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Pollux (star)
Pollux (β Gem, β Geminorum, Beta Geminorum) is a star in the northern constellation of Gemini. It is an evolved giant star with an orange hue. At an apparent visual magnitude of 1.1,〔 Pollux is the brightest star in the constellation; brighter even than its neighbor Castor (Alpha Geminorum). Parallax measurements made with the Hipparcos astrometry satellite〔〔 place it at a distance of about from Earth.〔 Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified.〔 In 2006, Pollux was confirmed to have an extrasolar planet orbiting it.〔
==Properties==
This star is larger than the Sun, with about two times its mass and almost nine times its radius.〔 Once an A-type main sequence star,〔 Pollux has exhausted the hydrogen at its core and evolved into a giant star with a stellar classification of K0 III.〔 The effective temperature of this star's outer envelope is about ,〔 which lies in the range that produces the characteristic orange hue of K-type stars.〔 Pollux has a projected rotational velocity of .〔 The abundance of elements other than hydrogen and helium, what astronomers term the star's metallicity, is somewhat uncertain, with estimates ranging from 85% to 155% of the Sun's abundance.〔〔
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.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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