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In the sky, Gemini is distinguished by Castor and Pollux, the Twins. These two bright stars make the constellation easy to find.
In Greek mythology, Castor was the mortal brother of Polydeuces (Pollux in Latin). Queen Leda was the mother of both, but Castor was fathered by her husband King Tyndarus, and Polydeuces was fathered by the god Zeus who had seduced Leda disguised as a swan. Both were also brothers to Helen of Troy. In legend, the mortal Castor was killed in a fight over a woman. Polydeuces implored his father Zeus to let them both reside together on Mount Olympus, so Zeus placed them both in the sky next to each other.
In Roman legend, the constellation of Gemini is connected with Romulus and Remus, the twin brothers who founded Rome.
Alpha Geminorum is Castor. It is a spectral-class A-type blue-white star that shines at an apparent magnitude of 1.58. It is located 49.8 light-years away from the Earth. Castor is a binary star. Its components are magnitude 2.0 and 2.9 and are separated by 6 arcseconds, making it a beautiful double that can be split by modest sized telescopes under good seeing conditions. Each of the two main components is itself a spectroscopic binary. The spectral classes of the two bright components of Castor are A1V, and A2Vm.
Another faint M-class dwarf star lies 72 arcseconds away that is an eclipsing binary whose magnitude varies from 9.3 to 9.8. This star is also part of the Castor system, making it a sextuple-star system.
Beta Geminorum is Pollux. At magnitude 1.16 it is brighter than Alpha Geminorum. It is located 33.8 light-years away from the Earth. Pollux is a spectral class K0 IIIb red giant star that is twice as massive as the Sun, but nine times bigger.
Zeta Geminorum is a Cepheid-type variable star whose brightness ranges from magnitude 3.6 to 4.2 over a 10.15 day period.
Gemini contains several interesting deep-sky objects. M35 is a large open cluster at the foot of the constellation's figure on the right, between Propus (Eta Geminorum) and 1 Geminorum. NGC 2392 is the Eskimo or Clown Face planetary nebula. IC 443, the Jellyfish Nebula, is a large, but faint, supernova remnant just to the east of Propus.
The Sun moves through the constellation of Gemini from June 21 to July 20.
Gemini was cataloged by the Greek astronomer Ptolemy in the second century C.E. (Common Era). It is the 30th largest of today's 88 modern constellations, covering 514 square degrees of sky.
North is to the top right in the above image.