VV Cephei
 Location of VV Cephei in Cepheus constellation |
|
| Observation data Epoch J2000 Equinox J2000 |
|
|---|---|
| Constellation | Cepheus |
| Right ascension | 21h 56m 39.14385s |
| Declination | +63° 37′ 32.0174″ |
| Apparent magnitude (V) | 4.91(4.80 - 5.36) |
| Characteristics | |
| U−B color index | +0.43 |
| B−V color index | +1.73 |
| Variable type | EA + SRc |
| A | |
| Spectral type | M2 Iab |
| U−B color index | +2.07 |
| B−V color index | +1.82 |
| B | |
| Spectral type | B0-2 V |
| U−B color index | −0.52 |
| B−V color index | +0.36 |
| Astrometry | |
| Parallax (π) | 1.33 ± 0.20mas |
| Distance | 4.9k ly (1.5kpc) |
| Absolute magnitude (MV) | −6.93 |
| Orbit | |
| Period (P) | 7,430.5 days |
| Semi-major axis (a) | 16.2 ± 3.7" (24.8 AU) |
| Eccentricity (e) | 0.346 ± 0.01 |
| Inclination (i) | 84° |
|
Semi-amplitude (K1) (primary) |
19.43 ± 0.33 km/s |
|
Semi-amplitude (K2) (secondary) |
19.14 ± 0.68 km/s |
| Details | |
| A | |
| Mass | 18.2 M☉ |
| Radius | 1,050 R☉ |
| Luminosity | 200,000 L☉ |
| Surface gravity (log g) | 0.0 cgs |
| Temperature | 3,826 K |
| B | |
| Mass | 18.4 M☉ |
| Radius | 13 R☉ |
| Metallicity | −0.14 |
| Age | 25 Myr |
| 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. It is both a B[e] star and shell star.
VV Cephei is an eclipsing binary with the second longest known period. A red supergiant fills its Roche lobe when closest to a 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 supergiant primary, known as VV Cephei A, is currently recognised as one of the largest stars in the galaxy, with an estimated radius of 1,050 R☉.
VV Cephei experiences both primary and secondary eclipses during a 20.3 year orbit. The primary eclipses totally obscure the hot secondary star and last for nearly 18 months. Secondary eclipses are so shallow that they have not been detected photometrically since the secondary obscures such a small proportion of the large cool primary star. The timing and duration of the eclipses is variable, although the exact onset is difficult to measure because it is gradual. Only Epsilon Aurigae has a longer period among eclipsing binaries.
VV Cephei also shows semiregular variations of a few tenths of a magnitude. Visual and infrared variations appear unrelated to variations at ultraviolet wavelengths. A period of 58 days has been reported in UV, while the dominant period for longer wavelengths is 118.5 days. The short wavelength variations are thought to be caused by the disc around the hot secondary, while pulsation of the red supergiant primary caused the other variations. It has been predicted that the disc surrounding the secondary would produce such brightness variability.
The spectrum of VV Cep can be resolved into two main components, originating from a cool supergiant and a hot small star surrounded by a disk. The material surrounding the hot secondary produces emission lines, including [FeII] forbidden lines, the B[e] phenomenon known from other stars surrounded by circumstellar disks. The hydrogen emission lines are double-peaked, caused by a narrow central absorption component. This is caused by seeing the disk almost edge on where it intercepts continuum radiation from the star. This is characteristic of shell stars.
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