The Less Acclaimed of the Twins

Who are the most famous of all twins? Remarkably, they are not historical people. They are characters in Greek mythology. And their names have been given to the most famous of all pairings of bright stars in our sky: Pollux and Castor.

But if you are a reader of mythology, you might be immediately jolted by something I just did: I gave the name of Pollux first. When discussing the mythological figures it is customary to say "Castor and Pollux." A person would be no more likely to say "Clark and Lewis" or "Costello and Abbott" than "Pollux and Castor."

Yet in astronomy it is possible to speak for quite a while about one of these stars without mentioning the other and, when the two are mentioned together, wouldn't it make sense to say first the name of the star that appears brighter? That star is Pollux. The stars Pollux and Castor are not really twins

Gemini, featuring Pollux and Castor.

in brightness, for Pollux is noticeably brighter even to a casual view. More notably, the dividing line between first magnitude and second magnitude comes at 1.50 and this falls between the brightnesses of Pollux and Castor— so Pollux gets its own chapter in this book, while Castor doesn't.

Pollux's magnitude of 1.16 edges out Fomalhaut by only 0.01 magnitude for the title of seventeenth-brightest star in the heavens. But Pollux glows 0.42 magnitude brighter than the combined radiance of the Castor system.

Castor is not even quite the brightest of the 2nd-magnitude stars, by the way. Its 1.58-magnitude light is outshined by 1.50-magnitude Adhara (Epsilon Canis Majoris)—though the latter is overlooked in favor of Sirius in its constellation and is far enough south to be always a little dimmed by atmospheric extinction for observers at midnorthern (and of course higher northern) latitudes.

But guess what? Four hundred years ago, when the Bayer letters were being assigned, Castor got designated Alpha Geminorum. Pollux had to settle for being Beta Geminorum. There are many departures from the letter of the law (letters in order of brightness) in the Greek lettering of the stars that was done by Johannes Bayer and others. But in this particular case, the reason for Castor getting precedence in the lettering was probably the tradition of calling the two mythological brothers "Castor and Pollux"—Castor first, Pollux second.

Now you would think as astronomy developed as a science, learning more about the stars, and especially as amateur observational astronomy became established as a discrete hobby or pastime, the greater brightness of Pollux would overcome the old order-of-naming bias and make Pollux at least as popular an object of talk and attention as Castor.

But that's not so. The reason is that Pollux seems to be a single star but telescopic observation shows Castor to be a spectacular double star—a magnitude 1.93 and 2.97 pair now 41z2" apart (and slowly widening). In fact, if you know where to look, there is a third, distant visual companion in the Castor system. And spectroscopic studies have revealed that all three of these stars of Castor are themselves close doubles—a six-star stellar system!

No wonder then that Castor has attracted more interest among amateur astronomers than Pollux.

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