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Pollux (star)

Coordinates:Sky map07h45m19.4s,28° 01′ 35″
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Pollux
Location of Pollux (circled)
Observation data
EpochJ2000.0EquinoxJ2000.0
Constellation Gemini
Pronunciation /ˈpɒləks/[1]
Right ascension 07h45m18.94987s[2]
Declination +28° 01′ 34.3160″[2]
Apparent magnitude(V) 1.14[3]
Characteristics
Evolutionary stage Red clump[4]
Spectral type K0 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 type Suspected[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 ± 0.27mas[2]
Distance33.78 ± 0.09ly
(10.36 ± 0.03pc)
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(logg)2.685±0.09[11]cgs
Temperature4,586±57[10]K
Metallicity[Fe/H]–0.07 to +0.19[11]dex
Rotation660±15d[12]
Rotational velocity(vsini)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,GJ286,HD62509,HIP37826,HR2990,SAO79666,LFT548,LHS1945,LTT12065[14]
Database references
SIMBADdata
ARICNSdata

Polluxis the brightest star in theconstellationofGemini.It has theBayer designationβ Geminorum,which isLatinisedtoBeta Geminorumand abbreviatedBeta Gemorβ Gem.This is an orange-hued,evolvedgiant starlocated at a distance of 34light-years,making it theclosestgiant to the Sun. Since 1943, thespectrumof this star has served as one of the stable anchor points by which other stars are classified.[15]In 2006 anexoplanet(designatedPollux bor β Geminorum b, later named Thestias) was announced to be orbiting it.[11]

Nomenclature[edit]

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

β Geminorum(LatinisedtoBeta Geminorum) is the star'sBayer designation.

The traditional namePolluxrefers to the twinsCastor and PolluxinGreekandRoman mythology.[16]In 2016, theInternational Astronomical Unionorganized 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 includedPolluxfor this star.[18]

The Sun viewed from Pollux (in red circle) in theconstellationSagittarius.Made with Celestia.

Castorand 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 UnionlaunchedNameExoWorlds,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 bytheSkyNetofAustralia;namelyLeda,Pollux's mother. At the request of the IAU, 'Thestias' (thepatronymof Leda, a daughter ofThestius) was substituted. This was because 'Leda' was already attributed toan asteroidand 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 intoLatinasPosterior Brachii,meaningthe end in the paw.[25]

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

Physical characteristics[edit]

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

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

Parallaxmeasurements by theHipparcosastrometry satellite[30][31]place Pollux at a distance of about 33.78light-years(10.36parsecs) from theSun.[2]

The star is larger than the Sun, with about two[9]timesits massand almost nine timesits radius.[11]Once anA-type main-sequence starsimilar toSirius,[32]Pollux has exhausted the hydrogen at its core andevolvedinto agiant starwith astellar classificationof K0 III.[5]Theeffective temperatureof this star's outer envelope is about4,666K,[11]which lies in the range that produces the characteristic orange hue ofK-type stars.[33]Pollux has aprojected rotational velocityof2.8 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][34]

An old estimate for Pollux's diameter obtained in 1925 byJohn Stanley Plaskettvia interferometry was 13 million miles (20.9 million km, or 18.5R☉), significantly larger than modern estimates.[35]A more recent measurement by theNavy Precision Optical Interferometergive a radius of 9.06R☉.[10]Another estimate that uses Pollux'sspectral linesobtained 8.9R☉.[36]

Evidence for a low level ofmagnetic activitycame from the detection of weak X-ray emission using theROSATorbiting telescope. The X-ray emission from this star is about 1027erg s−1,which is roughly the same as the X-ray emission from the Sun. A magnetic field with a strength below 1gausshas 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 starwith a much stronger magnetic field.[32]The star displays small amplituderadial velocityvariations, but is notphotometricallyvariable.[37]

Planetary system[edit]

Since 1993 scientists have suspected anexoplanetorbiting Pollux,[38]from measuredradial velocityoscillations. 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 activitycannot be ruled out.[12]

The Pollux planetary system[11]
Companion
(in order from star) 		Mass Semimajor axis
(AU) 		Orbital period
(days) 		Eccentricity Inclination Radius
b (Thestias)(disputed[12]) >2.30±0.45MJ 1.64±0.27 589.64±0.81 0.02±0.03 — —

References[edit]

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