PHI VEL (Phi Velorum). Just-barely fourth magnitude (3.54) Phi Velorum (of no proper name) has a rather surprising number of things to set it apart, including some "negatives." The first positive is in its location as one of the three stars that define the southern classic outline of Vela, the Sails of the Ship Argo (visible south of about 35 degrees north latitude), the others brighter Kappa and Delta Vel, these two part of the "False Cross" that resembles Crux, the proper Southern Cross. Better, it sits practically on the Galaxy's equator, the center line of the Galactic disk, just 0.1 degree to the north of it. Phi Vel is also a blue-white and very luminous class B (B5) supergiant (albeit a lesser one) that appears dim only because of its great distance of 1590 light years (give or take 70). Oddly, even though it sits smack within the Milky Way, the star appears to be in a clear path that has little, if any, interstellar dust absorption to dim it further. Distance and a temperature of 13,600 Kelvin, needed to account for ultraviolet light, reveal a luminosity 18,200 times that of the Sun and a radius 24 times solar. The rotation speed is, at 33 kilometers per second, fairly low, giving it a rotation period under 37 days, which is not much of a constraint. Then we come to the star's greatest claim to stellar fame, its mass, which comes in (from luminosity, temperature, and theory) right on 10 times solar. The figure represents something of an upper limit to the mass at which stars turn into white dwarfs instead of exploding as supernovae, Phi Vel right on the line and thus in an unusual position. (In truth, the limit is somewhere between 8 and 12 solar masses, most astronomers opting for 8, making the star a potential exploder.) Whatever its forecast, Phi Vel began life some 20 million years ago as a much hotter B0.5-B1 dwarf, its cooler and larger status now the result of the depletion of hydrogen in its nuclear burning core. On the "negative" side, in which the star has been rejected, are a couple of memberships. Just over half a minute of arc away is a twelfth "companion," Phi Vel B, whose slow drift relative to Phi Vel itself seems to rule it out as a real binary member. What really does it is that the fainter star is touted as a class K giant, whose brightness would put it 7800 light years away, five times farther than Phi Vel itself (though closer if dimmed by interstellar dust). So no double. Early ideas also had the star as a member of the huge, expanding Scorpius-Centaurus association of hot blue stars. Rejected again, it isn't, its motion not in line with real members. What will happen to it, only time will tell, though it might be a million or more years in the future before some of our distant descendants may know. Before anything dramatic happens, however, the star, still cooling and expanding before it fires up helium in its core, will eventually double its luminosity and grow to some 500 times the solar size, to a radius of roughly two Astronomical Units.
Written by Jim Kaler 4/09/10. Return to STARS.