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.