VV CEP (VV Cephei). Two of the most magnificent, and largest, stars of the sky lurk close together and rather anonymously within the dark interstellar dust clouds of Cepheus (the King): Herschel's Garnet Star (Mu Cephei) and the extraordinary variable and binary, VV Cephei. Both are huge red supergiants. Mu Cep stands at only fourth magnitude (4.08), VV fainter at fifth (4.91). Were it not for the dimming effects of the dust, they would respectively shine at second (2.0) and third (2.9) and might have been parts of the formal constellation. Mu Cep has a current estimated radius somewhere between 1450 and 1650 times that of the Sun, or 6.7 to 7.7 Astronomical Units, considerably bigger than the orbit of Jupiter. Though VV may well top it, the uncertainties preclude accurate assessment. Such problems aside, VV Cep is a terrific example of a mass- exchanging eclipsing binary, in which a distorted, swollen red class M (M2) supergiant orbits with a fainter but much hotter shrouded blue- white star whose assigned class runs from B8 dwarf to B6 giant, or even hotter into class O. The pair orbits with a period of 20.4 years. Separated by 25 Astronomical Units (80 percent the distance between Neptune and the Sun), a high eccentricity takes them between 17 and 34 AU apart. When the blue star goes in back of the supergiant, the visual light dips by about 20 percent. The supergiant is so huge that the blue dwarf is eclipsed for the better part of a year, 250 days. The binary is hard to study, as the interval between eclipses gives only a couple of them in a working astronomical career. Analysis of the spectrum and the eclipses give radii for the supergiant between 1600 and 1900 solar (7.5 and 8.8 AU). The bigger estimate gives us a star 92 percent the size of Saturn's orbit, making it among the largest known. The temperature, not well known, falls between 3300 and 3650 Kelvin. A radius of 7.9 AU and a temperature of 3500 Kelvin give a luminosity of almost 400,000 Suns, which in turn yields a mass about 35 times solar. The distance is estimated through the star's membership in the Cepheus OB2 association of hot blue stars, which gives 2400 light years to within about 20 percent. The Hipparcos parallax, which at this distance is subject to high error, closely agrees. For all the discussion of radius, however, there is a serious problem. VV Cephei A (the red supergiant) is not spherical, in fact rather far from it. The star instead seems to be distorted into a teardrop shape and to fill its tidal surface, from which it sends matter into a disk around the smaller, much hotter, companion, resulting in overestimates of average dimension (and making even the concept of dimension problematic). The companion, which dominates the star's ultraviolet spectrum (as the supergiant does in the visual and infrared), is even more mysterious, as we are not even certain of its class and mass, but it's probably relatively high. The mass exchange, which could be as high as a few hundredths of a solar mass per year (and which must alter the evolution of both stars), is probably at the heart of sudden changes in orbital period. The flowing matter makes the two into "emission line stars." Typical of supergiants, VV Cep is also a pulsating semi-regular variable that changes by a few hundredths to a few tenths of a magnitude with suggested recognized periods of 58, 118, and 349 days plus one of 13.7 years. While the various parameter ranges are unfortunately large (showing how hard it is to study such rare stars), it is clear that the supergiant (now probably fusing helium into carbon in its deep core) will "soon" blow up as a grand supernova, perhaps ejecting its companion back into the cosmos as a single star that had quite a career behind it. VV Cephei is included in Jim Kaler's "The Hundred Greatest Stars." Thanks to Jose Rodriguez, who suggested it.

Written by Jim Kaler 10/28/05; revised 5/21/13. Return to STARS.