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Astronomical Observatory: Cool Images

Images: Great Globular Star Cluster in Hercules

Great Globular Star Cluster in Hercules

Contents: The Great Globular Star Cluster in the constellation of Hercules is the finest star cluster visible in the northern sky. High overhead in the early summer, it appears as a fuzzy patch of brightness comparable to a 6th magnitude star when viewed with binoculars in a dark sky. It was first described by Edmund Halley (of comet fame) in 1715, and listed by Charles Messier as the 13th object in his famous catalog (hence the name M13). (As a comet hunter, Messier compiled a catalog of objects that appear at first glance like comets, but are not. Although, he considered it a list, therefore, of objects to avoid, in reality it is a list of the most accessible and fascinating objects in the night sky.)

The English astronomer William Herschel was the first with a telescope adequate to resolve some of the individual stars. Many hundreds can be discerned in our image, and it is estimated that the cluster has over a million stars altogether.

The distance to the cluster is estimated to be 25,000 light years. At that distance the width of our image is 100 light years. If there are planets orbiting the cluster stars, the nighttime view from such a planet would include thousands of stars brighter than Venus, some as bright as the full moon.

What is a globular cluster? The many stars in a globular cluster all formed about the same time. They are clustered so densely that their mutual gravity binds them together throughout their lifetime. Dozens of globular clusters are distributed spherically about the core of the Milky Way galaxy. It is thought that they formed before the disk of our galaxy (the site of continuing star formation to this day) and so are older than nearly all other stars in our galaxy. Mapping out the distribution of clusters as observed from Earth was key to first identifying the direction to the galactic center. (Direct observation of the disk is limited by the large quantities of obscuring dust.)

Processing: This image was made by Philip Ammar, a freshman, and Larry Molnar. Four sets of three 20-second exposures were made between 11:50 pm and 12:02 am on the night of July 1/2, 2001 with no filter. Each image was dark subtracted and flat field corrected. Each trio was added together for an effective exposure time of one minute. Each of the four trios had the cluster core in a different corner, so that our final image, a mosaic of the four, covers a wider field of view than any of the individual shots.

Orientation and scale: North is up and East is to the left. The image covers a field of 14.3 by 10.1 arcminutes. The brightest stars seen are 11th magnitude.

The Great Cluster is found in the constellation of Hercules. Hercules is about midway between Vega and Arcturus, the brightest stars in the northern sky. Arcturus may be found by following the curve of the handle of the Big Dipper out one Big Dipper's length from the tip of the handle. Vega is the brightest of the three stars in the summer triangle. The most prominent part of Hercules is a trapezoid of 3rd magnitude stars. The cluster is along the side of the trapezoid closer to Arcturus. The celestial coordinates of M13 are 16h41m42s,+36deg28'0" (epoch 2000).

Content updated 7/3/01





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