Star Facts: Pollux

Pollux
Image Credit: scienceblogs.com

Pollux is the brightest star in the constellation Gemini, and the 17th brightest in the entire night sky. However, contrary to convention, Pollux bears the designation Beta Geminorum, which by rights should apply to Castor (Alpha Geminorum), which is the second brightest star in Gemini, and only the 23rd brightest star in the night sky. At a distance of just 33.78 light years away, Pollux also has the distinction of being the closest giant star to the Sun.

Since 1943, this orange giant has been one of the stable standards by which other stars spectra are classified, and along with Castor, Pollux features prominently in literature throughout history, including in a romance-era poem by Shelley called ‘Homer’s Hymn to Castor and Pollux’ in which Pollux is referred to thus:

“Ye wild-eyed Muses, sing the Twins of Jove,
Whom the fair-ankled Leda, mixed in love
With mighty Saturn’s Heaven-obscuring Child,
On Taygetus, that lofty mountain wild,
Brought forth in joy: mild Pollux, void of blame,
And steed-subduing Castor, heirs of fame.”

Quick Facts

• Constellation: Gemini
• Coordinates: RA 7h 45m 19s | Dec +28° 1′ 35?
• Distance: 33.78 light years
• Star Type: Orange Giant (K0 III)
• Mass: 2.04 sol
• Radius: 8.8 sol
• Apparent Magnitude: +1.14
• Luminosity: 43 sol
• Surface Temperature: 4,666
• Rotational Velocity: 2.8 km/s (One rotation = 558 days)
• Age: 724 million years
• Other Designations: Beta Geminorum, 78 Geminorum, BD+28°1463, GCTP 1826.00, Gliese 286, HD 62509, HIP 37826,

Location

Gemini ConstellationPollux is the brightest star in Gemini, a northern constellation that can be seen from latitudes of between +90° and -60°. It is a first magnitude star (+1.14), which together with the star Castor (+1.58), mark the heads of the legendary twins from Greek mythology. The rest of the stars in the constellation represents their bodies, with Gemini easily located northeast of Orion, with other nearby constellations including Leo to its east, Taurus to its west, and Canis Minor to its south.

Visibility

Since there are no other bright stars immediately around either Pollux or Castor when observing from low to mid-northern latitudes, the two stars are conspicuous and therefore easily spotted. However, since the two stars are relatively close to each other, it is easy to confuse one with the other, so to make sure the right star is observed bear in mind that Pollux appears as pale yellow, whereas Castor is white, with a pale blue tinge at times. Pollux is also the brighter star of the pair, which makes its identification easier.

Best Seen

Pollux becomes visible for most northern observers from about mid-October, when it rises above the north-eastern horizon before midnight, with the actual time depending on the observer’s location. It can then be seen throughout winter and spring in the northern hemisphere, with its best time for viewing during its midnight culmination around the 15th of January each year when the star is visible right through the night. On that day, Pollux crosses the meridian at around local midnight, which means the star crosses an imaginary line drawn from due north through the zenith, to a position due south, during which it can be seen high in the sky from much of the planet’s surface. Bear in mind also that Pollux is a circumpolar star from central Alaska, the northern reaches of Canada, and northward of central Scandinavia.

Physical Properties

Size, Mass, Luminosity

Apart from being more than forty times as bright as the Sun, Pollux has just more than twice the Sun’s mass, and almost nine times its size. Having consumed all, or most of its hydrogen fuel, Pollux, has now evolved off of the main sequence into a giant star with an effective surface temperature of 4,666K, which is typical of K-type, orange giant stars.

Metallicity and Magnetic Field

Pollux’s metalicity (which refers to elements in a star other than hydrogen and helium), is still somewhat uncertain, with some investigators estimating the total metal abundance to be no more than about 85% of that of the Sun, while others have shown the total metal abundance to be as high as 155% of that of the Sun. Investigations are continuing.

Recent studies have revealed the presence of weak magnetic activity on Pollux, based on measurements of X-ray emissions from the star that were made with the ROSAT orbiting telescope. Interestingly, the detected 1027 erg s-1 X-ray emission from Pollux is about the same as that produced by the Sun. Nonetheless, the magnetic field of Pollux turns out to be well below 1 Gauss in strength, which is the weakest magnetic field ever to have been discovered on any star. By comparison, the magnetic field of Jupiter is on average about 10 Gauss, which is roughly 20,000 times stronger than Earth’s 0.5 Gauss-strength magnetic field.

The presence (and strength) of Pollux’s magnetic field suggests that Pollux was once an Ap-type main sequence star, which is a chemically-peculiar class of main sequence stars (hence the “p” in the classification) that display large overabundance of metals such as strontium, chromium and europium in their spectra. Together with Bp-class stars, Ap-type stars typically have very low rotational velocities, which in the case of Pollux rotates at 2.8 km/sec, seeming to confirm Pollux’s past life as a chemically peculiar Ap-class star.

Exoplanet

In June of 2006, the presence of a planet with an orbital period of 589.64 ± 0.81 days was confirmed. Named Thestias, the planet has just more than twice the mass of Jupiter, and orbits Pollux in a nearly circular orbit at a distance of 1.64 astronomical units. Interestingly, Thestias was the grandfather of Pollux in Greek mythology.

History

Pollux and Castor
Image Credit: Greg Parker

According to Greek Mythology, Polydeukes and Kastor, whose Latin names are Pollux and Castor, were the identical twin son’s of Queen Leda of Sparta, whose other children included Helen of Troy, and Clytemnestra, who later became the wife of King Agamemnon of Mycenae. While Castor was the son of Leda’s husband King Tyndareus, Pollux was the offspring of the adulterous Zeus, and was therefore immortal.

Castor was a skilled warrior horseman, while Pollux was a formidable boxer, and throughout their lives, Pollux and Castor were inseparable, and enjoyed many adventures together, including helping Jason and the Argonauts steal the Golden Fleece. In terms of etymology, Polydeukes means “very sweet” or even “charming” (according to some sources) in Greek, which is difficult to reconcile with the brutal and bloody sport that boxing was in ancient Greece.

On account of their striking appearance together, the stars Pollux and Castor have symbolized twins in many cultures throughout history. In ancient India, for instance, they represented the Ashwins, or twin horsemen of the dawn; in Persia they were Du Paikar, the two figures; in Phoenicia they depicted two gazelles or kid-goats; while in China they symbolized Yin and Yang, the opposite but interconnected forces of nature.

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