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VV Cephei, also known as HD 208816, is an eclipsing binary star 4,900 light years away from Earth. VV Cephei A, the main companion, is a red supergiant that fills the system's Roche lobe when closest to a companion star, a blue main sequence star called VV Cephei B. Matter flows from the red supergiant into orbiting the blue companion for at least part of the orbit and the blue star is blocked from view by a large disk of material. VV Cephei A is currently recognised as one of the largest stars in the Milky Way with a diameter pulsating between 1,050 to 1,900 solar radii (4.9 to 8.8 au; 730,000,000 to 1.32×109 km). Thus, if VV Cephei A were placed in the center of the Solar System, it would engulf the orbit of Saturn, possibly even the orbit of Uranus.
The spectrum of VV Cep can be broken down into two main components, one for VV Cephei A and VV Cephei B. The material surrounding VV Cephei B generates emission lines, including [FeII] forbidden lines which is not common for other stars surrounded by circumstellar disks. The hydrogen emission lines are double-peaked due to a narrow central absorption component. This is caused by seeing the disk almost by its edge on where it intercepts continuum radiation from the star.
The distance has been estimated by a variety of astronomical techniques to be around 1,5 kiloparsecs, while the Hipparcos parallax measurement produces a distance considerably below 1 kiloparsec. From the distance, with the extinction measured at 1.24 magnitudes, the absolute magnitude of the VV Cephei system as a whole is fairly well defined.
The angular diameter of VV Cephei A can be estimated using photometric methods and has been calculated at 0.00638 arcseconds across. This allows a direct calculation of the actual diameter, which is in good agreement with the 1,050 D☉ derived from a complete orbital solution and eclipse timings. Observations of earlier eclipses had given diametrical values between 1,200 D☉ and 1,600 D☉ and an upper limit of 1,900 D☉. The size of VV Cephei B is highly uncertain since it is physically and photometrically blocked by a much larger disc several hundred D☉ in diameter. The secondary is definitely much smaller than both the primary or the disc, and has been calculated to have a diameter between 13 D☉ to 25 D☉ from the orbital solution.
Despite VV Cephei A being an extremely large star (possibly the second largest known), showing high mass loss and having some emissions lines, it is not scientifically considered to be a hypergiant. The emission lines are produced from the accretion disc around the hot secondary and the absolute magnitude is typical for a normal red supergiant.
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- Table 4 in Levesque, Emily M.; Massey, Philip; Olsen, K. A. G.; Plez, Bertrand; Josselin, Eric; Maeder, Andre; Meynet, Georges (2005). "The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not as Cool as We Thought". The Astrophysical Journal 628 (2): 973. arXiv:astro-ph/0504337. Bibcode 2005ApJ...628..973L. doi:10.1086/430901.
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