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HD 17156 is a yellow subgiant star approximately 240 light-years away〔 in the constellation of Cassiopeia. The apparent magnitude is 8.17, which means it is not visible to the naked eye but can be seen with good binoculars.〔 A search for a binary companion star using adaptive optics at the MMT Observatory was negative.〔 The star is more massive and larger than our Sun while Its absolute magnitude of 3.70 and spectral type of G0, show that it is both hotter and more luminous. Based on asteroseismic density constraints and stellar isochrones, it was found that the age is 3.37 billion years making it about two thirds as old as the sun. Spectral observations show that the star is metal-rich.〔〔 An extrasolar planet was discovered with the radial velocity method in 2007, it was subsequently discovered to transit the star. At the time it was the transiting planet with the longest period.〔〔 == Planetary system == It is the first star in Cassiopeia around which an orbiting planet was discovered (in 2007) using the radial velocity method. Later observations showed that this planet also transited the star.〔 In February 2008, a second planet, HD 17156 c, was proposed, with a 5:1 mean motion resonance to the inner planet HD 17156 b.〔 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 17156」の詳細全文を読む スポンサード リンク
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