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HD 218566 is a star in the equatorial zodiac constellation of Pisces. With an apparent visual magnitude of 8.6,〔 this ninth magnitude star can not be viewed with the naked eye. However, it can be readily seen even with a small telescope.〔 HD 218566 is a smaller star than the Sun, with about 81%〔 of the Sun's mass and 86% of the radius of the Sun.〔 It is a K-type main sequence star with a stellar classification of K3 V〔 that is generating energy by the nuclear fusion of hydrogen at its core. HD 218556 is radiating around 35% of the luminosity of the Sun from its outer envelope at an effective temperature of 4,849 K.〔 This heat gives the star the characteristic orange-hued glow of a K-type star.〔 Compared to the Sun, this star has an unusually high abundance of elements other than hydrogen and helium, what astronomers term the metallicity. Based upon the abundance of iron, the metallicity is 2.4 times as high as in the Sun.〔 It is much older than the Sun, with estimates of its age ranging from 8.5〔 to 11.5〔 billion years. It appears to have a negligible rate of spin as its projected rotational velocity is too small to measure.〔 This star belongs to the thick disk population of the Milky Way. In the galactic coordinate system, it has space velocity components of (V, W ) = [77, –61, . HD 218556 is following an orbit through the galaxy with an eccentricity of that carries it as close as and as far as from the Galactic Center. The orbital tilt carries this star as much as from the galactic plane.〔 Based upon high resolution measurements performed at the W. M. Keck Observatory, HD 218566 shows cyclical variations in radial velocity that suggest gravitational perturbation by orbiting companion. This candidate object is estimated to be orbiting the parent star with a period of days at an eccentricity of . The semi-major axis for this Keplerian orbit is an estimated 0.6873 Astronomical Units. Because the inclination of the orbit remains unknown, the mass of this companion has not been determined. However, it can be constrained to have a mass of at least 21% the mass of Jupiter. There is no evidence of additional companions in the system.〔 ==References== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「HD 218566」の詳細全文を読む スポンサード リンク
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