculis. M13 spans well over 145 light-years, containing at least several thousand, but possibly over one million stars. The majority of these stars are concentrated into a core region with a diameter of nearly 100 light-years. The stars in the center are almost 500 times more concentrated than in those in the solar neighborhood, leaving the average distance between stars inside the cluster to only be about 1 light-year. Estimates of the age of the cluster range from 14-24 billion years. The M13 cluster contains a young blue star (or a ‘blue straggler’), which is considered rare for a cluster that old. It is speculated that the star, Barnard 29, was captured by the cluster. The brighter stars in the cluster are red giants. The galaxy NGC 6207 can also be seen in the vicinity of M13. Its luminosity is over 300,000 times that of the Sun, and the sun would likely not be visible from M13.
The first recorded observation of M13 occurred by accident in 1714 by Edmond Halley. Halley found another cluster, the much larger Omega Centauri, thirty-seven years earlier. M13 has a small visual magnitude, making it difficult to see from the Earth, especially when the Moon is visible, or in areas affected by light pollution. With the best instruments of his time, Halley noted that the “Nebula in Hercules…is but a little Patch, but it shews it self to the naked Eye, when the Sky is serene and the Moon absent.” When observed with small telescopes, it appears as a misty patch with a denser center. When M13 was catalogued by Charles Messier in 1764, with a four and a half foot telescope, he called it “A nebula without a star,” unaware of the hundreds of thousands of stars beyond the visual reach of his Newtonian telescope. When William Herschel observed it twenty-three years later on a twenty foot telescope, he would note that that M13 “is a most beautiful cluster of stars.”
On November 16, 1974, a frequency modulated radio wave was directed at the cluster. From the Arecibo radio telescope, a single beam was aimed at M13 to mark the telescopes reopening after a remodeling. The logic behind it was that M13 contained a large number of stars, thus increasing the odds of receipt. Other factors for choosing M13 as a destination for the message, also weighed into the decision, the foremost being that it was in the sky at the time and place of the ceremony. The three minute binary message of ones and zeroes was written by Dr. Frank Drake of Cornell University and Carl Sagan. The message contained a good deal of information relating to humanity’s understanding of math and the makeup and disbursement of the human species, as well as representations of the Solar System. Rather than an outright attempt at interspecies contact, the message was more a demonstration of the capabilities of the new technology.
NGC-6205 = M13
Dreyer’s description: !! glob. cl. , eB, vRi, vgeCM, st 11…; = M13
Cross Identifications: GC 4230, h 1968. Hercules Globular Cluster; Halley (1714), Bode 30; BD +36 2768
visual magnitude: 5.9
apparent diameter: 23′
actual diameter: 165 light-years
distance: 25,000 light-years
position: R.A. 16h 41.7m, Dec. +36° 28′