A yellowish star shining at magnitude 1.17, Pollux the brightest star in Gemini, and the 17th brightest star in the sky. Pollux is in fact brighter than Castor, the α star of the constellation, even though it carries the Bayer designation β Gem. It has been suggested that one of the these stars has changed in brightness since Bayer assigned their designations early in the 17th century; but this is astrophysically unlikely.
As of 2006, Pollux was confirmed to have an extrasolar planet orbiting it.
Name and Mythology
Castor and Pollux are the two "heavenly twin" stars which give the constellation Gemini (which means "twins" in Latin) its name. Pollux's original Greek name was Polydeuces, the immortal twin sons of Zeus and Leda. The star also bears Arabic name Al-Ras al-Tau'am al-Mu'akhar, literally meaning, "The Head of the Second Twin".
Historically, the Chinese recognized Pollux as Yang, which in their ancient philosophy was one of the two fundamental principles upon which all things depend. Castor and Pollux together correspond to the Nakshatra Punarvasu in Hindu astronomy. Since astrologers associate Pollux with prosperity and celebration with wine, the name Pollux is mistakenly thought by some people to carry the meaning "much wine".
Properties and Evolution
Physically, Castor and Pollux are nothing alike. Pollux a single, cool, yellow-orange giant; Castor is a complex sextuple system comprised of hot, bluish-white A-type stars and dim red dwarfs. Pollux is also closer, approximately 34 light-years away, as opposed to 50 for Castor.
Pollux is spectral class K0 IIIb, with a rather cool surface temperature of 4,500 K, giving the star its fine orange-yellow color. It has a total luminosity (including infrared radiation) of about 46 suns. The star's diameter is 8.8 times the Sun's, and it contains 1.7 solar masses. Like many other stars with planets, Pollux is metal-rich, with an iron content relative to hydrogen that is 155% solar; its spectrum also shows "pronounced emissions" of magnesium-II. Evidence has been found for a a hot, outer, magnetically supported corona around Pollux, and the star is known to be an X-ray emitter.
Because Pollux is more massive than the Sun, it is more evolved, even though much younger than the Sun. Pollux's mass suggests that it has not yet suffered substantial mass loss, and is only starting its first ascent to the red giant branch of the H-R diagram. As a star that has evolved out of the main sequence, Pollux has fully shifted from fusing hydrogen into helium at its core, to fusing helium into carbon and oxygen. This helium-burning stage is relatively brief, lasting a few hundred million years. Eventually, the star will lose much of its current mass, from an stellar wind that puffs its outer gas envelopes into interstellar space, forming a planetary nebula. The remaining core will be a planet-sized white dwarf, that gradually cools and fades from the shutdown of thermonuclear fusion.
Pollux has as many as six optical, line-of-sight companions that are not gravitationally related.
Pollux b
In 2006, astronomers confirmed the presence of a Jupiter-class planet, Pollux b, that was first detected in 1993. Assuming that Pollux contains 1.7 solar masses, Pollux b has a minimum mass 2.3 times Jupiter's. It moves around Pollux at an average distance of 1.64 AU, in a nearly circular orbit (e=0.02) that takes 589.64 days (1.6 years) to complete.
Pollux is of the very few giant stars with planetary companions. From the planet, which is bathed with 16 times the amount of radiation intensity we get from the Sun, Pollux would glower in the sky with an angular diameter of nearly 3 degrees - 5.7 times bigger than we see the Sun.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]