To the unaided eye, the star Castor in the constellation Gemini the Twins appears as a bright pinpoint of light. But it’s actually three pairs of binary stars – six stars in all – in a complex dance about a common center of mass.
Even a fairly small telescope will show Castor as two stars. A much-fainter star nearby can be glimpsed, too; it’s also part of the Castor system. Each of these three stars – called Castor A, B and C – is also double. Their doubleness can’t be seen directly in telescopes, but it can be discerned via spectroscopic data, that is, by splitting starlight into its component colors.
The mass of all six stars together is, very roughly, about six times that of the sun.
Pollux is not one of the 6 stars in the Castor system. When we speak of the six stars in the Castor system, we mean the single point of light we see as Castor is six stars. But Pollux is nearby, and about equally bright as Castor. The early stargazers identified Castor and Pollux as “twins.”
Here are two ways to find Pollux and Castor. From a Northern Hemisphere location face generally northward to find the Big Dipper asterism in the constellation Ursa Major. Draw an imaginary line diagonally through the bowl of the Big Dipper, from the star Megrez through the star Merak. You are going in the direction opposite of the Big Dipper’s handle.
This line will point to Castor and Pollux.
Here’s the second way to find Caster and Pollux. Face generally southward in the winter months from a Northern Hemisphere location – or generally overhead in those same months, the summertime months in the Southern Hemisphere – and look for the very noticeable constellation Orion the Hunter. You’ll spot it easily by looking for the three medium-bright stars that make up Orion’s Belt. A line drawn from Rigel through Betelgeuse in Orion – extending perhaps three times the distance between them – passes near Gemini’s twins.
At 34 light-years, Pollux is closer to us while Castor lies 51 light-years away. Thus Pollux and Castor aren’t gravitationally bound, but only near each other along our line of sight. Their proximity in our sky is what makes them easy to spot.
The twin stars are close to the moon’s path across our sky. The moon passes no more than 15 degrees south, and sometimes fewer than 5 degrees south, of Castor on some day every month. For this reason, the moon might help you find Castor.
The sun passes closest to Castor on or near July 14, and consequently the star cannot be seen for several weeks before that date to several weeks afterward. Other than for that time period, intrepid observers can find Castor at some time of night for about 10 months of the year.
History and mythology of Castor. Castor is designated Alpha Geminorum. Most alpha stars are the brightest in their constellations, but Castor is slightly fainter than Pollux.
The reason for the name Castor is unclear, although there appears no specific connection with a beaver, which is what the word means in Latin. It’s also not clear why this star holds the appellation of alpha, a label typically put on a constellation’s brightest star. Castor is clearly secondary in brightness to Pollux, which is officially called Beta Geminorum.
There is much mythology associated with these two stars, typically only in conjunction with each other. They are usually considered to be twins. In Greek mythology Pollux is immortal, the son of Zeus, and Castor is mortal, the son of King Tyndareus of Sparta. Thus they were really half-brothers rather than true twins, with a common mother in Queen Leda. Their conception and birth, however, was a complicated and unlikely affair, with their mother succumbing to both Zeus (disguised as a swan) and King Tyndareus on the same night, with the resulting birth not only of Castor and Pollux, but of their sister Helen of Troy. Castor and Pollux later were among the Argonauts who sailed with Jason in search of the Golden Fleece, and due to their mutual devotion, Zeus placed them both in the heavens on their death, so that they could remain together forever.
Although many cultures saw Castor and Pollux as twins, early Christians sometimes called them David and Jonathan, whereas the Arabs knew them as peacocks. Perhaps the most unexpected connotation for the twins (along with the rest of Gemini) was as a “pile of bricks” as reported by Richard Hinckley Allen. Apparently the pile of bricks stood for the foundation of Rome, and in that context Castor and Pollux were associated with Romulus and Remus, the city’s legendary twin founders.
Castor’s position is RA: 07h 34m 36s, dec: +31° 53′ 19″
Bottom line: The star Castor, which appears as one of two bright stars in the constellation Gemini the Twins, is actually a six-star system.
Larry Sessions has written many favorite posts in EarthSky's Tonight area. He's a former planetarium director in Little Rock, Fort Worth and Denver and an adjunct faculty member at Metropolitan State University of Denver. He's a longtime member of NASA's Solar System Ambassadors program. His articles have appeared in numerous publications including Space.com, Sky & Telescope, Astronomy and Rolling Stone. His small book on world star lore, Constellations, was published by Running Press.