PHI AND CHI CEN (Phi and Chi Centauri), a two-for-one special. Not a gravitational double, but from their motions probably sharing a birthplace, Phi and Chi Centauri (of central Centaurus, the Centaur) make a fairly prominent fourth magnitude (3.83 and 4.36) pair, Chi 1.7 degrees to the east of its brighter neighbor. Easy to find, the duo lies just to the east of a brighter vertical pairing, third magnitude Mu and Nu Centauri, to which they are also mostly likely loosely related. Three of the four (Mu, Nu, Phi) are classified as hot B2 subgiants, Chi as a B2 dwarf (though in evolutionary terms all are actually hydrogen-fusing dwarfs, such discrepancies common). All are purported to belong to the Upper Centaurus grouping of the vast Scorpius-Centaurus association of massive stars, though none is actually listed in the latest compilation of more specific subgroups. At a distance of 525 light years (give or take 14), with a temperature of 22,540 Kelvin, and with a correction for 0.16 magnitudes of dimming by interstellar dust, Phi Cen shines with the luminosity of 5100 times that of the Sun. A radius of 4.7 Suns and a projected equatorial rotation speed of 102 kilometers per second yield a rotational period under 2.3 days. Theory then gives a mass of 9 times that of the Sun (similar to Mu and Nu) and shows the star to be a dwarf roughly halfway through its hydrogen-fusing lifetime of 25 million years. Nominally somewhat closer, 510 light years (give or take 17), warmer (23,100 Kelvin), and afflicted with slightly more interstellar dimming, Chi Cen at 3220 solar luminosities and 8 solar masses is just a bit the lesser star. It too is a dwarf (as augured by its class), but younger, theory suggesting it is just born, though that may be a product of incorrect parameters. A radius of 3.6 times that of the Sun and a projected equatorial rotational speed of 24 kilometers per second shows Chi to rotate with a period of under 7.4 days, though it could spin much faster as we do not know the axial tilts of either. An unconfirmed couple percent variation over a period of under an hour suggests that Chi might be a Beta Cephei type variable, while Phi was considered as one, then dropped. Separated from Chi by 85 seconds of arc is an eighth magnitude "companion" that from its motion appears to be just a line of sight coincidence. The concentration of bright hot stars in this part of the sky is remarkable. From a planet orbiting any of them, the view would be spectacular. Given the uncertainties, Phi and Chi could be at the same distance. If so, they would be but 14 light years apart, and either would shine with roughly the brightness of our Venus in each others' skies. But given the high ultraviolet luminosities and short lifetimes of such stars, don't expect anybody to be actually watching. Both (all four for that matter) are near the limit at which their cores might collapse, causing the stars to explode as supernovae, though they might also turn into massive white dwarfs, perhaps with advanced neon/oxygen compositions.

Written by Jim Kaler 6/28/13. Return to STARS.