ALPHERATZ (Alpha Andromedae).
Andromeda runs like a string of pearls to the northeast
of the Great Square of Pegasus.
Since its three principal stars
have almost the same brightness, Beyer simply lettered them Alpha,
Beta, and
Gamma in order, Alpha and Beta tied for the lead in
apparent magnitude. Mid-second magnitude (2.06) Alpheratz, the Alpha star,
connects Andromeda (the Lady) with
Pegasus, the Flying Horse that
carried Perseus to Andromeda's rescue. As such, Alpheratz is also
the northeastern star of the
Great Square of Pegasus. The name has
a confused origin possibly transferred from another star, and is
commonly taken to mean "the horse's shoulder" or "naval," showing
that the star originally belonged more to Pegasus, though now it is
formally within the boundaries of Andromeda. As a linking star,
Alpheratz is one of two that carries two Greek letter names, and is
also Delta Pegasi, though the name is no longer used. (The other
star is Elnath, Beta Tauri, which links Taurus to
Auriga and is
also called Gamma Aurigae.) If you draw a line through Alpheratz
and Algenib,
Gamma Pegasi to the south, it will pass just to the
east of the vernal equinox in Pisces. Alpheratz, at the cool end
of the B star range (B8 subgiant) , is still hot, with a temperature of about
13,000 Kelvin. At a distance of 97 light years, we calculate a
total luminosity, including the star's ultraviolet radiation, of
about 200 times that of the Sun.
Alpheratz stands out in a couple of
ways. It is a "spectroscopic binary," a close double that can be
investigated only through the examination of its spectrum, the pair
orbiting each other every 96.7 days. The much dimmer companion
seems to be about a tenth the brightness of the principal star.
The bright member of the pair, the one that makes the impression on
the human eye, is also chemically peculiar, and is the brightest
member of the odd class of "mercury-manganese" stars. Compared to
our standard, the Sun, and to the vast majority of other stars, the
atmosphere of these stars have vast enrichments of these elements,
mercury overabundant by a factor of tens of thousands. Other
elements such as gallium and europium are hugely enriched as well,
while some others are depressed. We believe we are seeing a
separation of elements as a result of the inward pull of gravity
and the outward pressure of radiation that act differently on
different elements. (Thanks to Monica Shaw, who helped research
this star.)