Only four of the 88 constellations in our sky contain more than one of the 25 brightest stars.
These would be Orion (Rigel and Betelgeuse), Centaurus (Rigel Kentaurus and Hadar), Crux (Acrux, Mimosa and Gacrux), and Gemini (Pollux and Castor). Of these, only Orion and Gemini are visible from mid-northern latitudes. If February is the best month to see Orion in the early evening, then March belongs to Gemini.
Go out around 8:30 p.m. every night in March and look straight up at the zenith. There you cannot miss the twin stars of Gemini, Pollux and Castor.
The Gemini twins are positioned in the sky above the great hunter Orion, as he battles Taurus the Taurus. The stars Pollux and Castor represent the heads of the famous twins and the fainter stars Propus and Alhena mark their feet.
Gemini is one of the 12 constellations of the zodiac, which means they lie along the path that the sun, moon, and planets follow in their travels around the sky. The sun’s northernmost position is just below the Gemini twins, near the magnificent star cluster M35.
Pollux and Castor were fraternal twins born to Leda. The ancient Greeks called them the Dioscuri, or sons of Zeus, although only one of the mythological twins is said to have been fathered by the king of the gods.
Castor’s father was Leda’s real husband, Tyndareus, while Pollux’s father was Zeus. Thus, Pollux was immortal like his father, but his brother Castor was only a mere mortal.
Even though they weren’t conjoined twins, they were nonetheless inseparable. The two brothers were also very close to their two sisters, Helena (of Troy) and Clytemnestra. Pollux and Castor shared many adventures in Greek mythology, even sailing with Jason and the Argonauts on their quest for the Golden Fleece.
The Gemini twins were thought to bring good luck to sailors, sometimes appearing as a bright blue glow atop the ship’s mast. St. Elmo’s fire, as it is now known, is caused by a buildup of static electricity on a sharp object, such as a mast, during a thunderstorm.
When Castor was mortally wounded in battle, Pollux begged Zeus to grant his brother immortality as well, so he wouldn’t die. Zeus heard the call and immortalized the two brothers in the sky, side by side forever, like our constellation of Gemini, the Twins.
Close inspection reveals that the star Pollux, representing the immortal twin, is slightly brighter than Castor, the mortal twin. That’s how I remember which is which.
Point a medium-sized telescope at Castor and you might think you’re seeing double. In fact, you are. Castor is a fine example of a binary star, two stars that appear as one to the naked eye.
Each of these stars is double again and is circled by a third pair of faint stars, making Castor a rare sextuple star system, about 51 light-years from Earth. Pollux is a little closer to us, 33 light years away, and has no known star companions.
Recent studies, however, show that Pollux has at least one major planet orbiting it. This planet, officially named Thestius, is more than twice the size of the largest world in our solar system, Jupiter.
The beautiful cluster of stars at the twins’ feet is called Messier 35, or M35 for short. M35 is estimated to be 175 million years old – quite young for a star cluster. It contains over 400 stars and covers an area of the sky equivalent to a full moon.
Although M35 is visible to the naked eye as a blur, binoculars or a small telescope are needed to show its twinkling stars. M35 is 3870 light years from Earth. A much more distant star cluster, NGC 2158, appears at the edge of M35.
The Geminid meteor shower erupts from this constellation in mid-December each year, from a point near the star Castor. Up to 90 meteors per hour can be seen on the night of the peak, making it one of the best annual meteor showers.
NASA’s Gemini program in the 1960s was named after the celestial twins because each Gemini mission launched two astronauts to the stars.
For more information about astronomy-related events in Steamboat Springs, including public star parties at CMC’s Ball Observatory, contact Physics and Astronomy Instructor Paul McCudden at pmccudden@coloradomtn .edu or 970-870-4537 or visit the SKY Club webpage at http://www.coloradomtn.edu/skyclub.
Jimmy Westlake is assistant professor of physical sciences at Colorado Mountain College and former director of the Rollins Planetarium at Young Harris College in Georgia and the Planetarium at St. Charles Parish Library, Luling, Louisiana. His “Celestial News” column appears monthly in Steamboat Today. Check out Jimmy’s astrophotography website at http://www.jwestlake.com.