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Template:PAGEBANNER:Pollux-Sun comparison.png

Castor[lower-alpha 1] and Pollux[lower-alpha 2] (or in Greek, Polydeuces[lower-alpha 3]) were twin brothers in Greek and Roman mythology, known together as the Dioscuri.[lower-alpha 4]

Their mother was Leda (who had a moon of Jupiter named after her). but they had different fathers; Castor was the mortal son of Tyndareus, the king of Sparta, while Pollux was the divine son of Zeus, who seduced Leda in the guise of a swan. Though accounts of their birth are varied, they are sometimes said to have been born from an egg, along with their twin sisters Helen of Troy and Clytemnestra.

Gemini constellation map.svg
The position of Pollux within the constellation of Gemini.
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Gemini
Right ascension 07h 45m 18.94987s[2]
Declination +28° 01′ 34.3160″[2]
Apparent magnitude (V) 1.14[3]
Evolutionary stage Giant star
Spectral type K0 III[4]
U−B color index +0.86[3]
B−V color index +1.00[3]
Variable type Suspected[5]
Radial velocity (Rv)+3.23[6] km/s
Proper motion (μ) RA: –626.55[2] mas/yr
Dec.: –45.80[2] mas/yr
Parallax (π)96.54 ± 0.27[2] mas
Distance33.78 ± 0.09 ly
(10.36 ± 0.03 pc)
Absolute magnitude (MV)+1.08±0.02[7]
Mass1.91±0.09[8] M
Radius8.8±0.1[9] R
Diameter8.8±0.1[9] D
Luminosity43[10] L
Surface gravity (log g)2.685±0.09[9] cgs
Temperature4666±95[9] K
Metallicity [Fe/H]–0.07 to +0.19[9] dex
Rotation558 days[11]
Rotational velocity (v sin i)2.8[12] km/s
Age724[13] Myr
Other designations
Beta Geminorum, 78 Geminorum, BD+28°1463, GCTP 1826.00, Gliese 286, HD 62509, HIP 37826, HR 2990, LFT 548, LHS 1945, LTT 12065, SAO 79666.[14]
Database references

Pollux, also designated Beta Geminorum (β Geminorum, abbreviated Beta Gem, β Gem), is an orange-hued evolved giant star approximately 34 light-years from the Sun in the northern constellation of Gemini. It is the brightest star in the constellation of Gemini and the closest giant star to the Sun.

Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified.[15] In 2006, an extrasolar planet (designated Pollux b or β Geminorum b, later named Thestias) was confirmed to be orbiting it.[9]


β Geminorum (Latinised to Beta Geminorum) is the star's Bayer designation.

Castor and Pollux are the two "heavenly twin" stars giving the constellation Gemini (Latin, 'the twins') its name. The stars, however, are quite different in detail. Castor is a complex sextuple system of hot, bluish-white A-type stars and dim red dwarfs, while Pollux is a single, cooler yellow-orange giant. In Percy Shelley's 1818 poem Homer's Hymn To Castor And Pollux, the star is referred to as "..mild Pollux, void of blame."[16]

Following its discovery the planet was designated Pollux b. In July 2014 the International Astronomical Union launched a process for giving proper names to certain exoplanets and their host stars.[17] The process involved public nomination and voting for the new names.[18] In December 2015, the IAU announced the winning name was Thestias for this planet.[19] The winning name was based on that originally submitted by theSkyNet of Australia; namely Leda, Pollux's mother. At the request of the IAU, 'Thestias' (the patronym of Leda, a daughter of Thestius) was substituted. This was because 'Leda' was already attributed to an asteroid and to one of Jupiter's satellites.[20][21]

In the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, this star was designated Muekher al Dzira, which was translated into Latin as Posterior Brachii, meaning the end in the paw.[22]

In Chinese, 北河 (Běi Hé), meaning North River, refers to an asterism consisting of Pollux, ρ Geminorum and Castor.[23] Consequently, Pollux itself is known as 北河三 (Běi Hé sān, English: the Third Star of North River.)[24]

Stellar characteristics[edit]

Parallax measurements made with the Hipparcos astrometry satellite[25][26] place Pollux at a distance of about 33.78 light-years (10.36 parsecs) from the Sun.[2]

The Sun viewed from the star Pollux (in red circle) in the constellation Sagittarius. Made with Celestia.

At an apparent visual magnitude of 1.14,[27] Pollux is the brightest star in the constellation, brighter even than its neighbor Castor (Alpha Geminorum). The star is larger than the Sun, with about two[8] times its mass and almost nine times its radius.[9] Once an A-type main sequence star,[28] Pollux has exhausted the hydrogen at its core and evolved into a giant star with a stellar classification of K0 III.[4] The effective temperature of this star's outer envelope is about 4666 K,[9] which lies in the range that produces the characteristic orange hue of K-type stars.[29] Pollux has a projected rotational velocity of 2.8 km·s−1.[12] The abundance of elements other than hydrogen and helium, what astronomers term the star's metallicity, is somewhat uncertain, with estimates ranging from 85% to 155% of the Sun's abundance.[9][30]

Evidence for a low level of magnetic activity came from the detection of weak X-ray emission using the ROSAT orbiting telescope. The X-ray emission from this star is about 1027 erg s−1, which is roughly the same as the X-ray emission from the Sun. A magnetic field with a strength below 1 Gauss has since been confirmed on the surface of Pollux; one of the weakest fields ever detected on a star. The presence of this field suggests that Pollux was once an Ap star with a much stronger magnetic field.[28] The star displays small amplitude radial velocity variations, but is not photometrically variable.[31]

Pollux is 6.69 degrees north of the ecliptic, so on rare occasions viewers in Earth's southern hemisphere see it occulted by the moon.

Planetary system[edit]

Since 1993, scientists have suspected an extrasolar planet orbiting Pollux,[32] from measured radial velocity oscillations. The existence of the planet, Pollux b, was confirmed and announced on June 16, 2006. Pollux b is calculated to have a mass at least 2.3 times that of Jupiter. The planet is orbiting Pollux with a period of about 590 days.[9]

The Pollux planetary system[9]
(in order from star)
Mass Semimajor axis
Orbital period
Eccentricity Inclination Radius
b (Thestias) > 2.30±0.45 MJ 1.64±0.27 589.64±0.81 0.02±0.03

See also[edit]


  1. Bloomsbury (1996), "Dioscuri", Dictionary of Myth, London: Bloomsbury Publishing
  2. 2.0 2.1 2.2 2.3 2.4 2.5 van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, arXiv:0708.1752, Bibcode 2007A&A...474..653V, doi:10.1051/0004-6361:20078357
  3. 3.0 3.1 3.2 Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues 2237: 0. Bibcode 2002yCat.2237....0D.
  4. 4.0 4.1 Morgan, W. W.; Keenan, P. C. (1973), "Spectral Classification", Annual Review of Astronomy and Astrophysics 11: 29–50, Bibcode 1973ARA&A..11...29M, doi:10.1146/annurev.aa.11.090173.000333
  5. Petit, M. (October 1990), "Catalogue des étoiles variables ou suspectes dans le voisinage du Soleil" (in French), Astronomy and Astrophysics Supplement 85 (2): 971, Bibcode 1990A&AS...85..971P.
  6. Famaey, B. et al. (January 2005), "Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters", Astronomy and Astrophysics 430 (1): 165–186, arXiv:astro-ph/0409579, Bibcode 2005A&A...430..165F, doi:10.1051/0004-6361:20041272
  7. Carney, Bruce W. et al. (March 2008), "Rotation and Macroturbulence in Metal-Poor Field Red Giant and Red Horizontal Branch Stars", The Astronomical Journal 135 (3): 892–906, arXiv:0711.4984, Bibcode 2008AJ....135..892C, doi:10.1088/0004-6256/135/3/892
  8. 8.0 8.1 Hatzes, A. P. et al. (July 2012), "The mass of the planet-hosting giant star β Geminorum determined from its p-mode oscillation spectrum", Astronomy & Astrophysics 543: 9, arXiv:1205.5889, Bibcode 2012A&A...543A..98H, doi:10.1051/0004-6361/201219332, A98.
  9. 9.0 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 Hatzes, A. P. et al. (2006), "Confirmation of the planet hypothesis for the long-period radial velocity variations of β Geminorum", Astronomy and Astrophysics 457: 335–341, arXiv:astro-ph/0606517, Bibcode 2006A&A...457..335H, doi:10.1051/0004-6361:20065445
  10. Mallik, Sushma V. (December 1999), "Lithium abundance and mass", Astronomy and Astrophysics 352: 495–507, Bibcode 1999A&A...352..495M
  11. Koncewicz, R.; Jordan, C. (January 2007), "OI line emission in cool stars: calculations using partial redistribution", Monthly Notices of the Royal Astronomical Society 374 (1): 220–231, Bibcode 2007MNRAS.374..220K, doi:10.1111/j.1365-2966.2006.11130.x
  12. 12.0 12.1 Massarotti, Alessandro et al. (January 2008), "Rotational and Radial Velocities for a Sample of 761 HIPPARCOS Giants and the Role of Binarity", The Astronomical Journal 135 (1): 209–231, Bibcode 2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209
  13. Takeda, Yoichi; Sato, Bun'ei; Murata, Daisuke (August 2008), "Stellar parameters and elemental abundances of late-G giants", Publications of the Astronomical Society of Japan 60 (4): 781–802, arXiv:0805.2434, Bibcode 2008PASJ...60..781T, doi:10.1093/pasj/60.4.781
  14. "POLLUX -- Variable Star", SIMBAD (Centre de Données astronomiques de Strasbourg), http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Beta+Geminorum, retrieved 2012-01-14
  15. Garrison, R. F. (December 1993), "Anchor Points for the MK System of Spectral Classification", Bulletin of the American Astronomical Society 25: 1319, Bibcode 1993AAS...183.1710G, http://www.astro.utoronto.ca/~garrison/mkstds.html, retrieved 2012-02-04
  16. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  17. NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars. IAU.org. 9 July 2014
  18. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  19. Final Results of NameExoWorlds Public Vote Released, International Astronomical Union, 15 December 2015.
  20. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  21. YOU helped name an exoplanet!, TheSkyNet, 2015-12-17
  22. Knobel, E. B. (June 1895). "Al Achsasi Al Mouakket, on a catalogue of stars in the Calendarium of Mohammad Al Achsasi Al Mouakket". Monthly Notices of the Royal Astronomical Society 55: 429–438. Bibcode 1895MNRAS..55..429K. doi:10.1093/mnras/55.8.429.
  23. (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
  24. (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived 2011-01-30 at the Wayback Machine., Hong Kong Space Museum. Accessed on line November 23, 2010.
  25. Perryman, M. A. C.Expression error: Unrecognized word "etal". (July 1997), "The Hipparcos Catalogue", Astronomy and Astrophysics 323: L49–L52, Bibcode 1997A&A...323L..49P
  26. Perryman, Michael (2010), "The Making of History's Greatest Star Map", Astronomers’ Universe (Heidelberg: Springer-Verlag), Bibcode 2010mhgs.book.....P, doi:10.1007/978-3-642-11602-5
  27. Lee, T. A. (October 1970), "Photometry of high-luminosity M-type stars", Astrophysical Journal 162: 217, Bibcode 1970ApJ...162..217L, doi:10.1086/150648
  28. 28.0 28.1 Aurière, M. et al. (September 2009), "Discovery of a weak magnetic field in the photosphere of the single giant Pollux", Astronomy and Astrophysics 504 (1): 231–237, arXiv:0907.1423, Bibcode 2009A&A...504..231A, doi:10.1051/0004-6361/200912050
  29. "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html, retrieved 2012-01-16
  30. The abundance is determined by taking the value of [Fe/H] in the table to the power of 10. Hence, 10−0.07 = 0.85 while 10+0.19 = 1.55.
  31. Henry, Gregory W. et al. (September 2000), "Photometric Variability in a Sample of 187 G and K Giants", The Astrophysical Journal Supplement Series 130 (1): 201–225, Bibcode 2000ApJS..130..201H, doi:10.1086/317346.
  32. A. P. Hatzes (1993). "Long-period radial velocity variations in three K giants". The Astrophysical Journal 413: 339–348. Bibcode 1993ApJ...413..339H. doi:10.1086/173002.

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