Earthsky Tonight
Castor: The fainter Twin star
June 7, 2009 - Brightest Stars
Castor, the “twin” of Pollux in the constellation Gemini the Twins, is a prominent white star roughly halfway between Capella in Auriga and Regulus in the constellation Leo. It is the 23rd or 24th brightest star in all the heavens, depending on whether or not you count Alpha Centauri B (see Regulus). In any event, at magnitude 1.58, it is technically a second magnitude star, and the brightest in that category. Gemini consists of two lines of stars to the northeast of Orion, with Pollux and Castor farthest away from Orion. Although designated Alpha Geminorum, Castor is slightly fainter than Pollux, and slightly farther to the north. The two make a distinct and easily identified pair.
Castor is well placed in the evening sky from late November through May, and is opposite the Sun in middle January. Around this time, the star rises at roughly sunset and stays up all night, reaching its highest point (culmination) to the south at midnight.
Castor rises about an hour after sunset in late December; is well up in the east an hour after sunset in late February; is high in the west an hour after sunset in late April; and is low to the west-northwest an hour after sunset in late May. Although not as close to the ecliptic as Regulus, Castor and Pollux are close enough to the moon’s path that it passes no more than 15 and sometimes fewer than 5 degrees south of Castor on some day every month. This fact could aid in identification..
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 afterwards. 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.
Science
Castor is a hot “main sequence” star, which designates it as a star in the mature stage of life. As an A1V star, it outshines our sun in total energy output (not just visual) by at least 30 times. What many people don’t realize is that 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 just 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. Allthough 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. 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. To make it even more interesting, each of these three stars (Castor A, B and C) is 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.
Castor’s position is RA: 07h 34m 36s, dec: +31° 53′ 19″