CAPH (Beta Cassiopeiae). At mid second magnitude (2.27)
and just barely the second brightest
star in Cassiopeia,
almost but not quite as bright as Shedar (the
Alpha star), Caph, the Beta star, stole its name from the whole "W"
of the constellation. The five stars of the "W" splay out like the
fingers of a stretched hand, the name "Caph" coming from an Arabic
phrase meaning "the stained hand," which refers to an ancient
cosmetic coloring by henna. The name eventually went to the Beta
star, leaving quite-bright
Gamma Cassiopeiae with no name at all.
Yet though Gamma is quite marvelous, Caph certainly deserves the
honor. In the geometry of the sky, a line through Caph and the
more southerly Alpheratz in
Andromeda points almost exactly at the
Vernal Equinox, the point on the
celestial equator where we find
the Sun on the first day of spring. Physically, Caph is a class F (F2)
giant (or subgiant, a not-quite-giant) with a 6700 Kelvin surface,
about 900 Kelvin warmer than the Sun.
From its very close
distance of only 54 light years, Caph is seen to shine with 28
times the brightness of the Sun. It is not unusual in having a
small stellar companion, about which little is known, that orbits
it every 27 days. Though called a "giant," the star is really not
all that large, only about four times as big as the Sun. The term
really refers to the state of Caph's evolution, hydrogen fusion in
its core having recently stopped. The star is not so much a
physical giant but becoming one, the core now contracting, the star
cooling and expanding (though no such changes could be seen in a
human lifetime). Caph is thus quite unusual in that it is in a
particularly rapid state of evolution, and is in a region of
temperature and luminosity called the "Hertzsprung Gap" (after the
famous Danish astronomer Ejnar Hertzsprung) in which we find few
other stars. It will spend only about one percent of its few
billion year lifetime in such a condition. With a mass of around
double the Sun's, Caph once probably looked much like Summer's
Altair. Of these "gap" stars, Caph has a particularly faint
"corona," a halo of hot, magnetically heated gas that radiates X-
rays (seen around the Sun during a total eclipse). Caph is also
unusual in being a "
Delta Scuti" type of variable star, in fact the
brightest of the breed. Slightly unstable, and rapidly pulsating,
a sort of low-mass version of the famed "Cepheid variables," the
star varies by about 6 percent in brightness over a 2.5 hour
period. (Note: Gamma Cas can be brighter than both
Caph and Shedar.)
Written by Jim Kaler 1/29/99. Return to STARS.