ABOUT THIS IMAGE:

This image, taken by the Advanced Camera for Surveys on the Hubble Space Telescope, shows the core of the great globular cluster Messier 13 and provides an extraordinarily clear view of the hundreds of thousands of stars in the cluster, one of the brightest and best known in the sky. Just 25,000 light-years away and about 145 light-years in diameter, Messier 13 has drawn the eye since its discovery by Edmund Halley, the noted British astronomer, in 1714. The cluster lies in the constellation of Hercules and is so bright that under the right conditions it is even visible to the unaided eye. As Halley wrote: "This is but a little Patch, but it shews it self to the naked Eye, when the Sky is serene and the Moon absent." Messier 13 was the target of a symbolic Arecibo radio telescope message that was sent in 1974, communicating humanity's existence to possible extraterrestrial intelligences. However, more recent studies suggest that planets are very rare in the dense environments of globular clusters.

Messier 13 has also appeared in literature. In his 1959 novel, The Sirens of Titan, Kurt Vonnegut wrote "Every passing hour brings the Solar System forty-three thousand miles closer to Globular Cluster M13 in Hercules - and still there are some misfits who insist that there is no such thing as progress." The step from Halley's early telescopic view to this Hubble image indicates some measure of the progress in astronomy in the last three hundred years.

This picture was created from images taken with the Wide Field Channel of the Advanced Camera for Surveys on the Hubble Space Telescope. Data through a blue filter (F435W) are colored blue, data through a red filter (F625W) are colored green and near-infrared data (through the F814W filter) are colored red. The exposure times are 1480 s, 380 s and 567 s respectively and the field of view is about 2.5 arcminutes across.

From Wikipedia:

Messier 13 (M13), also designated NGC 6205 and sometimes called the Great Globular Cluster in Hercules or the Hercules Globular Cluster, is a globular cluster of about 300,000 stars in the constellation of Hercules.

M13 was discovered by Edmond Halley in 1714, and cataloged by Charles Messier on June 1, 1764. Charles had a list of more than a 100 objects that later became known as the Messier Catalog. The French astronomer mistook these objects for comets, but compiled them into a list to help future scientists.

Less than halfway from Arcturus to Vega, four bright stars in the constellation of Hercules form the Keystone. M13 can be seen in the middle of Zeta Herculis and Eta Herculis. Only telescopes with a great light-gathering capability can best show the Hercules Globular Cluster. M13 is not clearly visible to the naked eye. With a low-power telescope, the Messier 13 looks like a comet. The cluster can be seen all year long at latitudes greater than 36 degrees north, on some nights more than others. However, in the months of August and September it brightens the sky for a longer period of time.

It is located at right ascension 16h 41.7m and declination +36° 28'. With an apparent magnitude of 5.8, it is barely visible with the naked eye on a very clear night. Its diameter is about 23 arc minutes and it is readily viewable in small telescopes. Nearby is NGC 6207, a 12th magnitude edge-on galaxy that lies 28 arc minutes directly north east. A small galaxy, IC 4617, lies halfway between NGC 6207 and M13, north-northeast of the large globular cluster's center.

Traditional binoculars make the Hercules Globular Cluster look similar to a round patch of light. A four to six inch telescope is recommended to observe M13 as the stars will be seen as small pinpoints of light. Larger telescopes provide a closer look and improve the view.

M13 is about 145 light-years in diameter, and it is composed of several hundred thousand stars, the brightest of which is a red giant, the variable star V11, with an apparent visual magnitude of 11.95. M13 is 22,200 light-years away from Earth.

It wasn't until 1779 that the single stars in this globular cluster were resolved. Compared to the stars surrounding the vicinity of the sun, M13 stellar population is more than a hundred times greater. These stars are so densely packed together that they often collide and produce new stars. The newly-formed, young stars, so-called "blue stragglers," are particularly interesting to astronomers.

The Arecibo message of 1974, which contained encoded information about the human race, DNA, atomic numbers, Earth's position and other information, was beamed from the Arecibo Observatory radio telescope towards M13 as an experiment in contacting potential extraterrestrial civilizations in the cluster. While the cluster will move through space during the transit time, the proper motion is small enough that the cluster will only move 24 light years, only a fraction of the diameter of the cluster. Thus, the message will still arrive near the center of the cluster.