Pollux (star)

Pollux /ˈpɒləks/,[14] designated β Geminorum (Latinised to Beta Geminorum, abbreviated Beta Gem, β Gem), is an orange-hued evolved giant star about 34 light-years from the Sun in the constellation of Gemini. It is the brightest star in Gemini and the closest giant star to the Sun.

Gemini constellation map.svg
Position of Pollux, in Gemini
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Gemini
Right ascension 07h 45m 18.94987s[1]
Declination +28° 01′ 34.3160″[1]
Apparent magnitude (V) 1.14[2]
Evolutionary stage Giant star
Spectral type K0 III[3]
U−B color index +0.86[2]
B−V color index +1.00[2]
Variable type Suspected[4]
Radial velocity (Rv)+3.23[5] km/s
Proper motion (μ) RA: −626.55[1] mas/yr
Dec.: −45.80[1] mas/yr
Parallax (π)96.54 ± 0.27[1] mas
Distance33.78 ± 0.09 ly
(10.36 ± 0.03 pc)
Absolute magnitude (MV)+1.08±0.02[6]
Mass1.91±0.09[7] M
Radius9.06±0.03[8] R
Luminosity32.7±1.6[8] L
Surface gravity (log g)2.685±0.09[9] cgs
Temperature4586±57[8] K
Metallicity [Fe/H]–0.07 to +0.19[9] dex
Rotation558 days[10]
Rotational velocity (v sin i)2.8[11] km/s
Age724[12] Myr
Other designations
β 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[13]
Database references

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]


Pollux is one of the two brightest stars in the constellation of Gemini (lower left).

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

The traditional name Pollux refers to the twins Castor and Pollux in Greek and Roman mythology.[16] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[17] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN, which included Pollux for this star.[18]

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

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."[19]

Originally the planet was designated Pollux b. In July 2014 the International Astronomical Union launched NameExoWorlds, a process for giving proper names to certain exoplanets and their host stars.[20] The process involved public nomination and voting for the new names.[21] In December 2015, the IAU announced the winning name was Thestias for this planet.[22] 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.[23][24]

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.[25]

In Chinese, 北河 (Běi Hé), meaning North River, refers to an asterism consisting of Pollux, ρ Geminorum, and Castor.[26] Consequently, Pollux itself is known as 北河三 (Běi Hé sān, English: the Third Star of North River.)[27]

Physical characteristicsEdit

Size comparison of Pollux (left) and the Sun (right)

At an apparent visual magnitude of 1.14,[28] Pollux is the brightest star in its constellation, even brighter than its neighbor Castor (α Geminorum). Pollux is 6.7 degrees north of the ecliptic, presently too far north to be occulted by the moon and planets. Lunar occultations were possible as recently as about 2,000 years ago.[definition needed][clarification needed][citation needed]

Parallax measurements by the Hipparcos astrometry satellite[29][30] place Pollux at a distance of about 33.78 light-years (10.36 parsecs) from the Sun.[1]

The star is larger than the Sun, with about two[7] times its mass and almost nine times its radius.[9] Once an A-type main-sequence star,[31] Pollux has exhausted the hydrogen at its core and evolved into a giant star with a stellar classification of K0 III.[3] The effective temperature of this star's outer envelope is about 4,666 K,[9] which lies in the range that produces the characteristic orange hue of K-type stars.[32] Pollux has a projected rotational velocity of 2.8 km·s−1.[11] The abundance of elements other than hydrogen and helium, what astronomers term the star's metallicity, is uncertain, with estimates ranging from 85% to 155% of the Sun's abundance.[9][33]

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.[31] The star displays small amplitude radial velocity variations, but is not photometrically variable.[34]

Planetary systemEdit

Since 1993 scientists have suspected an extrasolar planet orbiting Pollux,[35] 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 alsoEdit


  1. ^ a b c d e f 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, S2CID 18759600
  2. ^ a b c 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.
  3. ^ a b 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
  4. ^ Petit, M. (October 1990), "Catalogue des étoiles variables ou suspectes dans le voisinage du Soleil", Astronomy and Astrophysics Supplement (in French), 85 (2): 971, Bibcode:1990A&AS...85..971P.
  5. ^ 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, S2CID 17804304
  6. ^ 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, S2CID 2756572
  7. ^ a b 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, S2CID 53685387, A98.
  8. ^ a b c Baines, Ellyn K.; Armstrong, J. Thomas; Schmitt, Henrique R.; Zavala, R. T.; Benson, James A.; Hutter, Donald J.; Tycner, Christopher; van Belle, Gerard T. (2017). "Fundamental parameters of 87 stars from the Navy Precision Optical Interferometer". The Astronomical Journal. 155 (1): 16. arXiv:1712.08109. Bibcode:2018AJ....155...30B. doi:10.3847/1538-3881/aa9d8b. S2CID 119427037.
  9. ^ a b c d e f g h 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 (1): 335–341, arXiv:astro-ph/0606517, Bibcode:2006A&A...457..335H, doi:10.1051/0004-6361:20065445, S2CID 14319327
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  11. ^ a b 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
  12. ^ 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, S2CID 16258166
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  14. ^ Kunitzsch, Paul; Smart, Tim (2006), A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.), Cambridge, Massachusetts: Sky Pub, ISBN 978-1-931559-44-7.
  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, retrieved 2012-02-04
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  18. ^ Bulletin of the IAU Working Group on Star Names, No. 1 (PDF), retrieved 28 July 2016.
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  31. ^ a b 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, S2CID 14295272
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  33. ^ 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.
  34. ^ 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, CiteSeerX, doi:10.1086/317346.
  35. ^ A. P. Hatzes; et al. (1993), "Long-period radial velocity variations in three K giants", The Astrophysical Journal, 413: 339–348, Bibcode:1993ApJ...413..339H, doi:10.1086/173002.

External linksEdit

Coordinates:   07h 45m 19.4s, 28° 01′ 35″