PORRIMA (Gamma Virginis). Follow the curve of the handle of the
Big Dipper to the
south as it first passes through orange
Arcturus and then south of
the sky's equator through blue-white Spica.
Just up and to the
right of Spica lies dimmer, third magnitude Porrima, Virgo's Gamma
star (or Gamma Virginis). Unlike most star names, which are
Arabic, this one is Latin and honors a Roman goddess of prophecy.
A telescope shows a remarkable sight, one of the finest double
stars in the sky. The components are almost perfect identical
twins, both white stars with surface temperatures of about 7000
degrees, significantly warmer than the Sun. They orbit each other
on highly elliptical paths in only 170 years, and as a result, a
single observer can watch them easily move over the course of a
lifetime. They are now about 3 seconds of arc apart, and will make
their closest approach to each other in the year 2007. Thirty-
eight light years away, the stars average 40 astronomical units
from each other, about the distance between the
Sun and Pluto.
Both stars, like the Sun, belong to the "main sequence," that is,
they radiate as a result of the fusion of internal hydrogen into
helium. They are each about 50 percent more massive than the Sun,
which results in their higher surface temperatures and in
luminosities about four times solar. These otherwise ordinary
stars are close to two important transition points. First, cooler
stars, including those like the Sun, rotate slowly. But just
cooler than the Porrima pair, stars begin to spin much faster. In
the 20,000 degree range, they can rotate so fast that they come
close to breaking up. Cooler stars possess the structures needed
to create magnetic fields that provide the means of braking them
down; warmer stars do not. Second, cooler, lower mass stars run
mostly on a nuclear reaction chain that for the most part simply
slams protons together to make helium. At Porrima's twin masses,
a much more vigorous "carbon cycle" takes over that uses carbon to
aid in the creation of helium from hydrogen.