VV Cephei

VV Cephei

VV Cep A as it appears on Celestia.
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
Distance4.9k ly
(1.5k[5] pc)
Absolute magnitude (MV)-6
Details
Mass<25[1] / <20[3][foot 1] M☉
Radius1050[5]-1900[6][foot 1] / 5-8[3] R☉
Luminosity200,000[1]-320,000[5] / ~10,000[foot 1] L☉
Temperature3,800[1] / ~25,000 K
Other designations
HIP 108317[4]
Database references
SIMBADdata

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 sun compared to VV Cep A

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.

Red circle indicates location in Cepheus constellation

Notes

  1. ↑ 1.0 1.1 1.2 1.3 Size, mass and luminosity estimates are all considerably uncertain due to insufficient knowledge of the Cephei star system. (Kaler)

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Bennet, P. D. (2010). "Chromospheres and Winds of Red Supergiants: An Empirical Look at Outer Atmospheric Structure". ASP Conference Series 425: 181. arXiv:1004.1853. Bibcode:2010ASPC..425..181B.
  2. ↑ Bauer, W. H.; Bennett, P. D.; Brown, A. (2007). "An Ultraviolet Spectral Atlas of VV Cephei during Total Eclipse". Astrophysical Journal Supplement Series 171 (1): 249–259. Bibcode:2007ApJS..171..249B. doi:10.1086/514334.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Hopkins, J. L.; Bennett, P. D. (2006). "Single Channel UBV Photometry of Long Period Eclipsing Binary VV Cephei" (PDF). Proceedings for the 25th Annual Conference of the Society for Astronomical Sciences: 105. Bibcode:2006SASS...25..105H.
  4. ↑ 4.0 4.1 "Hipparchos catalogue: query form". CASU Astronomical Data Centre. Cambridge Astronomical Survey Unit. 2006. Retrieved March 10, 2009.
  5. ↑ 5.0 5.1 5.2 Bauer, W. H.; Gull, T. R.; Bennett, P. D. (2008). "Spatial Extension in the Ultraviolet Spectrum of Vv Cephei". The Astronomical Journal 136 (3): 1312. Bibcode:2008AJ....136.1312H. doi:10.1088/0004-6256/136/3/1312.
  6. ↑ 6.0 6.1 Saito, M.; Sato, H.; Saijo, K.; Hayasaka, T. (1980). "Photometric Study of VV Cephei during the 1976-1978 Eclipse". Publications of the Astronomical Society of Japan 32: 163. Bibcode:1980PASJ...32..163S.
  7. ↑ Habets, G. M. H. J.; Heintz, J. R. W. (1981). "Empirical bolometric corrections for the main-sequence". Astronomy and Astrophysics Supplement Series 46: 193–237. Bibcode:1981A&AS...46..193H.
  8. ↑ 8.0 8.1 Leedjärv, L.; Graczyk, D.; Mikolajewski, M.; Puss, A. (1999). "The 1997/1998 eclipse of VV Cephei was late". Astronomy and Astrophysics 349: 511–514. Bibcode:1999A&A...349..511L.

External links

Wikimedia Commons has media related to VV Cephei.
Preceded by
Mu Cephei		 Largest known star
1998 — 2007 		Succeeded by
VY Canis Majoris

Coordinates: 21h 56m 39.14s, +63° 37′ 32″