How to see it
Pollux, otherwise known as Beta Geminorum, is the 17th brightest star in the sky, prominent in evening skies from late fall through spring each year. Pollux and its nearby companion, Castor, pass high overhead from mid-north latitudes (e.g. the central US), and there are no bright stars immediately around them. This makes them stand out and easy to identify.
However, there are plenty of bright stars in this part of the sky. A line drawn from Regulus in Leo to Capella in Auriga passes near Pollux and Castor. Similarly, a line drawn from Rigel through Betelgeuse in Orion, and extending perhaps 3 times the distance between them also passes near Gemini’s “twin” stars. Pollux is a yellowish color, whereas Castor is white with perhaps a tinge of pale blue. One other way to distinguish which is which is to notice that Pollux is slightly brighter than Castor.
Pollux is opposite the sun (opposition) on about January 15. This means that it is rising as the sun sets, and reaches its highest point at about local midnight. This situation is called a “midnight culmination,” and marks the time when the star crosses the meridian, an imaginary line drawn from due north, through the zenith overhead, down to the horizon due south, at midnight. Traditionally the night of midnight culmination is considered the best time for observation because it is the time when the star is in the sky all night long. However, as early as mid-October, Pollux can be seen rising in the northeastern sky before midnight (daylight savings time). By mid-May, it is low in the northwestern sky at the same time (midnight Daylight time).
From central Alaska, northern Canada and parts of Scandinavia northward, Pollux is circumpolar.
History and Myth
Of the 20 brightest stars, four are “Beta” stars, a term normally reserved for the second brightest star in a given constellation. But as with Rigel in Orion, this is not the case for Pollux, which noticeably outshines the Alpha star, Castor. Being so close together in the sky, it is easy to compare the two and to most eyes, Pollux is quite obviously brighter. It is possible that one or both stars have altered in brightness since German astronomer Johann Bayer assigned the designation about 300 years ago. But it also has been suggested that Bayer sometimes labeled the stars in their order of rising. Here Castor rises slightly before Pollux, and hence Castor, the dimmer star, received the Alpha. This explanation clearly fits for Betelgeuse and Rigel in Orion, as viewed from the latitude of Germany, because the Alpha star, Betelgeuse, rises slightly before the truly brighter star, Rigel. However, there is a geographical dependency here. From some locations south of the Equator, both Rigel and Pollux rise first
Pollux is of Greek origin and apparently refers to a boxer. The original Greek word seems at odds with this idea, however, as it apparently means “very sweet,” which may allude to the warm fraternal relationship between the two brothers. In any event, it is simply the proper name of one of the two brothers that comprise the constellation Gemini, the Twins. Both twins figure prominently as argonauts who sailed with Jason. By most accounts, they were sons of Leda, Queen of Sparta, but Castor had a mortal father and hence was mortal himself. Pollux was the son of Zeus and immortal. Pollux also had a famous sister, Helen of Troy. Castor’s name, by the way, is the Latin spelling for the Greek word meaning “beaver”!
There are many variants in the story, but by one account, Castor was killed in battle and Pollux could not bear to live without him and begged Zeus to let him die, too. Zeus could not grant the gift quite as asked, but decreed that Pollux would spend every other day in Olympus with the gods, and the rest of the time in the underworld with his brother. To honor this devotion, Zeus placed their constellation in the sky as a remembrance.
Pollux and Castor are sometimes identified with Apollo and Hercules, or the founders of Rome, the brothers Romulus and Remus. While in many cultures they were the twins, India they were the Horsemen, and in Phoenicia they were the two gazelles or two kid-goats. It is said that in China they were associated with Yin and Yang, the contrasts and complements of life.
Science
Pollux is classified as a “K0IIIb” star. The K0 means that it is somewhat cooler than then sun, with a surface color that is a light yellowish orange. (Keep in mind that the color a star appears depends significantly on the sensitivity of the observer’s eyes, and that color is difficult to discern with most point sources.) The “III” is a “luminosity” class designator, indicating basically how much energy it is putting out, which is largely dependent on size. A type-III star is considered a “normal” giant or just a giant. Finally, the “b” indicates that Pollux is slightly below the average luminosity for this class.
A relatively close 34 light-years away, Pollux is about 31 times as bright as the sun in visible light, but Pollux also pumps out a good bit of energy in non-visible infrared radiation. With all forms of radiation counted, Pollux is about 46 times more energetic than Sol. According to Dr. James Kaler, Pollux is just under 10 times the diameter of the sun, making it a little less than 8 million miles across, and not quite twice the solar mass.
Although Castor is only a couple dozen light years from it, Pollux has no companion star. However, a large planet, several times larger than Jupiter, has been confirmed. This planet, sometimes just called “Pollux b”, is not likely to harbor intelligent life, but at 34 light years distance, it is one of the nearest of the several hundred extrasolar planets discovered so far.
Pollux’s position is RA: 7h 45m 20s, dec: +28° 01′ 35″.
