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.