Hercules is a large constellation in the northern hemisphere. It was originally included on the list of constellations by the Greek astronomer Ptolemy in the 2nd century.
Once known as Engonasin or the Kneeler in Greece, the constellation is now identified with the Greek hero Heracles, or Hercules in Roman mythology, and depicted as a hero figure standing triumphantly over a slain dragon, holding a club. The dragon is associated with Ladon, the beast with a hundred heads that guarded the Garden of the Hesperides. The playwright Aeschylus interpreted the celestial image as showing Heracles kneeling, wounded after his battle with the Ligurians.
The myth of Heracles, son of Zeus and Alcmene, the wisest and most beautiful woman in the world, is a very long and complex one. As an infant, he drunk the milk of the goddess Hera and became immortal. He was the biggest and strongest of all men, but suffered greatly at the hands of Hera, who had sworn to make his life hard. Under her spell, he killed his own children. Realizing what he had done, he visited the Oracle at Delphi and was told that, to atone for his act, he had to serve Eurysteus, king of Mycenae, for 12 years. It was the Oracle that gave him the name Heracles. It means, ironically, "the glory of Hera."
Eurysteus gave Heracles a number of tasks to accomplish, known as the Labours of Heracles. These included killing the Nemean lion and the Lernaean Hydra, stealing the Girdle of Hippolyte and the Apples of Hesperides, and capturing the Ceryneian Hind, the Erymanthian Boar and Cerberus, the multi-headed hound that guarded the entrance to Hades. Hercules suffered a tragic fate in the end, being poisoned by his wife Deianeira, who was deceived by the centaur Nessus into believing that his blood could be used as a love potion.
The constellation Hercules occupies an area of 1225 square degrees and contains seven stars with known planets. It can be seen at latitudes between +90° and -50° and is best visible at 9 p.m. during the month of July.
Even though Hercules is the fifth largest constellation in the sky, it does not abound in bright stars. 
alpha Herculis, also known as Rasalgethi ("the kneeler’s head"), is a red giant that varies in luminosity between third and fourth magnitude and has a yellow-white giant star for a companion.
beta and 
delta Herculis represent Hercules' shoulders. Beta Herculis, a binary with a G-type giant for its primary component, is the brightest star in the constellation. It is also known as Kornephoros ("club bearer"). 
delta Herculis has the traditional name Sarin.
Four stars in Hercules – 
eta and 
pi Herculis – form a distinctive asterism known as the Keystone, which marks Hercules’ pelvis. 
epsilon Herculis is a spectroscopic binary star. 
zeta Herculis is another double star composed of a subgiant and a faint companion star. 
eta Herculis is a G-class giant that acts as a gateway to Messier 13, the Great Cluster. 
pi Herculis, also known as Fudail ("excellent in character"), is an orange giant.
Hercules’ left leg is marked by 
theta Herculis at the knee and 
iota Herculis at the shin, while the star 
tau Herculis marks Hercules’ right knee.
The most notable deep sky object in the constellation is Messier 13 (NGC 6205), a globular star cluster, the brightest and finest one of its kind in the northern hemisphere. It contains several hundred thousand stars. 145 light-years in diameter, it is easily spotted in small telescopes. It is also called the Great Globular Cluster or the Hercules Globular Cluster.
Lying relatively nearby is another notable Messier object, Messier 92 (NGC 6341). Approximately 26,000 light-years distant, it is one of the brightest globular clusters in the northern sky.
The constellation Hercules also contains an interesting planetary nebula, Abell 39. Lying about 6,800 light-years away, Abell 39 appears as an almost perfect sphere, one with a radius spanning approximately 2.5 light-years.
Hercules belongs to the Hercules family of constellations, along with Sagitta
, Corona Australis
, Triangulum Australe
Constellations directly bordering Hercules are Draco
, Corona Borealis
, Serpens Caput