At magnitude 2.27, Caph (or β Cas) is the second brightest star in the "W" of Cassiopeia. The five stars in the "W" splay out like the fingers of an outstretched hand; the name Caph comes from an Arabic phrase meaning "the stained hand," which refers to an ancient cosmetic coloring by henna.
A line from Caph though Alpheratz (α And) points almost exactly at the Vernal Equinox. This is the point on the celestial equator where we find the Sun on the first day of spring.
Properties
Physically, Caph is a class F2 III-IV giant (or subgiant) with a 6700 K surface, about 900 K warmer than the Sun. From its nearby distance of only 54 light years, Caph shines with 28 times the luminosity of the Sun. It has a small stellar companion, about which little is known, that orbits it every 27 days.
Caph is unusual in being a δ Scuti-type variable star, in fact the brightest of the breed. Slightly unstable, and rapidly pulsating, the star varies by about 6% in brightness over a 2.5 hour period. It is a sort of low-mass version of the famed Cepheid variables.
Evolution
With a mass around double the Sun's, Caph was once a class A star much like Aquila's Altair. Though classed as a giant, Caph is really not all that large, only about four times the radius of the Sun.
The term really refers to the state of Caph's evolution. Having recently stopped fusing hydrogen, its core is now contracting, and its outer layers are cooling and expanding. Caph is in a particularly rapid state of its evolution, with an unusual combination of temperature and luminosity called the "Hertzsprung Gap". Caph will only spend about 1% of its several-billion-year lifetime in such a condition.
Of these "gap" stars, Caph has a particularly faint corona - a halo of hot, magnetically heated gas that radiates X-rays. This is much like the Sun's corona, which is visible during a total solar eclipse.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]