EPS CEN (Epsilon Centauri). Deep in southern Centaurus, Epsilon Cen provides the link between the
sprawling northern part of the huge constellation and the pair of
first magnitude stars (Rigil-Kent and
Hadar) that both anchor it and lead the
eye to the Southern Cross. Bright and
prominent, this second magnitude (2.30), hot (23,900 Kelvin),
sparkling blue class B (B1) giant (but see below) is the fourth
brightest star without some kind of proper name (after Delta Velorum, Beta
Gruis, and Epsilon Scorpii). Rather distant at 375 light
years, the star is dimmed by a few hundredths of a magnitude by a
bit of intervening interstellar dust, whose absence would render
Eps Cen at magnitude 2.20. Its membership in the Lower-Centaurus-
Crux association is uncertain. Allowance for a whopping amount of
ultraviolet light leads to an impressive luminosity (typical for
its class) of 11,400 times that of the Sun,
which in turn (with temperature) gives a radius of 6.25 solar
(hardly that of a "giant") and (from stellar structure theory) a
large mass of 11 times solar. An equatorial rotation period of at
least 114 kilometers per second gives the star a rotation period of
less than 2.7 days. Like many stars of its class, it is rather low
in metals compared with the Sun, in this case 72 percent solar.
Theory also rather firmly shows that the star is not actually a giant, but a hydrogen-fusing
dwarf less than 10 million years old (though one closing in on
being a helium-core subgiant). To add to this mild confusion, Eps
Cen is a subtly-vibrating "Beta Cephei"
star, one that chatters away, changing by only a hundredth of a
magnitude or so with multiple periods, here 0.170, 0.177, 0.210,
and 0.191 days. Other oscillation periods are surely present.
Such stars are usually evolving giants or subgiants (in which core
hydrogen fusion has ceased), not dwarfs. Perhaps the distance (or
other parameter) is wrong. Direct measure of angular diameter,
however, gives 6.1 times solar, which strongly suggests that errors
are minimal. So the star gives us a bit of an unsolved conundrum,
giant or dwarf? Also a bit of a mystery is a nearby, dim (13th
magnitude) companion 39 seconds of arc away, which from its
absolute brightness (if the two are at the same distance) appears
to be a class K (K6) dwarf. A projected separation of 4500
Astronomical Units gives an orbital period of at least 89,000
years. However, the relative motion between the two is higher than
expected, and the coupling is probably just a line-of-sight
coincidence. If they really are a pair, from Eps Cen proper, the
companion would shine with but the light of a bit more than our
Venus, while from the companion, Eps Cen would glare with the
radiance of 70 full Moons. If they can stay together long enough,
the possible companion may well see its mighty mate explode as a supernova (which would almost
certainly eject the lesser star from the system). Eps Cen's mass
is close to the edge for such an event. If not over that ill-
defined edge, the star will at least probably turn into a rare,
massive, neon-oxygen white
dwarf.