SIGMA CRB (Sigma Coronae Borealis). We have a fascination for solar type stars, perhaps wondering if there is anyone out there on an orbiting planet looking back at us. Science fiction can have great fun with double solar type stars. Here is the acme of the class, a TRIPLE-solar star -- or at least the closest we are probably ever going to see -- and one just 71 light years away, the Sigma star in Corona Borealis, the Northern Crown. Through the telescope, we observe two very solar- looking hydrogen-fusing dwarf stars just 7 seconds of arc apart, from west to east seventh magnitude (just barely, 6.66) class G0 Sigma-1 and sixth magnitude (again barely, 5.64) class G1 Sigma-2, which together make the apparent fifth magnitude (5.3) Sigma CrB that is unresolved with the naked eye. (Sigma-1 has been called as cool as class G4.) The spectrograph then shows something yet more remarkable, that Sigma-2 is ITSELF double, consisting of F9 and G0 dwarfs (Sigma-2 A and Sigma-2 B) that take a mere 1.14 days (1 day 3 hours 21 minutes) to orbit each other. Their respective characteristics are temperatures of 6000 and 5900 Kelvin, luminosities 1.17 and 1.02 times that of the Sun, radii 1.14 and 1.10 solar, and masses 1.108 and 1.08 solar. The period combined with the total mass of the system shows that they are only 0.028 Astronomical Units (AU) - - just 6 solar radii -- apart. The closeness causes tides that have forced each of the stars to spin very rapidly, with periods of 1.157 days, nearly synchronized with the orbital period. Solar rotation and up and down convection of the solar gases cause active solar magnetism, sunspots, magnetic flares, and so on. The rapid spins of the Sigma-2 pair, just over 20 times greater than that of the Sun, cause much greater activity, making the system one of a class of "RS Canum Venaticorum stars" (after the prototype) like Epsilon Ursae Minoris and Lambda Andromedae. Sophisticated analysis with the spectrograph reveals huge starspots that march across the line of sight as the stars orbit and rotate. There is so much activity that the system is even observed in the radio spectrum. Now out to Sigma-1. With a temperature if around 5800 Kelvin and a luminosity of around 85 percent that of the Sun, the star carries just about a solar mass. Orbital motion observed over the past two centuries gives orbital characteristics: a period around the inner pair of 890 years, an average separation of 128 AU, and a high eccentricity that carries it as close as 31 AU and as far as 225 AU from the inner binary. The luminosity, temperature, and lithium content (an element slowly destroyed in stars by nuclear reactions) show the system to be very young. A hypothetical planet could orbit the inner pair, giving a double sun with another off in the far distance. The activity from the double would make life hazardous, however. Or, a planet could orbit outer Sigma-1 with the binary off in the distance. From Sigma-1, Sigma-2 would appear as double whose maximum separation varied from 25 seconds of arc to 3 minutes of arc, the latter visible to the naked eye, each of the inner pair shining with (on average) 70 times the light of the full Moon. No planet, however, is in evidence. But what a scenario for a story!
Written by Jim Kaler. Return to STARS.