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VV Cephei, also known as HD 208816, is an eclipsing binary star system located in the constellation Cepheus, approximately 5,000 light years from Earth.〔〔Size, mass and luminosity estimates are all considerably uncertain due to insufficient knowledge of the Cephei star system. (Kaler)〕 A red supergiant fills the system's Roche lobe when closest to its companion blue star, the latter appearing to be on the main sequence. Matter flows from the red supergiant onto the blue companion for at least part of the orbit and the hot star is obscured by a large disk of material. The red supergiant primary, known as VV Cephei A, is currently recognised as one of the largest stars in the galaxy, with an estimated solar radius of 1,050. Although VV Cephei is an extremely large star showing high mass loss and having some emissions lines, the spectral luminosity class and absolute magnitude do not qualify it as a hypergiant.〔 〕 It should be possible to calculate the masses of eclipsing binary stars with some accuracy, but in this case mass loss, changes in the orbital parameters, a disk obscuring the hot secondary, and doubt about the distance of the system have led to wildly varying estimates. Calculations before the most recent primary and secondary eclipses had started to settle on masses of around 20 for both stars, but more recent papers have suggested that even this is too high.〔〔 Evolutionary models show that the initial mass of a star reaching the red supergiant stage as a star like VV Cephei would be no more than 25 times the Sun,〔 and obviously considerably lower now. Like Betelgeuse, VV Cephei A is plagued by phenomena intrinsic to large diffuse stars that make them difficult to measure with precision. VV Cephei is not entirely spherical, being surrounded by opaque shells of a highly extended atmosphere, which, coupled with limb darkening, unstable luminosity, and other factors, make it difficult to determine its true size. Some researchers still hold that VV Cephei is a normal AGB star about 500 times the radius of the Sun, and not a hypergiant, including results from the most recent eclipse.〔 〕 ==Size== The angular diameter of VV Cephei A can be estimated using photometric methods and has been calculated at 0.00638 arcseconds.〔 This allows a direct calculation of the actual diameter, which is in good agreement with the 1,050 solar radii derived by other methods. Analysis of the orbit and eclipses places a firm upper limit on the possible size at 1,900 solar radii.〔 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「VV Cephei」の詳細全文を読む スポンサード リンク
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