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.