The constellation Hercules is overhead when darkness falls. Although it holds no first magnitude stars, it is easy to distinguish the “Keystone” asterism.
Hercules contains two of the most conspicuous globular clusters: M13, the brightest globular cluster in the northern hemisphere, and M92.
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is derived from the Latin globulus—a small sphere. A globular cluster is sometimes known more simply as a globular.
Globular clusters, which are found in the halo of a galaxy, contain considerably more stars and are much older than the less dense galactic, or open clusters, which are found in the disk. Globular clusters are fairly common; there are about 158 currently known globular clusters in the Milky Way, with perhaps 10–20 more undiscovered. Large galaxies can have more: Andromeda, for instance, may have as many as 500. Some giant elliptical galaxies, such as M87, may have as many as 10,000 globular clusters. These globular clusters orbit the galaxy out to large radii, 40 kiloparsecs (approximately 131 thousand light-years) or more.
The Great Globular Cluster in Hercules was discovered by Edmond Halley in 1714, and catalogued by Charles Messier on June 1, 1764.
Its real diameter is about 145 light-years, and at its distance of 25,100 light years, its angular diameter of 20' corresponds to a linear 145 light years - visually, it is perhaps 13' large, and it is composed of several hundred thousand stars.
There are seven stars in Hercules known to be orbited by extrasolar planets. They were discovered in 1996, 2005, 2006, and the last four in 2007.
Four of the planets in our solar system are visible in the night sky, Venus setting in the west, closely followed by Saturn and Mars, and Jupiter can be seen in the south, close to the “Teapot” of Sagittarius.
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