Hercules is the 5th largest constellation, and contains 22 stars which span an impressive 1,225 square degrees of sky. Because the constellation contains no stars above third magnitude, it can be difficult to identify, although the large asterism called the “Keystone” is useful in this regard. The constellation is named after Heracles, the legendary strongman from Greek mythology, with Hercules being his Roman equivalent.
The constellation can be seen by observers located between +90° and -50° of latitude, with the best viewing time for the Northern Hemisphere from April to November, and for the Southern Hemisphere between June and September. Hercules is located between the night sky’s 4th and 5th brightest stars, namely Arcturus (-0.04mag) in Boötes to its west, and Vega (0.03mag) in Lyra to its east. To the constellation’s north lies Draco; and to its south Ophiuchus.
Hercules Constellation Family
Hercules is a member of the Hercules family of constellations, together with Aquila, Ara, Centaurus, Corona Australis, Corvus, Crater, Crux, Cygnus, Hydra, Lupus, Lyra, Ophiuchus, Sagitta, Scutum, Sextans, Serpens, Triangulum Australe and Vulpecula.
The origin of the constellation is widely assumed to have its roots in Sumerian times, and is associated with the hero Gilgamesh, a mythical hero famed for his great strength. Similarly, the constellation according to Greek mythology is associated with perhaps the greatest hero of them all, Heracles, the son of Zeus and Alcmene, who was a mere mortal. However, soon after his birth, Zeus placed him at the breast of the sleeping Hera, and having suckled the milk of a goddess, Heracles became a demi-god, and thus immortal.
Understandably, Hera was somewhat upset with both Zeus’s philandering, and the fact that she was tricked into suckling the child of a mortal woman, and since she could not murder the child outright, she took a vow to make his life as difficult as was godly possible. For a start, Hera bewitched Heracles, which made him insane and as a result, he killed his wife and children. Realizing what he had done, Heracles then consulted the Oracle of Delphi, who sent him to enter the service of Eurystheus, the king of Mycenae, for a period of twelve years to atone for his crime, and it was during this period that he acquired the name “Heracles”, which translates into “The glory of Hera”.
This was also the time when he performed his famous twelve labors, but in the end he died at the hands of his wife Deianeira, who poisoned him at the instigation of Nessus, a centaur who tricked her into believing that his blood could be turned into a potent love potion that would ensure Heracles’ fidelity. The centaur’s blood, was tainted with poison, though, and after suffering unrelenting pain, Heracles, resigned to his fate, lay upon a pyre where his mortal half perished, but his immortal half was carried to heaven. Zeus then honored his son by placing him amongst the stars as the constellation now known as Hercules.
The Tau Herculids are the sole meteor shower associated with the constellation, and is linked with the periodic comet called 73P/Schwassmann–Wachmann. The shower is active from May 19th to June 19th, with its peak occurring on the 9th, when an average of about four meteors per hour can be seen.
Despite being the 5th largest constellation in the night sky, Hercules has no stars of first or even second magnitude.
– Kornephoros (Beta Herculis), the constellation’s brightest star, is a yellow star situated 139 light years away with a visual magnitude of 2.81. It is actually a binary system whose primary component is a yellow giant that has 17 times the diameter of our sun, three times its mass, and shines 175 times brighter. The two stars have an orbital period of 410 days.
– Zeta Herculis, the second brightest star in Hercules, is a multiple star system found 35 light years from our solar system with a visual magnitude of 2.81. Its main component star is a subgiant with 2.6 times the Sun’s radius, 1.45 times its mass, and 6 times its luminosity, while its companion is a yellow dwarf not unlike our own sun.
– Sarin (Delta Herculis), the constellation’s third brightest star, is a multiple star system located 75 light years distant with a magnitude of 3.126. Its primary star is a white sub-giant with twice the Sun‘s radius and mass.
The constellation contains a number of other notable stars, including four which comprise an asterism known as the Keystone, representing Hercules’s torso, namely the 3rd magnitude stars Pi Herculis and Zeta Herculis, and the 4th magnitude stars Epsilon Herculis and Eta Herculis.
Other stars of interest in Hercules includes Rasalgethi (alpha Herculis), a variable red giant with a giant yellow-white companion; the orange giant 109 Herculis; the blue subgiant Rukbalgethi Shemali (Tau Herculis); the yellow-white giant Nu Herculis; as well as several dwarf stars, including Gliese 649 (red), 14 Herculis (orange), and HD 155358 (yellow).
Notable Deep Sky Objects
The constellation of Hercules contains two Messier objects, namely the globular clusters M13 and M92, as well as a number of other notable deep-sky objects.
– The Hercules Cluster (Abell 2151) consists of a group of about 200 galaxies about 500 million light years away, and is part of the much larger Hercules Super Cluster (SCI 160) of galaxies.
– Messier 13 (M13, NGC 6205) also known as “The Hercules Globular Cluster”, is located 25,000 light-years away, is 145 light years across and contains around 300,000 stars. M13 can be seen in small telescopes, but with a visual magnitude of 5.8 is difficult to see without the use of an optical aid, even under the best seeing conditions.
– Messier 92 (M92, NGC 6341) is a globular cluster around 27,000 light years from Earth with a visual magnitude of 6.3. It is estimated to be 14.2 billion years old, which makes it one of the oldest clusters of the Milky Way galaxy, and almost as old as the universe itself.
Other objects of interest in Hercules includes the planetary nebulae Abell 39 and NGC 6210; the colliding spiral galaxies called Arp 272; and the elliptical galaxy NGC 6166.