Keystone Asterism Identifies Hercules the Kneeling Hero Constellation

Summary: The four star Keystone asterism identifies Hercules the Kneeling Hero constellation in the northwest at mid-northern latitudes in September.

Hercules the Kneeling Hero constellation as illustrated in Alexander Jamieson’s A Celestial Atlas, Plate VIII (1822): Public Domain, via U.S. Naval Observatory

The four star Keystone asterism identifies Hercules the Kneeling Hero constellation, appearing in the northwest, below bluish Vega, Lyra the Lyre constellation’s brightest star, at mid-northern latitudes in September.

The four star Keystone asterism marks the lower abdominal and pelvic regions of ancient Greece’s greatest hero in his starry abode. The asterism’s name derives from its resemblance to the critical, wedge-shaped, architectural feature known as a keystone. The asterism’s namesake, which is the last piece fitted in place during construction, is responsible for locking all other stones into position in a masonry arch.

The four stars that anchor the Keystone asterism present a mix of apparent magnitudes, or apparent brightness values. The quartet separates into pairs, each sharing a magnitude value with its diagonal opposite. Two shine as third magnitude and two as fourth magnitude stars.

Zeta Herculis (ζ Her; ζ Herculis) marks the Keystone asterism’s southwestern corner. Zeta Herculis shines as the asterism’s brightest star and as the second brightest star in Hercules the Kneeling Hero constellation. Zeta Herculis is actually a double star, with a subgiant, shining modestly at third magnitude, as primary and a dwarf, shining more faintly at sixth magnitude, as close companion. The third magnitude star system participates in the Keystone asterism from a distance of 35 light years.

Pi Hercules (π Her; π Herculis) anchors the Keystone asterism’s northeastern corner. Pi Hercules shines as the asterism’s second brightest star. Fluctuations in apparent magnitude qualify Pi Hercules as a variable star. Possible causes for variability include an orbiting companion, inward and outward pulsations of the stellar surface or a rotational period affecting the field of view, from Earth’s perspective. University of Illinois Emeritus Professor of Astronomy James B. “Jim” Kaler suggests pulsation-driven variability. The third magnitude star contributes to the Keystone asterism from a distance of over 370 light years.

Eta Herculis (η Her; η Herculis) resides at the Keystone asterism’s northwestern corner. Eta Herculis shines as the asterism’s third brightest star. American astronomer Jim Kaler notes that measurements taken between 1873 and 1998 nix the notion of Eta Herculis as a binary star system. Its suspected 12.5 magnitude companion is a visual double, with no physical relationship and with only an apparent alignment from Earth’s perspective. The fourth magnitude star associates with the Keystone asterism from an estimated distance of around 112 light years.

Epsilon Herculis (ε Her; ε Herculis) rests at the Keystone asterism’s southeastern corner. Epsilon Herculis rates as the asterism’s dimmest star. Identified as a spectroscopic double by way of component velocities, the double star measures an orbital period of 4.0235 days. The fourth magnitude star joins the Keystone asterism from a distance of 155 light years.

The Keystone asterism eases identification of Hercules the Kneeling Hero. Although placing as fifth largest among the 88 modern constellations established by the International Astronomical Union (IAU), the vast constellation lacks first and second magnitude stars and favors moonless, rural skies.

At mid-northern latitudes in September and October, Hercules the Kneeling Hero appears in the northwest. The Keystone’s placement south of bluish Vega, brightest star in Lyra the Lyre constellation and second brightest in the celestial northern hemisphere, favors identification of the four star asterism.

The takeaway for the four star Keystone asterism that identifies Hercules the Kneeling Hero constellation is the starry quartet’s fortuitous placement below bluish Vega, second brightest star in the Northern Hemisphere’s night skies.

four star Keystone asterism and other stars in Hercules the Kneeling Hero constellation: International Astronomical Union and Sky & Telescope magazine, CC BY 4.0 International, 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:

Hercules the Kneeling Hero constellation as illustrated in Alexander Jamieson’s A Celestial Atlas, Plate VIII (1822): Public Domain, via U.S. Naval Observatory @ http://aa.usno.navy.mil/library/

four star Keystone asterism and other stars in Hercules the Kneeling Hero constellation: International Astronomical Union and Sky & Telescope magazine, CC BY 4.0 International, via International Astronomical Union (IAU) @ https://www.iau.org/public/images/detail/her/

For further information:

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Available @ http://earthsky.org/tonight/two-stars-lead-to-constellation-hercules

Fisher, Mark. “Epsilon Herculis.” Glyph Web eSky (Electronic Sky) > Stars.
Available @ http://www.glyphweb.com/esky/stars/epsilonherculis.html

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Available @ http://www.constellation-guide.com/constellation-list/hercules-constellation/

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Available @ http://www.space.com/29717-globular-clusters.html

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Available via U.S. Naval Observatory Library @ http://aa.usno.navy.mil/library/

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Available @ http://stars.astro.illinois.edu/sow/her1-p.html

Kaler, James B. (Jim). “Eps Her (Epsilon Herculis).” University of Illinois Astronomy Department > Star of the Week.
Available @ http://stars.astro.illinois.edu/sow/epsher.html

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Available @ http://stars.astro.illinois.edu/sow/etaher.html

Kaler, James B. (Jim). “Pi Her (Pi Herculis).” University of Illinois Astronomy Department > Star of the Week.
Available @ http://stars.astro.illinois.edu/sow/piher.html

Kaler, James B. (Jim). “Zeta Her (Zeta Herculis).” University of Illinois Astronomy Department > Star of the Week.
Available @ http://stars.astro.illinois.edu/sow/zetaher.html

Marriner, Derdriu. "Curious George Co-Creator Hans Rey Drew Keystone as Head of Hercules." Earth and Space News. Wednesday, July 9, 2014.
Available @ https://earth-and-space-news.blogspot.com/2014/07/curious-george-co-creator-hans-rey-drew.html

McClure, Bruce. “Find the Keystone in Hercules.” EarthSky > Tonight. May 25, 2016.
Available @ http://earthsky.org/tonight/vega-guide-star-to-the-keystone-and-hercules-star-cluster

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Available @ http://www.space.com/29780-hercules-constellation-skywatching-guide.html

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Available @ http://www.ianridpath.com/startales/hercules2.htm

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