The most remarkable of the northern hemisphere globular clusters is called "the Hercules cluster", but known to astronomers as Messier 13 or M13.
It was labeled No. 13 in the first catalog of remarkable "nebulae" formed by Messier, the famous discoverer of comets. It was also viewed by Halley in 1714.
The brighter of the two clusters in Hercules is known as M13, the Great Hercules Cluster. Under favorable conditions that is, on a clear, dark night, when there is no moonlight it may be seen without the aid of a telescope as a small, faint patch of light. One would never suspect from such a view what a wonderful object this cluster becomes when seen with the aid of a telescope. Photographs taken with the telescopes show this faint wisp of light as a magnificent assemblage of thousands of stars, each a sun many times more brilliant than
our own sun. The crowded appearance of the stars in the cluster is due partly to the fact that it is very distant from the earth, though neighboring stars in the cluster are indeed much nearer to one another than are the stars in the vicinity of our solar system. It has been found that this cluster is so far away that its light takes over thirty-six thousand years to reach the earth. At the distance of this cluster, a sun equal in brightness to our own sun would be so faint that the must powerful telescope in the world would not show
it. So we know that the stars that are visible in the Hercules cluster are far more brilliant than our sun.
A fair-sized telescope will show about thousands of stars in this cluster, but the greatest telescopes show over one hundred thousand in it, and there are without doubt many more too faint to be seen at all. The Hercules cluster is called a globular cluster, because the stars in it are arranged nearly in the form of a sphere. There are in the heavens about ninety such clusters whose distances have been found, and they are among the most distant of all objects. Most of them are very faint, and a few are over two hundred thousand light-years distant from the earth. The Hercules cluster is one of the nearest and is the most noted of all of these globular clusters. It is considered to be one of the finest objects in the heavens. The other cluster in Hercules is also very fine, but not to be compared with this one.