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.
Written by Jim Kaler. Return to STARS.