UPS CAR (Upsilon Carinae). Double stars are among the most interesting and useful creatures of the starry skies. From them and their interactions we can deduce stellar masses, radii, and all sorts of other properties. At the same time, pairs can be amazingly frustrating. Bright, third magnitude (2.95, the two components combined) Upsilon Carinae, at the southern edge of the rich southern Milky Way in Carina (the Keel of the great Ship Argo) about 10 degrees southwest of the Carina Nebula and Eta Carinae, is a case in point. The two components, an A6 third magnitude (3.02) lesser supergiant and a sixth magnitude (6.03) B3-4 subgiant (but see below) 4.9 seconds of arc apart, interfere enough with each other's light that until recently we had little idea of the lesser star's (Ups Car B's) class. In addition, the pairing prevented the Hipparcos satellite from measuring a parallax (the shift in position caused by the orbital motion of the Earth), leaving Ups Car with no directly measured distance. We are thus stuck with adopting the average absolute magnitudes (what the apparent magnitudes would be at 32.6 light years) of the spectral classes. Comparison with the observed magnitudes (after allowing for 0.35 magnitudes of dimming by interstellar dust) gives 1100 light years for Ups Car A (the supergiant) and 1400 for Ups Car B (pretty good agreement) for an average of 1250 light years. At a measured temperature of 7600 Kelvin, Ups Car A needs little correction for non-visual radiation, while at 23,000 K (both temperatures a bit cool for the classes) "B" produces quite a lot of its light in the invisible ultraviolet. "A" and "B" then have rather similar absolute luminosities of 7700 and 6100 Suns, which with temperatures lead to radii of 50 and 5 times solar. From theory, Ups Car A carries a mass of between 7.5 and 8.5 Suns depending on whether it is swelling with a dead helium core or stably fusing helium into carbon and oxygen, while "B" comes out to be a nine solar mass hydrogen fusing dwarf. Since higher mass stars die first, we have an impossible situation of the less massive being the more evolved. Given the various uncertainties in the input data, however, that the masses come out as close as they do is in fact rather satisfying and not a source of conflict. But clearly the star, one of the brightest in the sky with so little known about it, is ripe for study. That the two stars are truly connected is unquestioned as they have been tracking each other at their current separation for nearly 200 years. They must be at least 1900 Astronomical Units apart, and given that they are each around 9 solar masses must take at least 20,000 years to make a full orbit of each other. Given that separation, from "A," "B" would shine about 10 times that of Venus, while from"B," "A" would carry the luster of a quarter Moon. Both are at the edge of the limit above which stars blow up as supernovae and below which they will turn into massive white dwarfs. At this point nobody really knows.

Written by Jim Kaler 3/15/13. Return to STARS.