Pollux is the brightest star in the
constellation of
Gemini. It has the
Bayer designationβ Geminorum, which is
Latinised to Beta Geminorum and abbreviated Beta Gem or β Gem. This is an orange-hued,
evolvedgiant star located at a distance of 34
light-years, making it the
closest giant 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.[10]
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-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 characteristics
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. The last lunar occultation visible from Earth was on 30 September 116 BCE from high southern latitudes.[29]
An old estimate for Pollux's diameter obtained in 1925 by
John Stanley Plaskett via interferometry was 13 million miles (20.9 million km, or 18.5
R☉), significantly larger than modern estimates.[35]
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.[32] The star displays small amplitude
radial velocity variations, but is not
photometricallyvariable.[36]
Planetary system
Since 1993 scientists have suspected an
extrasolar planet orbiting Pollux,[37] 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.[10]
^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,
ISBN978-1-931559-44-7.
^
abcdeDucati, 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,
doi:10.26093/cds/vizier, VizieR Cat. II/237/colors.
^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.
^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
^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
^"The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from
the original on March 18, 2012, retrieved 2012-01-16
^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.