GAMMA CAS (Gamma Cassiopeiae). High overhead in northern autumn
evenings lies the W-shaped constellation
of Cassiopeia, in mythology the Queen and
mother to Andromeda, to us brilliant
against the background of the Milky Way and
fully circumpolar from most of the
United States and Europe and all of Canada. At the center of the
"W" lies our star. It stands out in that, oddly, it is among the
brightest stars in the sky that carries no proper "western" name, though known as Tsih (the whip) in
old China. We do not know why at second magnitude (variable and
now at 2.1) the ancients paid it so little attention. The four
eastern stars of the "W" are all about the same brightness and were
together known through Arabic as "the stained hand." The shortened
version of the term, "Caph," eventually went to the eastern-most
(the Beta) star, so perhaps Gamma is a victim of early
collectivization. Making it all even odder, it's a star of
superlatives. At the extreme hot end of class B (B0, more likely
B0.5, but still almost class O), the star is a rarity. It's
usually listed as a subgiant, though theory (see below) tells
otherwise. The distance of 549 light years (give or take 11 in the
second Hipparcos reduction) is well known. The temperature is not,
a rounded-off 30,000 Kelvin (from a range of 25,000 to 34,000)
appropriate to the B0.5 class adopted here; as on a slippery road,
caution is advised. In the Milky Way, Gamma Cas is dimmed by
roughly 0.35 magnitudes by interstellar dust. After further
correction for a whopping amount of ultraviolet radiation, the
luminosity totals around 65,000 times that of the Sun (as compared to the visual luminosity of
"only" about 5000 Suns). Temperature and luminosity then conspire
to yield a radius of 9.4 times solar. An equatorial rotation speed
of at least 280 kilometers per second gives a rotation period under
1.7 days. Fluctuations in X-ray radiation (again, see below) show
the true rotation period to be 1.21 days, which then gives an axial
tilt of 45 degrees to the line of sight and reveals a true rotation
speed of almost 400 kilometers per second, 200 times that of the
Sun. As a result the star is notably oval, which makes temperature
even more problematic, as it must be hotter at the rotation poles
than it is at the equator. Theory then gives a great mass close to
20 times that of the Sun and shows that the star is really a dwarf
about halfway through its eight or so million year hydrogen fusing
lifetime. Not surprisingly, the star is associated with various
surrounding ionized interstellar clouds.
Gamma Cassiopeiae is more, however, than just another luminous
star. It's unpredictably variable. In 1937, it brightened almost
to first magnitude, and it has been as faint as third. The Bright
Star Catalogue gives 2.77, and it is now at about 2.15, making it
the constellation's brightest star, in conflict with its Greek letter name. Perhaps lack of
a proper name tells of ancient faintness. Then, in 1866 one of the
founders of the study of stellar spectra, Father Angelo Secchi, discovered
that the star radiated light in specific colors, those associated
with hydrogen. Gamma Cas thus has the distinction of being the
first known "Be star," the "e" standing for "emission." Be stars are fairly
common among the class and odd. All rotate with enormous speed (as
we see for Gamma Cas). The rotation, high luminosity, and perhaps
atmospheric oscillations in an unknown way drive mass from the star
into a surrounding disk that radiates the hydrogen emissions. Mass
loss is apparently related to the brightness variations. As noted
above, Gamma Cas also radiates X-rays, though no one is quite sure
why. Of one thing we are sure, that with a mass far above the
limit of 8 to 10 Suns, Gamma Cas will someday explode as a supernova. It will all be
watched by a much lower mass companion. Detected spectroscopically, it has an orbital
period of 203.5 days and therefore a separation of 1.8 Astronomical
Units, or a bit more depending on the companion's unknown mass.
Another likely companion, Gamma Cas B, an 11th magnitude F6 dwarf,
lies 2.2 seconds of arc (at least 370 AU) away, which gives it a
period of more than 1500 years. Thirteenth magnitude Gamma C at 54
seconds separation appears to be just a line of sight coincidence.
If any star deserves a name, surely this one does. (Thanks to
Monica Shaw, who originally helped research the star.)
Written by Jim Kaler 1/08/99; revised
5/03/13. Return to STARS.