Four Star Keystone Asterism Contains Hercules Globular Cluster

Summary: The four star Keystone asterism in constellation Hercules contains the celestial northern hemisphere’s famed Hercules Globular Cluster.

crowded heart of Hercules Globular Cluster (Messier 13; NGC 6205), imaged by Wide Field Channel of Hubble Space Telescope’s Advanced Camera for Surveys, Monday, July 5, 2010; blue-filtered (F435W) data are colored blue; red-filtered (F625W) data are colored green; near-infrared data (via F814W filter) are colored red; release date Monday, July 5, 2010; credit ESA/Hubble and NASA: CC BY 4.0 International, via Hubble Space Telescope

The four star Keystone asterism in Hercules the Kneeling Hero constellation contains the celestial northern hemisphere’s famed Hercules Globular Cluster, a dense, tightly gravitationally bound starry collection orbiting in the Milky Way’s halo, beyond the galaxy’s main body.

At mid-northern latitudes in September and October, Hercules the Kneeling Hero constellation appears in the northwest. Classed as fifth largest of the 88 modern constellations established in 1930 by the International Astronomical Union (IAU), the heroic constellation is herculean in size.

Yet, the constellation’s lack of first and second magnitude stars confuses easy visibility. Moonless rural skies ease identification.

The heroic constellation’s Keystone asterism especially helps with identification. A starry quartet marks the asterism’s four corners. Third magnitude stars Pi Herculis (π Her; π Herculis) and Zeta Herculis (ζ Her; ζ Herculis) anchor Keystone’s northeastern and southwestern corners, respectively. Fourth magnitude stars Eta Herculis (η Her; η Herculis) and Epsilon Herculis (ε Her; ε Herculis) represent Keystone’s northwestern and southeastern corners, respectively.

At mid-northern latitudes in September and October, the Keystone is easy to locate, with its distinctive shape, below bright Vega. The bluish star dominates Northern Hemisphere skies with a trio of honors. Vega rates as the brightest star in Lyra the Lyre constellation, the second brightest star in the Northern Hemisphere’s night skies and the fifth brightest star in the night skies of both the Northern Hemisphere and the Southern Hemisphere.

Locating the outstandingly bright star in neighboring Lyra the Lyre constellation and then finding the four star Keystone asterism in Hercules the Kneeling Hero constellation quickly lead to viewing the Hercules Globular Cluster. The Keystone asterism’s western side serves as gateway to the Hercules Globular Cluster.

The starry collection lies about one-third of the distance between Eta Hercules, anchor of the asterism’s northwestern corner, and Zeta Hercules, signal of the asterism’s southwestern corner. The globular cluster is found about 2.5 degrees south of Eta Hercules.

The Hercules Globular Cluster measures a distance of about 25,000 light years and a diameter of about 145 light years. Naked eye astronomy perceives the globular cluster as fuzzily shaped in darkened skies. Binocular and telescopic astronomy increase appreciation of this globular cluster.

English astronomer Edmond Halley (Nov. 8, 1656-Jan. 25, 1742) is credited with discovering the Hercules Globular Cluster in 1714. The cluster’s discoverer acknowledged the importance of a “serene” sky and an “absent” moon in naked eye visibility of the “little Patch.” Edmond Halley is the namesake of Halley’s Comet (1P/Halley), the icy solar system body whose short orbital period of approximately 76 years he determined.

On June 1, 1764, French astronomer and comet hunter Charles Messier (June 26, 1730-April 12, 1817) catalogued the Hercules Globular Cluster. He designated the cluster as M13 (Messier 13) in his influential deep sky catalog, Catalogue des Nébuleuses et des Amas d’Étoiles (“Catalog of Nebulae and Star Clusters”), first published in 1774. Compiled with the assistance of French astronomer Pierre François André Méchain (Aug. 16, 1744-Sept. 20, 1804), the catalog ultimately listed a total of 103 non-cometary astronomical objects.

The New General Catalogue of Nebulae and Clusters of Stars (NGC) designates the Hercules Globular Cluster as NGC 6205. Danish-Irish astronomer John Louis Emil Dreyer (Feb. 13, 1852-Sept. 14, 1926) published the catalogue in 1888 as a revision, correction and enlargement of General Catalogue of Nebulae and Clusters of Stars, published in 1864 by English astronomer Sir John Frederick William Herschel, 1st Baronet (March 7, 1792-May 11, 1871).

The takeaway for the four star Keystone asterism as gateway to the Hercules Globular Cluster is that naked eye astronomy rewards observers of Hercules the Kneeling Hero constellation with a distinctive asterism of third and fourth magnitude stars as well as with a fuzzy globular cluster sending light from a distance of over 25,000 light years.

M13, known popularly as Hercules Globular Cluster, in western end of Keystone asterism of Hercules the Kneeling Hero constellation: International Astronomical Union and Sky & Telescope magazine, CC BY 3.0 Unported, via International Astronomical Union (IAU)

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

crowded heart of Hercules Globular Cluster (Messier 13; NGC 6205), imaged by Wide Field Channel of Hubble Space Telescope’s Advanced Camera for Surveys; blue-filtered (F435W) data are colored blue; red-filtered (F625W) data are colored green; near-infrared data (via F814W filter) are colored red; release date Monday, July 5, 2010; credit ESA/Hubble and NASA: CC BY 4.0 International, via Hubble Space Telescope @ http://www.spacetelescope.org/images/potw1011a/

M13, known popularly as Hercules Globular Cluster, in western end of Keystone asterism of Hercules the Kneeling Hero constellation: International Astronomical Union and Sky & Telescope magazine, CC BY 3.0 Unported, via International Astronomical Union (IAU) @ https://www.iau.org/public/themes/constellations/ (image URL @ https://www.iau.org/public/images/detail/her/)

For further information:

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Available @ https://earth-and-space-news.blogspot.com/2017/09/keystone-asterism-identifies-hercules.html

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