18 SCO (18 Scorpii). Shining weakly at but sixth magnitude (5.50) among the myriad bright stars of Scorpius and surroundings, obscure 18 Scorpii (no Greek letter here, just a Flamsteed number) seems hardly worth a look. But oh what it tells us about ourselves, as it seems to be the long-sought "solar twin," a star exactly (or as close as one can get) like our class G (G2) Sun. Many are the class G stars, many are those near one solar mass, but factoring in all the solar properties cuts the numbers severely, leaving few candidates. The stars are either a bit too warm (those of Sigma Coronae Borealis for example), too cool (Achird, or Eta Cas A), double or multiple (both of these plus class G2 Alpha Centauri A). And here it is, just 45.7 light years away, practically on the northern border of Scorpius with Ophiuchus, its discovery the product of large searches that place solar-type stars in continua of various properties, most important that of age, and that tell us a great deal about our own star. Look at its amazing properties (those of the Sun in parentheses for comparison, where needed). Class G2 (G2); temperature 5789 Kelvin (5777); luminosity 1.05 solar; mass 1.012 solar; age 4.2 billion years (4.56); rotation period 23 days (25.7 days); magnetic activity cycle 9 to 13 years (11 years). Mass and age are found from the luminosity, temperature, and the theory of stellar evolution, since 18 Sco is distinctly single, with no binary companion to give its mass away; rotation is from spectral features associated with magnetically active areas (spots and the like) that rotate in and out of view. Age and activity are crucial. Magnetic activity is generated by rotation and outer- layer convection. A star's wind drags the magnetic field outward. The field, however, is still anchored at the star. As a result, the field produces a drag on the rotation and slows the star down, a process called "magnetic braking." Old stars clearly rotate more slowly than young stars. From the study of solar type stars, we see that the Sun must have been rotating much more quickly when it and its planets were young, and therefore must have been much more active. Theory shows that when the Sun was born it had only 70 percent its current luminosity. (Ordinary hydrogen-fusing dwarfs like the Sun slowly brighten as they age.) How then could the Earth have sustained liquid water for life? This "faint Sun paradox" could be resolved through increased activity, which showers us with X-ray and ultraviolet radiation. Little "18" thus shows us not just what we would look like from a distance, but helps put the whole solar experience into a larger context. (Stellar characteristics taken from work and compilation by R. T. Hamilton, E. F. Guinan, and L. E. DeWarf of Villanova University, and L. Seward of Florida Inst. of Tech.)
Written by Jim Kaler. Return to STARS.