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Hercules, the mighty strongman of the summer sky

A diagram shows Hercules in the late-July sky over New York City.
A diagram shows Hercules in the late-July sky over New York City. (Image credit: <a href="http://www.starrynight.com">Starry Night</a>)

A staple of mythology, the hero Hercules has a strange celestial story.

One of the best-known star patterns, Hercules stands high over our heads in the Northern Hemisphere at nightfall this week.

Like many of our oldest constellations, Hercules can be traced to the beginnings of recorded history about 5,000 years ago in the Middle East, specifically Mesopotamia (the region that today we call Iraq), although we refer to the ancient hero by his Latin name. Hercules has long represented a man of extraordinary strength and he appears in various forms in the legends of many peoples throughout the region, including the tale of Sampson in the Bible. 

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It looks like what? 

Depending on what astronomy guide book you are consulting, there are several variations on how Hercules is traced out. Initially, you might expect this mighty strongman to be a bright and conspicuous constellation, something along the lines of Orion the Hunter with his distinctive three-star belt. 

But this is not so. 

In his book "Find the Constellations" (Houghton Mifflin Co., 2008), author H.A. Rey perhaps said it best when he noted that "Hercules was ... famous for his strength, but as a constellation he is rather weak, without bright stars."

Mesopotamian skywatchers in 3000 BC saw Hercules standing upright high in the northern sky during the summer. Hercules' brightest star is third-magnitude Ras Algethi, which is Arabic for "Head of the Kneeler." And indeed, in the sky Hercules was portrayed posturing on one knee, with Ras Algethi, a red supergiant star, marking his head. 

But over the past 5,000 years, the wobbling of the Earth's axis (called "precession") has caused the position of the stars to shift in such a manner so that today Hercules appears to be performing acrobatics, with his head passing well south of the zenith. In other words, Hercules is now standing on his head! 

Rey reimagined this group of stars according to its current orientation as "a man swinging a club," he writes, "Hercules' favorite weapon." In Rey's version, a keystone-shaped quadrilateral — which the ancients envisioned as his waist and hips — ends up as Hercules' head. Meanwhile, the star Ras Algethi that the ancients considered the hero's head marks Hercules' left foot. Different strokes for different folks.

Meanwhile, in his book "Introducing the Constellations" (Viking Press, 1937), astronomer Robert H. Baker traced out Hercules as "a figure of six stars that outlines a butterfly with outstretched wings." This figure also somewhat resembles the letter "H," which of course is also the initial for Hercules. 

The legend of Hercules 

Hercules was known in Greek culture as Heracles, and the extensive legends surrounding him are among the best known of Greek mythology. We call him Hercules in keeping with the tradition of using Latin names for the constellations. 

Many Greek gods, heroes, heroines, and other legendary personalities were adopted by the Romans, who identified them with characters of their own. Thus, Zeus became Jupiter; Hera, Juno; Ares, Mars; and so forth. When astronomical bodies are given mythological names in modern times, the Latin version has generally been preferred. 

The great Roman author, statesman and philosopher Lucius Annaeus Seneca (4 BC-AD 65) relates in verse some of the leading Hercules legends in "Hercules Furens" ("Mad Hercules"), including a diatribe by Juno against Hercules — her husband's (Jupiter's) demigod son by another woman, the mortal Alcmene.

There is actually a connection between Hercules and two other constellations, the nine-headed serpentine water monster known as the Lernean Hydra, and a much smaller creeping sea creature. 

A jealous Juno summoned a crab (Cancer) to fatally bite Hercules. Her crustacean arrived just at that moment that Hercules was busy slaying the multiheaded Hydra, one of his 12 assigned superhuman "labors." 

But Cancer's bite was no more than a mere annoyance to our hero, who abruptly crushed the attacker under his heel. Infuriated with the crab's less-than-heroic fate, Juno banished this hapless creature to the heavens as one of the most inconspicuous of the traditional constellations. 

As for the Hydra, each time Hercules lopped off one head, two others grew in its place. But Hercules emerged victorious by having his nephew, Iolaus, burn the stump of each severed neck, preventing new heads from sprouting. Interestingly, in our current evening sky, as Hercules stands triumphantly at the top of the heavens, the tail of the constellation Hydra can be seen slithering below the southwest horizon and hurrying out of sight.  

A great ball of stars 

The object that always draws the most attention in Hercules and is regarded as a showpiece for Northern Hemisphere observers is M13, the Great Globular Cluster in Hercules. It is within the Keystone, about two-thirds of the way from the butterfly's head along the western edge of the northern wing. With a total light equivalent of about a sixth-magnitude star, it can be seen with the unaided eye in a very dark sky, as it was by Edmond Halley, who discovered this cluster in 1714. 

At a distance of roughly 22,000 light years, M13 is among the nearest globulars; scientists believe that this great swarm contains at least several hundred thousand stars. Binoculars will show it as a pale, colorless glow with a diameter as much as half that of the moon. But through telescopes it becomes a sight to behold. Small telescopes of 4 to 6-inches will reveal the outer stars, while larger telescopes of 8 or more inches reveal the entrancing beauty of a great ball of stars. 

The Hercules cluster is a celebrated object, often shown to those who might pay an evening summertime visit to an observatory. Next week (Aug. 5 to Aug. 8) will be the 85th annual Stellafane Convention which is held just outside of Springfield, Vermont. Weather permitting, assiduous amateur astronomers will set up their equipment under dark New England skies or congregate at the McGregor Observatory for views through the 13-inch Schupmann telescope or at the 12-inch Porter Turret Telescope. 

An oft-told story about M13 stars deep-sky authority Walter Scott Houston (1912-1993), who had a regular column in Sky & Telescope magazine for nearly half a century and was known to one and all as "Scotty." 

One evening he noticed a long line of people patiently waiting their turn to get a look through the Porter scope. "What are you folks looking at?" he asked as he poked his head through the observatory door. From out of the darkness, several people quietly murmured: M13. 

"M13?" replied Scotty, with a tinge of skepticism. "So many people have looked at it, you would think it'd be worn out by now!" 

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Follow us on Twitter @Spacedotcom and on Facebook.

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Joe Rao

Joe Rao is Space.com's skywatching columnist, as well as a veteran meteorologist and eclipse chaser who also serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications.