Gemini is one of the few constellations that actually looks like its namesake. It is one of the 48 constellations described by the 2nd century astronomer Ptolemy.
Gemini constellation is located in the northern hemisphere of the sky. It is the 30th largest constellation, occupying an area of 514 square degrees.
Gemini lies in the second quadrant of the northern hemisphere and can be seen at latitudes between +90° and -60°.
Pollux (Beta Geminorum) is the brightest star in Gemini and the 17th brightest star in the night sky. It is sometimes known as “The Head of the Second Twin,” from the Arabic Al-Ras al-Tau’am al-Mu’akhar. It is an evolved orange giant and is 33.78 light years distant. Beta Geminorum has twice the Sun’s mass and about nine times the solar radius. An extrasolar planet with a mass at least 2.3 times that of Jupiter was confirmed to be orbiting the star in June 2006.
The neighboring NGC 2158 is another open cluster in Gemini. It lies to the southwest of Messier 35. It is approximately 11,000 light years distant from the Sun and is believed to be about a billion years old.
The Medusa Nebula, also known as Abel 21 and Sharpless 274, is an old, large planetary nebula near the border with the constellation Canis Minor. It was once believed to be a supernova remnant. It got the name Medusa because the filaments of glowing gas are evocative of the serpent hair of the Gorgon Medusa, a monster from the myth of Perseus whose gaze would turn people to stone. The nebula was formed when a red giant turned into a hot white dwarf and shed its outer layers. The Medusa Nebula is approximately 1,500 light years distant from the solar system.
Finally, the Eskimo Nebula, NGC 2392 is a bipolar double-shell planetary nebula in Gemini. It got its name because it appears like a person’s head wearing a hood, with bright orange streams in the outer layer. The nebula is surrounded by a layer of gas that used to form the outer layers of a star similar to the Sun. It is sometimes also called the Clownface Nebula. The Eskimo Nebula is at least 2,870 light years distant from the solar system.
Where Gemini mythology comes into play is when Castor, being mortal, finally dies. Having spent their whole lives together, Pollux is distraught. He doesn’t want to live without his twin brother, but since he is immortal, there is nothing he can do. He begs his father, Zeus, for help.
Zeus decides that rather than killing Pollux so he can be with Castor, he makes Castor immortal also, and the two of them get to live together forever as the constellation Gemini.