ETA CRB (Eta Coronae Borealis). Solar type stars fascinate, as we see something of ourselves in the nighttime sky, and perhaps because the focus on naked-eye stars brings to us such an array of those that are vastly brighter than the Sun. Eta Coronae Borealis, in Corona Borealis (the Northern Crown), brings us a little bit back home. Twice. In western Corona Borealis, just off the western curve of the Crown, Eta CrB is made of a pair of sixth magnitude (5.64 for Eta CrB A, 5.95 for "B") class G (G1 and G3) dwarfs more or less similar to ours. Less than a second of arc apart, the two add to a fifth magnitude (5.03) single image that lies rather nearby, only 58 light years (give or take 1) away. Intensive observation shows that the two orbit each other every 41.63 years at a mean separation of 15.4 Astronomical Units, a significant eccentricity taking them from as close as 11.1 AU to as far apart as 19.7 AU. They were last closest in July of 1975 and are now orbiting away from each other.

Eta CrB Two "suns." Somewhat the fainter and less massive of the two, Eta CrB B orbits Eta-A (at the cross) every 41.63 years at an average distance of 15.4 Astronomical Units, the two in fact orbiting a common center of mass that lies nearly exactly between them. That "A" does not lie on the axis or focus of the apparent ellipse is caused by as 58 degree orbital tilt to the plane of the sky and the orbital orientation. (W. I. Hartkopf and B. D. Mason, Sixth Catalog of Orbits of Visual Binary Stars, US Naval Observatory Double Star Catalog, 2006.)

Kepler's Laws give a combined mass of 2.11 Suns, while an earlier measure of mass ratio suggests respective masses for "A" and "B" of 1.06 and 1.05 Suns, making them very similar, but still a bit brighter, than our own star. With respective temperatures of 6037 and 5949 Kelvin, they radiate total energies of 1.46 and 1.11 into space, which gives us radii of 1.1 and 1,0 solar. With a projected equatorial rotation speed estimated at 3 km/s, "A" rotates in under 19 days, and in under 16 if the tilt of the rotation pole is the same as that of the orbital axis. Masses from luminosity and temperature are a bit ambiguous, but fall roughly at 1.1 and 1.0 solar, very close to what is found from the orbit. Off in the distance lie three other stars, 13th magnitude Eta C (somewhat over a minute of arc away from the inner pair), 11th magnitude "D" (some 220 seconds away), and 17th magnitude "E" as bit closer in. All are irrelevant as they are just line of sight coincidences. Imagine now the possibilities. The near-twins are far enough apart that a planet might closely orbit either one of them and close enough together that a distant planet might orbit both. In either case, the sky would dazzle not with a single star, as does ours, but with a pair of them. Of course nothing of the sort has ever been associated with Eta CrB, but who really knows? In any case, the double star gives one a chance to look back on ourselves, not just once, but twice (the two nearly impossible to split with average telescopes).
Written by Jim Kaler 6/22/12. Return to STARS.