MU-1 AND MU-2 GRU (Mu-1 and Mu-2 Gruis), a two-for-one special. Separated by less than a third of a degree, by not much more than Ursa Major's Mizar and Alcor, the "Mu's" of modern Grus (the Crane, which stalks the skies to the southwest of Fomalhaut) must be a naked-eye double, a wide binary star, similar to the "Delta's," which lie to the southeast. Nope: like the Delta's, they are not physically related. They do, however, provide us with a remarkable set of coincidences that might lead us to think that they are. Set in central Grus, both are fifth magnitude, Mu-1 (the northwestern of the pair) 4.79, Mu-2 mag 5.10. Remarkably, they are of the same class, both class G (G8) orange giants, a coincidence in space and time reminiscent of the proximity of our class G2 Sun to similar Alpha Centauri A. Moreover, they are at close to the same distance from us, Mu-1 275 light years away, Mu-2 265 light years (with small errors that allow them to be at the SAME distance). It's plenty enough for one to conclude that they must be truly related. But here the overt similarities end. For any apparent double star to be considered a real (wide) binary, the components must have similar motions through space (since orbital motions for big separations are trivial). Mu-1 is moving at a rate of 0.05 seconds of arc per year to the northeast, while the other is going at a much lower rate to the southwest. Mu-1 is also approaching us at 7 kilometers per second, while Mu-2 is going oppositely at 13 km/s. The two just happen to be at about the same place at the same time. Mu-1 is brighter than Mu-2 in part because it is itself double, accompanied by a close "G star" roughly a tenth of a second of arc away about which nothing is really known except that it contributes maybe 20 percent of the light.

Physically, Mu-1 (with a temperature of about 4900 Kelvin) shines at a rate of 76 times that of the Sun (after deducting the guess for the companion's contribution), Mu-2 (also at 4900 K) coming in at a similar 66 Suns. Radii are then respectively 12.1 and 11.4 solar. Theory reveals similar masses of about 2.5 Suns and shows that they are both helium-fusing "clump giants" (since in a graph of luminosity vs. temperature they would sit amongst a crowd of others) with ages of about 650 million years. Given a separation between Mu-1 and Mu-2 of just 10 light years, Mu-2 would shine it Mu-1's sky at minus second magnitude, while from Mu-2, Mu-1 (plus its "mystery star") would make the minus third. Both were born much farther apart as cool-end class B dwarfs essentially out of sight of each other and now just happen to be visiting. (Thanks to Jerry Diekmann, who suggested these stars.)
Written by Jim Kaler 12/09/11. Return to STARS.