VV Cephei
| Observation data Epoch J2000 Equinox J2000 |
|
|---|---|
| Constellation | Cepheus |
| Right ascension | 21h 56m 39.144s |
| Declination | +63° 37′ 32.01″ |
| Apparent magnitude (V) | 4.91[1] (Naked eye) |
| Characteristics | |
| Spectral type | M2 Iab[2] / B0-2V[3] |
| U−B color index | 0.3 (variable)[3] |
| B−V color index | 1.6 (variable)[3] |
| Astrometry | |
| Parallax (π) | 1.33 ± 0.20[4] mas |
| Distance | 4.9k ly (1.5k[5] pc) |
| Absolute magnitude (MV) | -6 |
| Details | |
| Mass | <25[1] / <20[3][foot 1] M☉ |
| Radius | 1050[5]-1900[6][foot 1] / 5-8[3] R☉ |
| Luminosity | 200,000[1]-320,000[5] / ~10,000[foot 1] L☉ |
| Temperature | 3,800[1] / ~25,000 K |
| Other designations | |
| Database references | |
| SIMBAD | data |
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.[1][foot 1] 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.[7]
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.[1][8] 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,[1] 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.[8]
Size
The angular diameter of VV Cephei A can be estimated using photometric methods and has been calculated at 0.00638 arcseconds.[1] 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.[6]
Distance
The distance has been estimated by a variety of techniques to be around 1.5kpc, which places it within the Cepheus OB2 association. Although some older studies found a larger distance and consequently very high luminosity and radius, it now seems that the distance is unlikely to be significantly more than 1.5kpc. The Hipparcos parallax measurement produces a distance considerably below 1kpc, although there are reasons to think that Hipparcos consistently underestimates the distance to this type of object. From the distance, with relatively little extinction, the absolute magnitude of the VV Cephei system is fairly well defined. Since the primary contributes the vast majority of the energy output, its luminosity is also reasonably well defined.
Temperature
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 of 3,500-3,600K. More recent calibration of the temperature scales for supergiants based on observations across a wider range of wavelengths gives an effective temperature of 3,800-3,900K. The secondary star is heavily obscured by a disk of material from the primary, and its spectrum is almost undetectable. It is apparently an early B main-sequence star, but likely to be abnormal in several respects due to mass transfer from the supergiant. A normal star of that type would be around 10,000 times the luminosity of the Sun, 5-8 times the radius of the Sun, 15-18 times the mass of the Sun, and around 25,000K.
Notes
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Lua error in package.lua at line 80: module 'strict' not found.
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- ↑ 3.0 3.1 3.2 3.3 3.4 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 4.0 4.1 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 5.0 5.1 5.2 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 6.0 6.1 Lua error in package.lua at line 80: module 'strict' not found.
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- ↑ 8.0 8.1 Lua error in package.lua at line 80: module 'strict' not found.
External links
 |
Wikimedia Commons has media related to VV Cephei. |
- Hposoft.com
- Space.Com
- Universe Today
- VV Cephei at Kempten observatory
- Aladin image of VV Cephei
| Preceded by | Largest known star 1998—2007 |
Succeeded by VY Canis Majoris |
