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 (1.97) and third (2.91) and might have been parts
of the formal constellation. Mu Cep has a current estimated radius
somewhere between 1200 and 1650 times that of the
Sun, or 5.6 to
7.7 Astronomical Units, bigger than the orbit of Jupiter. Though
VV may well top it out, the uncertainties preclude accurate
assessment. Above all, VV Cep is a terrific example of a
mass-exchanging eclipsing
binary, in which a distorted, swollen red class M (M2) bright
supergiant orbits with a much fainter class B (B8) blue-white
hydrogen-fusing dwarf (one that has been classified as hot as O8),
which tidally distorts its vastly bigger, and much less dense,
companion. 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 the largest known. The temperature, not well
known, falls between 3300 and 3650 Kelvin. Radius and temperature
give a luminosity that falls somewhere between 275,000 and 575,000
times that of the Sun, which in turn give masses between 25 and 40
times solar. These figures can be checked through visual
brightness, distance, and temperature. Much too far away for
parallax, 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. However, one study claims
the star does not belong to the association at all, so in truth we
really do not know. At 2400 light years, the temperature range
(from which we estimate the amount of infrared light) gives similar
luminosities of between 163,000 and 535,000 Suns, a radius between
1000 and 2200 Suns (4.7 and 10.4 AU), and an accordingly similar
mass range. The smaller values (from the higher temperature) seem
the more reasonable, which then do not accord well with the values
from the spectrum and the eclipses. VV Cephei A (the red
supergiant), however, is not spherical. It 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 the basic concept of radius problematic). The star may also
be much farther away, perhaps twice the distance of Cepheus OB2.
The companion is even more mysterious, as we are not even certain
of its class and mass, which is probably a few times that of the
Sun. 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 the
lesser star), 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 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.