Gemini is a major constellation in the winter sky. It is easy to find by locating the two bright stars Castor and Pollux. Gemini is one of the few constellations with two first magnitude stars. The constellation is best seen from December through May. Clyde Tombaugh discovered the dwarf planet Pluto in this constellation in January 1930.
History and Mythology
Gemini is the third constellation of the Zodiac. In Greek Mythology, Gemini was named for the twin sons of Leda. The stars Castor and Pollux mark the heads of the twins. They had different fathers; Pollux was the son of Zeus, and Castor was the son of Leda's husband King Tyndareus. They were the brothers of Helen of Troy.
Castor and Pollux sailed with Jason and his Argonauts to find the Golden Fleece. It was thought that Pollux was immortal and Castor was mortal. When Castor died, Pollux was so grief-stricken that he wished to join his brother in death. Zeus was so touched by the devotion of the two brothers that he immortalized both in the sky on the edge of the Milky Way.
The Romans identified the Twins with Romulus and Remus, the brothers who were believed to be the founders of Rome. The Chinese associate Castor and Pollux with yin and yang, the dual forces of nature.
Notable Stars
Alpha Geminorum, or Castor, is a close double star with components of 2nd and 3rd magnitude, separated by about 3 arcseconds. These two stars are also close binary systems, with orbital periods of orbit of 3 and 9 days. There is a third system, composed of a close pair of red dwarfs, which orbit Castor in several thousand years, making a total of stars in the system. The Castor system is 50 light-years away.
Pollux, or Beta Geminorum, shines at magnitude 1.2 and is the 17th brightest star in the sky. It is 35 light-years from the Earth. Pollux is somewhat cooler than the Sun, but is 30 times more luminous. Pollux is one of the few giant stars known to have a Jupiter-sized planet, which orbits Pollux in a nearly circular path with a period of 590 days.
Eta Geminorum, at the southwest corner of Gemini, is a variable red giant star whose magnitude fluctuates between 3.15 and 3.9 over a period of 234 days. Its Greek name, Propus, means "forward foot". Zeta Geminorum is one of the brightest Cepheid variables in the sky, and ranges between magnitude 3.6 and 4.1 over a period of 10 days. It is a white supergiant 1200 light years away; its traditional name, Mekbuda, comes from an Arabic phrase meaning "the lion's folded paw".
Clusters, Nebulae, and Galaxies
Located along the northern Milky Way, Gemini is rich in open clusters. M 35 is a beautiful cluster which can be seen with the naked eye. The cluster contains over 100 stars from 6th magnitude, and it is about 2,800 light-years away. The much smaller, fainter cluster NGC 2158 lies in the same binocular field of view, but is more than five times further away.
NGC 2392, known as the Eskimo Nebula or Clown Face Nebula, is an 8th magnitude planetary nebula about 40" in diameter. A modest telescope will show a small greenish ball and reveal the 9th magnitude star at the center of the nebula.
The Medusa Nebula, PK 205+14.1, is a very old planetary nebula in Gemini, on the border with Canis Minor. This unusual planetary nebula is a very faint, rather large, irregular half-moon-shaped object. Its surface brightness is very low, so large telescopes are required to see it.