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Dupree, Ronald Gilliland,
NASA and ESA
(Larger ultraviolet image -- more).
A highly evolved, orange-red giant star, Pollux
is still much smaller than the red supergiant
Betelgeuse, at left. (See a Digitized Sky Survey
image of Pollux from the Nearby Stars Database.)
Pollux, or Beta Geminorum, is located about 33.7 light-years from Sol. Its original Greek name was Polydeuces, the immortal twin. Although Johannes Bayer (1572-1625) gave the first-rank Greek letter designation of Alpha to Castor around 1600, Pollux is actually the brightest star (7:45:19.0+28:1:34.3, ICRS 2000.0) of Constellation Gemini, the Twins. Hence, it has been suggested that one of these stars may have changed in luminosity during the past four centuries. As a highly evolved and relatively cool orange-red giant, single star, Pollux is not much like its "twin" star Castor, which is actually composed of three sets of binary stars (as many as four bluish-white, main sequence stars with two fainter companions). In any case, Pollux is the 17th brightest star in Earth's night sky and one of the stars of the Winter Circle. According to Robert Burnham, Jr. (1931-93), none of the faint visual companions to Pollux listed in the Aitken's Double Star (ADS) Catalogue -- published as the "New General Catalogue of Double Stars within 120 degrees of the North Pole" in 1932 by Robert Grant Aitken (1864-1955) in cooperation with William J. Hussey (1862-1926) -- are gravitationally bound. (See Akira Fujii's color photo of Pollux.) On June 16, 2006, astronomers confirmed the presence of a Jupiter-class planet around Pollux, that was first detected in 1993 (Hatzes et al, 2006; and Hatzes and Cochran, 1993 -- more below). (See an animation of the planet's orbit around Pollux, with a table of basic orbital and physical characteristics.)
Pollux is a orange-red giant star of spectral and luminosity type K0 IIIb. It has about 1.7 +/- 0.4 times Sol's mass (Allende-Prieto and Lambert, 2000; and Drake and Smith, 1990), 8.8 +/- 0.1 times its diameter (Hatzes et al, 2006), and 32 times its visual luminosity. Since 1990, it has been measured to be around 85 to 155 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Hatzes et al, 2006; and Drake and Smith, 1990). According to the Yale Bright Star Catalogue, 1991 5th Revised Edition notes entry for HR 2990, there are "pronounced emissions" of magnesium-II.
Given that Pollux has a higher mass and is more evolved than Sol, the star is likely to be much younger than Sol's 4.6 billion years. Its relatively high mass estimates, however, suggest that it has not yet suffered substantial mass loss. Thus, Pollux may have evolved to the first ascent of the red-giant branch or to the post-helium-flash clump-giant stage (Drake and Smith, 1990). Pollux is a New Suspected Variable star designated as NSV 3712, and it may have as many as six optical, stellar companions that are not gravitationally bound to the star. Useful catalogue numbers and designations for the star include: Bet Gem, 78 Gem, HR 2990*, Gl 286, Hip 37826, HD 62509, BD+28 1463, SAO 79666, FK5 295, LHS 1945, LTT 12065, LFT 548, and ADS 6335 A.
As a star that has evolved out of the "main sequence," Pollux has fully shifted from the fusion of hydrogen to helium at its core to the fusion of helium to carbon and oxygen, with trace activity of other nuclear processes. This helium-burning, orange-red giant stage is relatively brief, lasting tens to hundreds of million years (e.g., lasting around 700 million years for a star of one Solar mass like the Sun).
Eventually, the star will lose much of its current mass, from an intensified stellar wind that eventually puffs out its outer gas envelopes of hydrogen and helium (and lesser amounts of higher elements such as carbon and oxygen) into interstellar space as a planetary nebula. The result will be a planet-sized, white dwarf core that gradually cools and fades in brightness from the shutdown of thermonuclear fusion. (Nearby white dwarfs include solitary Van Maanen's Star and the dim companions of Sirius, Procyon, and 40 (Omicron2) Eridani.)
On June 16, 2006, a team of astronomers (including Artie P. Hatzes, William D. Cochran, E. Endl, E.W. Guenther, S.H. Saar, G.A.H. Walker, S. Yang, M. Hartmann, M. Esposito, and D.B. Paulson) confirmed the presence of a Jupiter-class planet "b" around Pollux, that was first detected in 1993 (Hatzes et al, 2006; and Hatzes and Cochran, 1993). Assuming that Pollux has 1.7 Solar-masses, planet b has a minimum mass of 2.3 +/- 0.45 Jupiter-masses. It moves around Pollux at an average distance of 1.64 +/- 0.27 AUs in a nearly circular orbit (e= 0.02 +/- 0.03) that takes 1.6 years (589.64 +/- 0.81 days) to complete. The presence of the planet has been confirmed by continued monitoring (Quirrenbach et al, 2011). (See an animation of the planet's orbit around Pollux, with a table of basic orbital and physical characteristics.)
Currently, the orbit of an Earth-like planet (with liquid water) around Pollux may be centered around 5.7 AU -- just outside the orbital distance of Jupiter in the Solar System. Astronomers would find it very difficult to detect an Earth-type planet in the water zone of this star using present methods.
The following table includes all star systems known to be located within 10 light-years (ly), plus more bright stars within 10 to 20 ly, of Pollux.
|Star System||Spectra &|
|VV Lyncis 3?||M3.5 Ve |
|* plus bright stars *||. . .|
|55 (Rho1) Cancri AB||G8 V |
|Chi1 Orionis AB||G0 V |
|SV Leonis Minoris AB||G8 V |
|Castor 6||A1 V |
Up-to-date technical summaries on this star can be found at: Jean Schneiders's Extrasolar Planets Encyclopaedia; the Astronomiches Rechen-Institut at Heidelberg's ARICNS and the Nearby Stars Database. Additional information may be available at Roger Wilcox's Internet Stellar Database.
Constellation Gemini, the Twins, is the northernmost of the zodiacal constellations and among the brightest. In Greek mythology, Zeus (the chief of the gods) seduced Leda (the wife of the King of Sparta, Tyndareos) on her wedding night by changing himself into a swan. In time, Leda gave birth to the twin boys immortal Pollux (by Zeus) and mortal Castor (by Tyndareos), and to a girl named Helena who became Queen of Sparta and was abducted by Paris to Troy which led to the Trojan War. The twins, on the other hand, sailed with Jason in the quest for the Golden Fleece; during a storm, they helped save their ship ARGO from sinking, and so the constellation became much valued by sailors. For more information about the stars and objects in this constellation and an illustration, go to Christine Kronberg's Gemini. For an illustration, see David Haworth's Gemini.
For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.
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