VV Cephei

VV Cephei (VV Cep, HD 208816, HIP 108317, BD+62 2007) is a supergiant star in the constellation of Cepheus.^[4] Along with Mu Cephei, VV Cephei is one of the largest stars in the night sky. It is actually an eclipsing binary system of two stars with the longest known period of 7,430 days, or 20.4 years.^[5] The two stars are designated VV Cephei A and VV Cephei B repectively to distinguish them, with VV Cephei A being the larger of the two. The high declination of VV Cephei means that it is a circumpolar system that can always be seen in the northern hemisphere.

The VV Cephei system is situated 2452.36 light years away from Earth.^[3] The system is composed of two stars, a red supergiant VV Cephei A and a blue dwarf VV Cephei B, with their orbits align to Earth such that one star can pass in front of the other. This results in the light from the two stars periodically dimming as one obscures light from the other. The dimming follows a cycle of 7,430 days (or 20.4 years) making it the second longest period for a variable star known, the longest being that of Epsilon Aurigae and a period of 9,890 days.^[5] The eclipse produced when the red supergiant passes in front of the blue dwarf lasts about 2 years and reduced the brightness of the system by 20%.^[4] As of 2019 the most recent eclipse began in August 2017. The long timescale over which an orbit occurs makes the system difficult to study.

The stars have a highly eccentric orbit with a semi-major axis of 25 astronomical units and an eccentricity of 0.346.^[6] This means the distance between the two stars varies between 17 AU up to 34 AU.^[4] The is thought to be some form of mass transfer between the stars with a jet of material travelling from the larger VV Cephei A to VV Cephei B. Up to a few hundreths of a solar mass of material is thought to be transfered between the stars each year.^[4]

VV Cephei A is thought to have a mass of 20 solar masses and a radius approximately 1,000 times that of the Sun, though more older measurements place it higher at 1,600 times the solar radius.^[5][7] For a comparison, if it were to replace the Sun in our solar system, its surface would easily extend to the orbit of Saturn or possibly Uranus.^[8] The temeprature of its surface it thought to be in the range 3,300-3,650 kelvin and its luminosity is estimated to be 400,000 larger than the Sun's.^[4] The star is a pulsating semi-regular variable star and changes in brightness by several hundredths to a few tenths of magnitude as it pulsates. These changes take place over several times periods 58, 118, and 349 days as well as an additional one of 13.7 years. VV Cephei A's shape is not thought to be spherical, but something closer to a teardrop shape, though more subtle. The star is sufficiently larger that there is a Doppler-shift in the hydrogen-alpha line in the star's spectrum. This is causes by the star's rotation as one side of the star is rotating towards us and so light from that side is blue shifted whereas light from the side rotating away from us is red shifted.^[5]

Although smaller than VV Cephei A, VV Cephei B is still quite large compared to the Sun with an estimated mass in the neighbourhood of 19 solar masses and radius of 13 solar radii.^[5][9] The star dominates the spectrum of the VV Cephei system in the ultraviolet and at higher wavelengths whereas VV Cephei A dominated in the infrared. The star appears blue-white in color and its class is a B6 dwarf.^[4]