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 class F (F0) dwarf stars with surface temperatures of about 7100 degrees Kelvin, significantly warmer than the Sun. They orbit each other on highly elliptical paths in only 169 years, and as a result, a single observer can watch them easily move over the course of a lifetime. Averaging They are now about 3 seconds of arc apart, and made their closest approach to each other halfway through the year 2005. Thirty-eight light years away, the stars average 43 Astronomical Units from each other, about the distance between the Sun and Pluto, the orbital eccentricity taking them between 81 and 5 AU.
Porrima The near-twin class F0 stars of Porrima (Gamma Virginis) go around each other every 169 years. In spite of the long interval, we've seen the full orbit. Here, for ease of visualization, the slightly dimmer one is charted as if it goes around the slightly brighter one (Porrima A), though in reality both go around a common center of mass. The scale on the axes is in seconds of arc. Separated on the average by 43 Astronomical Units, a high eccentricity takes the two between 81 and 5 AU apart. They were last closest in 2005. An orbital tilt of 31 degrees to the plane of the sky distorts the orbit and places Porrima A off the orbital focus. (W. I. Hartkopf and B. D. Mason, Sixth Catalog of Orbits of Visual Binary Stars, US Naval Observatory Double Star Catalog, 2006.)
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. From luminosity and temperature, they are each 1.5 times more massive than the Sun, which results in their higher surface temperatures and in luminosities about four times solar and radii 1.2 times that of the Sun. From the binary orbit and Kepler's Laws, we derive masses of 1.4 solar, satisfyingly similar. Both are very young. 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.
Written by Jim Kaler 5/15/98; revised 11/13/09. Return to STARS.