They're Twins, By Jiminy!

As twilight fades about 7 p.m. on February 1, Capella is low on the horizon. It travels around from east to north, but does not climb high into the northern sky. Just rising are twin stars (Castor and Pollux). They seem to pursue the mighty kangaroo Purra (Capella) as it skirts around the horizon. The hunters take a shortcut by crossing higher into the heavens in their chase, but by midnight Purra has hidden below the northwestern edge of the Earth out of reach. But each year as the weather changes to the hot, dry climate of the “great heat” the twin stars overtake, kill, and roast their quarry. Smoke from the fire—the original Australian “barbie” perhaps—causes the mirages seen in summers. At least this is what the aborigines of Queensland say.

In ancient China the twin stars of our modern constellation Gemini were the twin symbols of the eternal contrasts throughout life, Yin and Yang. Elements of mystery, Yin is the feminine principle, water, cold and dark, winter and the Moon; while Yang is masculine, fire, heat and light, summer and the Sun. They complement each other, and together they constitute the power of the seasons, the elements, the world above and the world below, and all of humanity.

The heroes of Greece and Rome, Castor and Pollux, enter into many of the myths and fables, including the adventures of the Argonauts in search of the Golden Fleece. On this voyage a fierce storm arose, and although the ship was sturdy and all the crew brave and fearless, still this was a storm such as none had experienced before. In the midst of the great peril as the fury of the sea was at its height, Orpheus took up his harp and began to play the melodies that only he could play to calm the storm. And as the winds died down and the clouds disappeared, suddenly there were two stars shining over the heads of the twins Castor and Pollux. From that time forward, the twins of Gemini have been the patron gods of sailors and of fishermen.

It was here in Gemini, a zodiacal constellation which lies between Taurus and Cancer, where both Uranus and Pluto were found. In 1781, William Herschel, then 42 years of age and a church organist, composer, and music teacher, was a dedicated amateur astronomer whose habit was to “unbend the mind” from his daily business by grinding telescope mirrors, or by using the telescopes he had made to see the heavens and the planets. On March 13, using one of his telescopes with a mirror 6½ inches in diameter, he noticed a “curious either nebulous star or perhaps a planet.” On the next clear night he found it to have changed location and so reported it to be a comet. Uranus, now to be seen among the stars of Capricornus, orbits the Sun once in 84 years. It will return to Gemini for only the third time since it was first seen about the year 2033. Meanwhile, from its discovery in 1930, Pluto has progressed only as far as from Gemini to Sagittarius in its trip around the Sun. It will not return “home” to Gemini until the year 2177. In mid-January, 1999, Pluto will recross the orbit of Neptune and again take its place as the most distant of the planets.

Gemini claims one of the finest open star clusters, M35. This is a nice object for viewing with binoculars during the evening hours in February. Another interesting feature of the constellation is the Eskimo Nebula, a planetary nebula (NGC 2392) with a hot white dwarf star in its center. In photographs taken with large telescopes, one can easily imagine the face of an Eskimo peering out from his seal-fur parka. The illusion comes from a series of nova explosions that resulted in concentric shells of luminous material, the outermost being half a light year in diameter.

Aside from their fame as the twin stars of Gemini, Castor and Pollux are interesting in other ways. Castor, Alpha Geminorum, is a twin of another sort. A binary star system, it is a pair of stars, Castor A and Castor B, revolving about their combined center of mass. And each of these is a spectroscopic binary. In fact, the entire system is comprised of six stars, including twin red dwarfs, Castor C (YY Gem), which slowly revolve around both A and B. Castor shines at magnitude 1.6, but it is outranked by Pollux at 1.2. Pollux has a diameter of from 12 to 20 times that of the Sun; it is the closest giant to us, about 35 light years away.

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