Gemini

Transit Date of principal star:
14 January

Gemini, the Twins, are really only half-brothers. They share the same mother (Leda) but have different fathers. Castor's father was a king of Sparta, Tyndareus - who would be chased from his throne but later rescued by Heracles (who nevertheless wound up killing him). The father of Pollux was none other than Zeus, or Jupiter. Zeus visited Leda on her wedding night in the guise of a swan. Thus the twins would be born. (In fact two twins came from this double union, but let's not complicate the matter even more...) It should be said, however, that Pollux had a sister as well by Leda and Zeus: the beautiful Helen, who would become Queen of Sparta, and whose abduction by Paris would lead to the Trojan War. Castor was a great horseman and fighter. One of his pupils was Heracles. Like Heracles, both Castor and Pollux would become Argonauts, that is, join Jason in his quest for the golden fleece. The twins spent their time raiding cattle and abducting young women, as Greek gods were wont to do. During one such cattle raid a cousin (Idas) became enraged at Castor and killed him. Zeus threw a thunderbolt at Idas, killing him instantly. Since Pollux was the son of Zeus, he was immortal. But Pollux mourned over his brother's loss to such a point that he wanted to follow Castor into Hades. Zeus was so stricken by Pollux's love for his brother, he allowed them both to share Hades and Olympus, (on alternate days). Later Greek writers had Zeus place the two in the heavens side by side. The stars of Gemini include two of the most recognisable in the heavens: the twins Castor and Pollux. Castor (alpha Geminorum) is the slightly dimmer star. It has a visual magnitude of 1.93 and is 52 light years distant. It isn't a particularly large star, at about twice the Sun's diameter. The star is a noted binary, discussed below. Pollux is the brighter of the two stars with a visual magnitude of 1.16 and a distance of 33.7 light years. It is also considerably larger, with an estimated diameter of about ten Suns. Castor and Pollux are 4.5 degrees apart, which helps observers estimate separation distances between other stars. Epsilon Geminorum is a supergiant at about 30 Sun diameters. This star may be as far away as 950 light years, but the combination of visual and absolute magnitudes suggests a much closer star, at only 190 light years. Zeta Geminorum is the most distant of the bright stars in this constellation, at over 1200 light years. This is a cepheid variable (see below). Eta Geminorum is a red giant, about 50 times the size of the Sun, at a distance of 280 light years. It is a visual binary and a variable (details below). Double stars in Gemini Alpha Geminorum is a well-known binary with the companion currently (2000.0) at a PA of 65º and separation 3.9". The visual magnitudes are 1.9 and 3.0. There is some disagreement over the precise period of the companion; one observer has it at 420 years, another at 511. More recent measurements put the orbit at 467 years and the orbit we've prepared uses this revised value. This was the first binary system that was so recognised, in 1802 (or 1803, accounts vary) by William Herschel. However there is considerable speculation that the star was a known double long before that, perhaps even a century before Herschel made his announcement. The companion, Castor B, is also a spectroscopic binary, with its companion revolving around Castor B every three days. In fact, the entire system is comprised of six stars, including a red dwarf, Castor C, which slowly revolves around both Castor A and Castor B. This star is also a variable (and therefore catalogued as YY Gem). Delta Geminorum: visual magnitudes 3.5, 8.2, PA 225º, separation 5.8". The period is estimated at 1200 years; the companion is an orange dwarf which may be difficult to resolve in smaller telescopes. Eta Geminorum is a visual binary that takes some work to resolve; the companion is only 8.8 (primary is 3.3), the PA is 266º and separation 1.4". This is nearly a fixed binary, with very little movement. Variable stars in Gemini Zeta Geminorum is a cepheid variable, from 3.62 to 4.18 every 10.15 days. Eta Geminorum is a semi-regular variable with an average period of 232.9 days. It ranges from 3.2 to 3.9. R Geminorum is a Mira-type long-period variable, with large variation from 6.0 to 14.0 every 370 days. The 2000 maximum should arrive in mid October. Deep Sky Objects in Gemini: The only Messier object in Gemini is M35 (NGC 2168). This is an open cluster easily enjoyed in small scopes. It lies just 2.5 degrees northwest of eta Geminorum. This cluster is extremely attractive, with gently curving rows of glittering stars. Several hundred stars make up the group, which is perhaps 2500 light years away. The Eskimo Nebula (NGC 2392) is one of the more distant nebulae at an estimated distance of 10,000 light years. There is a tenth-magnitude central star. If you do have a large enough scope, be prepared for anything: Burnham thought the Eskimo Nebula suggested "the classic and unforgettable features of W. C. Fields." While you can locate this blue-green object in small scopes, it takes a very large telescope to see the "face" of this nebula, the eyes, nose, and mouth and the "fur collar" that gave it its name. To find this rather small planetary nebula draw an imaginary line between kappa Geminorum and lambda Geminorum. Now draw a perpendicular line from delta Geminorum, and just about where this line meets the other one is where you'll find the Eskimo Nebula. For a closer look at Gemini, visit the Binocular Section. 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