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SIGMA CRB (Sigma Coronae Borealis). We have a fascination forsolar type stars, perhaps wondering if there is anyone out there onan orbiting planet looking back at us. Science fiction can havegreat fun with double solar type stars. Here is the acme of theclass, a TRIPLE-solar star -- or at least the closest we areprobably ever going to see -- and one just 71 light years away, theSigma star inCorona Borealis, theNorthern 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 G0Sigma-1 and sixth magnitude (again barely, 5.64) class G1 Sigma-2,which together make the apparent fifth magnitude (5.3) Sigma CrBthat is unresolved with the naked eye. (Sigma-1 has been called ascool as class G4.) Thespectrograph then showssomething yet more remarkable, that Sigma-2 is ITSELFdouble, consisting of F9 and G0dwarfs (Sigma-2 A and Sigma-2 B) that take a mere 1.14 days (1 day3 hours 21 minutes) to orbit each other. Their respectivecharacteristics are temperatures of 6000 and 5900 Kelvin,luminosities 1.17 and 1.02 times that of theSun, radii 1.14 and 1.10 solar, and masses1.108 and 1.08 solar. The period combined with the total mass ofthe system shows that they are only 0.028 Astronomical Units (AU) -- just 6 solar radii -- apart. The closeness causes tides thathave forced each of the stars to spin very rapidly, with periods of1.157 days, nearly synchronized with the orbital period. Solarrotation and up and down convection of the solar gases cause activesolar magnetism, sunspots, magnetic flares, and so on. The rapidspins of the Sigma-2 pair, just over 20 times greater than that ofthe Sun, cause much greater activity, making the system one of aclass of "RS Canum Venaticorum stars" (after the prototype) likeEpsilon Ursae Minoris andLambda Andromedae.Sophisticated analysiswith the spectrograph reveals huge starspots that march across theline of sight as the stars orbit and rotate. There is so muchactivity that the system is even observed in the radio spectrum. Now out to Sigma-1. With a temperature if around 5800 Kelvin anda luminosity of around 85 percent that of the Sun, the star carriesjust about a solar mass. Orbital motion observed over the past twocenturies gives orbital characteristics: a period around the innerpair of 890 years, an average separation of 128 AU, and a higheccentricity that carries it as close as 31 AU and as far as 225 AUfrom the inner binary. The luminosity, temperature, and lithiumcontent (an element slowly destroyed in stars by nuclear reactions)show the system to be very young. A hypothetical planet couldorbit the inner pair, giving a double sun with another off in thefar distance. The activity from the double would make lifehazardous, however. Or, a planet could orbit outer Sigma-1 withthe binary off in the distance. From Sigma-1, Sigma-2 would appearas double whose maximum separation varied from 25 seconds of arc to3 minutes of arc, the latter visible to the naked eye, each of theinner pair shining with (on average) 70 times the light of the fullMoon. No planet, however, is in evidence. But what a scenario fora story!
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