GAMMA CAS (Gamma Cassiopeiae).
High overhead in northern autumn evenings lies the W-
shaped constellation 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 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 (2.47) 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 (usually denoting
third brightest in a constellation) is a victim of early
collectivization. At a distance of 610 light years, this blue star
radiates with a huge luminosity some 40,000 times that of the
Sun,
after accounting for invisible ultraviolet radiation from the
25,000 Kelvin hot class B (B0) subgiant surface and a bit of absorption
by the dust of interstellar space that makes the star appear a bit
fainter than it would were space clear. Gamma Cassiopeiae is more,
however, than just another luminous star, even more than one that
is apparently approaching the end of its hydrogen-fusing life.
First, it is also unpredictably variable. In 1937, the star
brightened almost to first magnitude, and it has been as faint as
third. Perhaps its lack of a proper name tells us of ancient
faintness. Then, in 1866 the father of the study of stellar
spectra, Father Angelo Secchi, discovered that the star radiated
light in specific colors 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 and weird.
All rotate with enormous speed, Gamma Cas spinning at least 300
kilometers per second at its equator, 150 times the solar rate.
The rotation and high luminosity conspire to drive mass from the
star into a surrounding disk that radiates the "emissions," mass
loss apparently related to the brightness variations. Gamma Cas
also radiates X-rays, though no one is quite sure why, theories
including the transfer of lost mass to a compact companion and
magnetic effects similar to those found on the Sun. With a mass
of around 15 times that of the Sun, the star will most likely someday
explode as a supernova. If any star
deserves a name, surely this one does. (Thanks to Monica Shaw, who
helped research this star.)