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Pollux (star)

Pollux is the brightest star in theconstellation ofGemini. It has theBayer designationβ Geminorum, which isLatinised toBeta Geminorum and abbreviatedBeta Gem orβ Gem. This is an orange-hued,evolvedred giant located at a distance of 34light-years, making it theclosest red giant (andgiant star) to the Sun. Since 1943, thespectrum of this star has served as one of the stable anchor points by which other stars are classified.[15] In 2006 anexoplanet (designatedPollux b or β Geminorum b, later named Thestias) was announced to be orbiting it.[11]

Pollux
Location of Pollux (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
ConstellationGemini
Pronunciation/ˈpɒləks/[1]
Right ascension07h 45m 18.94987s[2]
Declination+28° 01′ 34.3160″[2]
Apparent magnitude (V)1.14[3]
Characteristics
Evolutionary stageRed clump[4]
Spectral typeK0 III[5]
U−Bcolor index+0.86[3]
B−Vcolor index+1.00[3]
V−Rcolor index+0.75[3]
R−Icolor index+0.50[3]
Variable typeSuspected[6]
Astrometry
Radial velocity (Rv)+3.23[7] km/s
Proper motion (μ)RA: −626.55mas/yr[2]
Dec.: −45.80mas/yr[2]
Parallax (π)96.54±0.27 mas[2]
Distance33.78 ± 0.09 ly
(10.36 ± 0.03 pc)
Absolute magnitude (MV)+1.08±0.02[8]
Details
Mass1.91±0.09[9] M☉
Radius9.06±0.03[10] R☉
Luminosity32.7±1.6[10] L☉
Surface gravity (log g)2.685±0.09[11] cgs
Temperature4,586±57[10] K
Metallicity [Fe/H]–0.07 to +0.19[11] dex
Rotation660±15 d[12]
Rotational velocity (v sin i)2.8[13] km/s
Age1.19±0.3[9] (0.9 – 1.7)[4] Gyr
Other designations
β Geminorum,78 Geminorum,BD+28°1463,GJ 286,HD 62509,HIP 37826,HR 2990,SAO 79666,LFT 548,LHS 1945,LTT 12065[14]
Database references
SIMBADdata
Exoplanet Archivedata
ARICNSdata

Nomenclature

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

β Geminorum (Latinised toBeta Geminorum) is the star'sBayer designation.

The traditional namePollux refers to the twinsCastor and Pollux inGreek andRoman mythology.[16] In 2016, theInternational Astronomical Union organized aWorking 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 includedPollux for this star.[18]

 
The Sun viewed from Pollux (in red circle) in theconstellationSagittarius. 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 type A stars and dim red dwarfs, while Pollux is a single, cooler yellow-orangegiant. InPercy Shelley's 1818 poemHomer'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 theInternational Astronomical Union launchedNameExoWorlds, 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 bytheSkyNet ofAustralia; namelyLeda, Pollux's mother. At the request of the IAU, 'Thestias' (thepatronym of Leda, a daughter ofThestius) was substituted. This was because 'Leda' was already attributed toan asteroid and toone of Jupiter's satellites.[23][24]

In the catalogue of stars in theCalendarium of al Achsasi al Mouakket, this star was designatedMuekher al Dzira, which was translated intoLatin asPosterior Brachii, meaningthe end in the paw.[25]

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

Physical characteristics

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Size comparison of Pollux (left) and theSun (right)

At anapparent visual magnitude of 1.14,[28] Pollux is thebrightest star in its constellation, even brighter than its neighborCastor (α Geminorum). Pollux is 6.7 degrees north of theecliptic, presently too far north to beocculted by the Moon. The last lunar occultation visible from Earth was on 30 September 116 BCE from high southern latitudes.[29]

Parallax measurements by theHipparcos astrometry satellite[30][31] place Pollux at a distance of about 33.78light-years (10.36parsecs) from theSun.[2] This is close to the standard unit for determining a star'sabsolute magnitude (a star's apparent magnitude as viewed from 10 parsecs). Hence, Pollux's apparent and absolute magnitudes are quite close.[32]

The star is larger than the Sun, with about two[9] timesits mass and almost nine timesits radius.[11] Once anA-type main-sequence star similar toSirius,[33] Pollux has exhausted the hydrogen at its core andevolved into agiant star with astellar classification of K0 III.[5] Theeffective temperature of this star's outer envelope is about4,666 K,[11] which lies in the range that produces the characteristic orange hue ofK-type stars.[34] Pollux has aprojected rotational velocity of2.8 km·s−1.[13] The abundance of elements other than hydrogen and helium, what astronomers term the star'smetallicity, is uncertain, with estimates ranging from 85% to 155% of the Sun's abundance.[11][35]

An old estimate for Pollux's diameter obtained in 1925 byJohn Stanley Plaskett via interferometry was 13 million miles (20.9 million km, or 18.5 R☉), significantly larger than modern estimates.[36] A more recent measurement by theNavy Precision Optical Interferometer give a radius of 9.06 R☉.[10] Another estimate that uses Pollux'sspectral lines obtained 8.9 R☉.[37]

Evidence for a low level ofmagnetic activity came from the detection of weak X-ray emission using theROSAT 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 1gauss 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 anAp star with a much stronger magnetic field.[33] The star displays small amplituderadial velocity variations, but is notphotometricallyvariable.[38]

Planetary system

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Since 1993 scientists have suspected anexoplanet orbiting Pollux,[39] from measuredradial 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 timesthat of Jupiter. The planet is orbiting Pollux with a period of about 590 days.[11]

The existence of Pollux b has been disputed; the possibility that the observed radial velocity variations are caused bystellar magnetic activity cannot be ruled out.[12]

The Pollux planetary system[11]
Companion
(in order from star)		MassSemimajor axis
(AU)		Orbital period
(days)		EccentricityInclinationRadius
b (Thestias)(disputed[12])>2.30±0.45MJ1.64±0.27589.64±0.810.02±0.03——

References

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  1. ^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.
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External links

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Wikimedia Commons has media related toPollux (star).


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