MU COL (Mu Columbae). Columba, the Dove,
is in a sense coupled to Orion, as the
Hunter makes a good guidepost for it, a line directly south from
Orion passing first through Lepus and
then through Columba. Curiously, there is also a powerful physical
connection between the two that is made by the fifth magnitude
(5.17) hot class O (O9.5) hydrogen-fusing dwarf star
Mu Columbae, one of the few of its
class easily visible to the naked eye. On its own, Mu is a
magnificent star. At a distance of 1300 light years, with a
temperature of 33,700 Kelvin, and allowing for a bit of
interstellar dust absorption (which dims the star by 0.1 magnitude)
and a lot of ultraviolet light, Mu Col radiates at a rate of 23,300
Suns. From that figure we derive a radius 4.5 times that of the
Sun, a rotation period less than 1.5 days (from the minimum
rotation speed of 140 or so kilometers per second, typical for such
stars), and a mass about a dozen times solar. Also typical, the
star blows a fairly strong wind with a mass loss rate of about 0.1
millionths of a solar mass a year. But none of these figures are
what gives Mu its intrigue. Mu Columbae and its partner
AE Aurigae
are the classic "runaway stars." Mu is moving at high speed -- 117
km/s -- relative to the Sun, and is moving directly away from AE
Aur at over 200 km/s. The two must once have been together, and
are now separated by some 70 degrees. Modern computers allow the
tracks of the two to be traced back in time, and show that the pair
crossed paths near Orion's current Trapezium (Theta-1 Ori) sometime around 2.5 million
years ago. The third actor in the drama seems to be Iota Orionis (Na'ir al Saif), a multiple
star whose main component is a very close double with an unusually
highly eccentric orbit. It seems that 2.5 million years ago --
before the Trapezium (only a million or so years old) itself was
even born -- two double stars crashed into each other, swapped two
members and ejected two others at high velocity, thus connecting
Columba, Orion, and Auriga. More oddly perhaps, runaways are not
all that unusual, as they constitute 10 to 15 percent of all O and
B stars. There seem to be two routes to runaway glory, double star
encounters as presented here, and supernova explosions in
doubles in which the exploder pops off off-center, going off in one
direction while ejecting its companion in the other, a classic case
being Zeta Ophiuchi.
Such will most likely be the fate
of Mu. The so-called "fixed stars"
are hardly so! (Read more about Mu Col in Jim Kaler's
Hundred Greatest Stars.