The story of Gemini is extraordinary even by mythological standards. The twins Kastor and Polydeukes (Castor and Pollux in Latin) were twins with different fathers. Their mother was Leda Queen of Sparta and wife of King Tyndareos. Kastor was the mortal son of Tyndareos, and Polydeukes was the immortal son of Zeus (who, let's face it, had an eye for the women). The twins were inseparable from birth. They lived as any great heroes of their generation, voyaging together with the Argonauts to win the Golden Fleece. But they were killed (or at least Kastor was as Polydeukes was immortal) before they could fight in the Trojan wars or succeed to the throne of Sparta.
The trouble began when they had a quarrel with their cousins Idas and Lynkeus, sons of Aphareus, King of Messenia. Accounts vary as to the cause of the quarrel. One account says that Idas and Lynkeus were to marry their cousins Hilaeira and Phoibe, daughters of Leukippos, brother to both Tyndaros and Aphareus, but that Kronos and Polydeukes seized the girls and carried them off to Sparta where both girls gave birth to sons. The second account says that the four cousins stole a large herd of cattle and it was given to Idas to divide the spoils. He cut a cow into four and said that half the cattle would go to whoever ate his share first and the rest to he who ate his second. He promptly ate both his share and that of his brother and drove the whole herd off to Messenia. Kastor and Polydeukes were aggrieved and marched an army to Messinia to recover the cattle and steal more. In the final battle, Lynkeus, who was gifted with superhuman vision and could even see through solid objects, ran to the top of Mount Taygetos and saw Kastor and Polydeukes hiding in a hollow tree. Idas crept up and stabbed his spear through the tree mortally wounding Kastor. Being immortal, Polydeukes was unhurt and chased his cousins and killed Lynkeus whilst Zeus flung a thunderbolt at Idas. Polydeukes rushed back to Kastor to find him dying. In tears he implored Zeus to allow him to die with his brother. Zeus offered him a choice, either Kastor would go to Hades whilst Polydeukes would take his place with the gods on Olympus, or he could share his immortality with Kastor on condition that each spent alternate days in the Underworld and on Olympus with the Gods. Without hesitation, Polydeukes chose the latter and Zeus immortalised the brothers in the sky as the constellation Gemini in recognition of their devotion to each other.
Gemini is a small constellation, but contains much of interest. The two main stars are Castor (α) and Pollux (β) of which Pollux is the brighter. Castor is a double star with a period of 420 years and easily resolved by a small telescope. Each component is a spectroscopic binary (two stars so close together that no telescope can resolve them) and there is a third component of the system, YY Geminorum, which is an eclipsing binary, so the whole system consists of six stars. There are three variable stars, ζ is a Cepheid with a period of 10.15 days from which its luminosity can be deduced as 5000 times that of the Sun. Propus (η) is a semi-regular variable with an extreme range of magnitude 3.1 to 3.9 and average period of 233 days. U Geminorum is the prototype of a small star which from time to time exhibits a sudden increase of brightness, these dwarf novas being known as U-Geminorum stars. It is very faint with a rest magnitude of 14.9 with outbursts every 100 days or so raising it to a magnitude no brighter than 8. Also in the constellation are M35, a very conspicuous cluster (just visible in my image) and NGC2392, the Eskimo Nebula, a rather faint planetary nebula.
The picture was taken with an Canon 1000D camera with an 18-mm lens at f/4 with 30 seconds exposure at ISO 1600 at 20:01 UT on 23 March 2011. The coloured light pollution has been removed and the brightness and contrast increased in PhotoImpact.