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.

Pollux

Position of Pollux, in Gemini
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Gemini
Right ascension	07^h 45^m 18.94987^s[1]
Declination	+28° 01′ 34.3160″[1]
Apparent magnitude (V)	1.14[2]
Characteristics
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]
Astrometry
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
Distance	33.78 ± 0.09 ly (10.36 ± 0.03 pc)
Absolute magnitude (MV)	+1.08±0.02[6]
Details
Mass	1.91±0.09[7] M☉
Radius	9.06±0.03[8] R☉
Luminosity	32.7±1.6[8] L☉
Surface gravity (log g)	2.685±0.09[9] cgs
Temperature	4586±57[8] K
Metallicity [Fe/H]	–0.07 to +0.19[9] dex
Rotation	558 days[10]
Rotational velocity (v sin i)	2.8[11] km/s
Age	724[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
SIMBAD	data
ARICNS	data

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]

Nomenclature

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 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 characteristics

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, too far north to be occulted by the moon and planets, but in the distant future it will be close enough.

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 4666 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 system

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]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b (Thestias)	> 2.30±0.45 MJ	1.64±0.27	589.64±0.81	0.02±0.03	—	—

See also

• Pollux in fiction

References

1. 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
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.
3. 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
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
7. 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.
8. 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.
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 (1): 335–341, arXiv:astro-ph/0606517, Bibcode:2006A&A...457..335H, doi:10.1051/0004-6361:20065445
10. 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
11. 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
13. "POLLUX -- Variable Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2012-01-14
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
16. "Pollux", STARS, University of Illinois, Urbana–Champaign Campus, 2008, retrieved 2009-05-29.
17. IAU Working Group on Star Names (WGSN), retrieved 22 May 2016.
18. Bulletin of the IAU Working Group on Star Names, No. 1 (PDF), retrieved 28 July 2016.
19. "Homer's Hymn To Castor And Pollux by Percy Bysshe Shelley", allpoetry.com, retrieved 13 April 2018.
20. "An IAU Worldwide Contest to Name Exoplanets and their Host Stars", NameExoWorlds, IAU, 9 July 2014, retrieved 2020-01-14.
21. "NameExoWorlds", nameexoworlds.iau.org, retrieved 13 April 2018.
22. "Final Results of NameExoWorlds Public Vote Released", NameExoworlds, International Astronomical Union, 15 December 2015, retrieved 2020-01-14.
23. "NameExoWorlds", nameexoworlds.iau.org, retrieved 13 April 2018.
24. "YOU helped name an exoplanet!", TheSkyNet, 2015-12-17, retrieved 2020-01-14.
25. 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 (8): 429–438, Bibcode:1895MNRAS..55..429K, doi:10.1093/mnras/55.8.429.
26. (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.
27. (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.
28. Lee, T. A. (October 1970), "Photometry of high-luminosity M-type stars", Astrophysical Journal, 162: 217, Bibcode:1970ApJ...162..217L, doi:10.1086/150648
29. Perryman, M. A. C.; Lindegren, L.; Kovalevsky, J.; et al. (July 1997), "The Hipparcos Catalogue", Astronomy and Astrophysics, 323: L49–L52, Bibcode:1997A&A...323L..49P
30. 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, ISBN 978-3-642-11601-8
31. 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
32. "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on March 10, 2012, retrieved 2012-01-16
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 10.1.1.40.8526, 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 links

• "Notes for star HD 62509". The Extrasolar Planets Encyclopaedia. Retrieved 2008-06-24.
• "Pollux". SolStation. Retrieved 2005-11-21.
• Sabine Reffert; et al. (2006-07-07). "Precise Radial Velocities of Giant Stars II. Pollux and its Planetary Companion". Astrophys. J. 652 (1): 661–665. arXiv:astro-ph/0607136. Bibcode:2006ApJ...652..661R. doi:10.1086/507516.