NU CRB (Nu-1 and Nu-2 Coronae Borealis). Nu Coronae Borealis (in Corona Borealis, the Nothern Crown) presents a rather striking sight, especially in binoculars, as it is not one star, but two. Both fifth magnitude, the eastern one of the pair (Nu-1, magnitude 5.20) is somewhat brighter than the western one (Nu-2, 5.39). Separated by only five minutes of arc, they appear as a naked-eye double. Given the true doubles Mizar and Alcor in Ursa Major at 12 minutes separation, and Epsilon Lyrae, whose naked-eye components are 3.5 minutes apart, it would appear that we have stumbled on another such physical pairing. We would be wrong. Instead, the two stars that make Nu CrB present us with one of the great coincidences of the sky. While physically unconnected, not only are they nearly the same apparent brightness, but they are actually similar in both distance, spectral class, and evolutionary development. They thus beautifully demonstrate that things are not always as they at first seem.
Nu Coronae Borealis is a remarkable naked-eye, line-of-sight, "double" in which the cool reddish stars have no physical relation with each other, even though they are at nearly the same distances, have similar apparent magnitudes, have the same masses, and are near the same points in the ageing process. The brighter and western star, Nu-1, is just five minutes of arc away from Nu-2. Sigma CrB is just above Nu, while Tau CrB is the brighter of the two stars toward the upper left. Xi CrB is at far right center.
Nu-1 and Nu-2 have measured distances of 555 and 545 light years, placing them but 10 light years apart. The measurement errors are such that they could actually be at the SAME distance (or considerably farther apart). Nu-1 is a class M (M2) giant, while Nu-2 is a similar class K (K5) giant. From their respective temperatures of 3765 and 3940 Kelvin (to account for a lot of infrared radiation), their luminosities come in at 817 and 479 times that of the Sun, radii at 67 and 47 solar (Nu-1 large enough to have had its angular diameter determined by interferometer, which gives the same radius). From theory, the stars both have masses close to 2.5 times that of the Sun. Both are advanced giants that have used up their internal helium and are brightening with dead carbon-oxygen cores. While the ages are very similar, about 750 million years, Nu-1 is a couple million years the older and has therefore brightened a bit more than Nu-2, showing how fast things can change when stars are in this stage of evolution. They look for all the world like a real pair that were born as class B9-A0 dwarfs and somehow became separated. When we look at their motions, however, we see otherwise, as the northern star is moving south, the southern star north. They actually seem to be getting closer together rather than farther apart, and are two similar stars that are just passing each other. There ARE some differences. As the more advanced in the ageing process, Nu-1 is slightly variable by about 0.01 magnitude over a 4.4 day period, which is rather odd, since we might expect considerably more. Nu-2 is somewhat deficient in metals, it's iron content 70 percent that of the Sun. The two are also moving at different velocities relative to the Sun, Nu-1 at 33 km/s, Nu-2 at a relatively high 56 km/s. Were you there on an orbiting planet (and there is no evidence for any), each star would dominate the other's skies, shining at the minus third magnitude, brighter than Jupiter does for us here on Earth.
Written by Jim Kaler 8/31/07. Return to STARS.