Science

The constellation Gemini the Twins has two stars that are noticeable for being bright and close together. The brighter and more golden star is Pollux. The fainter, whiter star is Castor.

Pollux on left. Castor on right. See the contrast in their colors? Pollux is golden, and Castor is white. (Warren Wilson College)

These two stars of nearly equal brightness are not really related in space. They are different kinds of stars and not gravitationally bound. Unlike Pollux, Castor is a multiple star. It contains three pairs of binary stars that all revolve around a common center of mass. In other words, the single bright light we see as Castor is really six stars in one.

Castor is a hot “main sequence” star, which designates it as a star in the mature stage of life. It outshines our sun in total energy output (not just visual) by at least 30 times. All the bright stars – and indeed most stars visible to the unaided eye – are brighter than our sun. Placed where Castor is, some 49 light-years away, our star would barely be visible under the best of conditions.

Castor has a surface temperature of about 9000 K (nearly 16,000 degrees F), compared to the sun’s roughly 6,000 K (11,000 degrees F). Hotter stars burn their available energy supplies much faster than cooler stars, so Castor’s expected lifetime is much shorter than the sun’s, perhaps only a few hundred million years compared to 10 billion years for the sun. Although we cannot say with precision exactly how old Castor is, we can say that it is a youngster compared to the sun, which has already lived 5 to 10 times as long as Castor’s entire life expectancy.

But here we speak of the characteristics of just one star, which in fact is just the brightest of a multiple star system. As we said, Castor is really six stars in one. Even a fairly small telescope shows that Castor appears double, and in fact an even fainter star nearby also is part of the same system. Each of these three stars (Castor A, B and C) is also double. This is not directly visible in telescopes but obvious through the use of a spectroscope. The two larger and visible components are hot A-type stars, whereas the smaller ones are cool, M-type red dwarf stars. The mass of all six stars together is, very roughly, about six times that of the sun.

How to see it

Castor is one of two bright stars in the constellation Gemini the Twins. It’s a prominent white star, noticeable for its nearness to its brother star, Pollux in Gemini. No other two such bright stars appear so close together on our sky’s dome.

At magnitude 1.58, Castor is technically a second-magnitude star and the brightest in that category. Its constellation, Gemini the Twins, consists of two lines of stars. Although designated Alpha Geminorum, Castor is slightly fainter than Pollux.

Castor is well placed in the evening sky from late November through May. It is opposite the sun in middle January. In February, Castor is particularly well placed for evening viewing. It is well up in the east an hour after sunset in late February – high in the sky throughout the evening hours.

Castor and Pollux are close to the moon’s path across our sky. The moon passes no more than 15 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 Myth

The reason for the name Castor is unclear, although there appears no specific connection with the beaver, which is what the word means in Latin. It also is not clear why this star holds the appellation of “Alpha,” because it is clearly secondary in brightness to Pollux, which is called “Beta Geminorum.” In any event there is much mythology associated with the two stars, typically only in conjunction with each other, and 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″