VV Cephei

VV Cephei A

### Other designations

VV Cep, HR 8383, HIP 108317, HD 208816, BD+62°2007, WDS J21567+6338, 2MASS J21563917+6337319, IRAS 21552+6323, AAVSO 2153+63

**VV Cephei**, also known as **HD 208816**, is an eclipsing binary star system located in the constellation Cepheus, approximately 4,900 light years from Earth.

A red hypergiant 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 hypergiant onto the blue companion.

## VV Cephei A

VV Cephei A, is a massive red hypergiant, is one of the largest stars known, and believed to be the forth largest star in the Milky Way after VY Canis Majoris and the red hypergiant WOH G64 in the Large Magellanic Cloud. It is of spectral type M2 and is poorly estimated at approximately 1,600–1,900 times the Sun's radius, or approximately 2,227,200,000 to 2,644,800,000 km across. If placed in the center of the solar system, the hypergiant would probably extend past the orbit of Jupiter and approach that of Saturn. VV Cephei is 275,000-575,000 times more luminous than the Sun. As is true for many red hypergiants, a stellar wind flows off the system at a velocity of approximately 25 kilometers per second. VV Cephei A's mass estimated from its orbital motion is about 100 solar masses, but its luminosity suggests a mass 25-40 solar masses. Consequently, the actual mass is unknown. This star also is know by SAO 19753, HD 208816, HIP 108317.

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 R☉ derived from a complete orbital solution and eclipse timings. Analysis of earlier eclipses had given radius values between 1,200 R☉ and 1,600 R☉ and an upper limit of 1,900 R☉. The diagrams of the roche lobe of VV Cephei A are contradictory, for example, the roche lobe is calculated to be about 1,800 R☉, thus the radius cannot be larger than this, although in another diagram, the roche lobe is calculated to be much larger at 3,000 R☉. The size of the secondary is even more uncertain, since it is physically and photometrically obscured by a much larger disc several hundred R☉ across. The secondary is certainly much smaller than either the primary or the disc, and has been calculated at 13 R☉ to 25 R☉ from the orbital solution.

The temperature of the VV Cephei stars is again uncertain, partly because there simply isn't a single temperature that can be assigned to a significantly non-spherical diffuse star orbiting a hot companion. The effective temperature generally quoted for stars is the temperature of a spherical blackbody that approximates the electromagnetic radiation output of the actual star, accounting for emission and absorption in the spectrum. VV Cephei A is fairly clearly identified as an M2 supergiant, and as such, it is given a temperature around 3,800 K. The secondary star is heavily obscured by a disk of material from the primary, and its spectrum is almost undetectable against the disc emission. Detection of some ultraviolet absorption lines narrow down the spectral type to early B and it is apparently a main-sequence star, but likely to be abnormal in several respects due to mass transfer from the supergiant.