NU CEN (Nu Centauri). Filled as Centaurus (the Centaur) is with bright blue stars, one
hardly knows which ones to pick. In the north-central part of the
huge constellation is a fine pair of blue third magnitude beauties
only three-quarters of a degree apart that almost look like a
physical pair, Nu Centauri (magnitude 3.41) to the near-exact north
of variable Mu Cen. While they are not a physical pair, they are
indeed related. Both are at about the same distance (437 light
years for Nu, 505 for Mu, making them but 68 light years apart),
both belong to the "Upper-Centaurus-Lupus" association of hot
stars, and both are classed as B2 subgiants while actually being B2
dwarfs of about the same age (this kind of discrepancy among hot B
stars very common). From each, the other would appear as a minus
first magnitude star. Then their properties diverge. Nu Cen is
known from its spectrum to be double, with a companion in
circular orbit with a period of a mere 2.625 days. Nothing is
known about the companion, and we assume it does not add much if
any light to that of the B2 primary star, Nu Cen proper, so for now
we ignore it. With a temperature of 22,950 Kelvin, Nu Centauri
produces a good fraction of its radiation as ultraviolet light.
Taking that into account, along with 6 percent dimming by
interstellar dust, the star produces energy at a rate of 4970 times
that of the Sun, which yields a radius of
4.5 times solar and (from a projected equatorial rotation speed of
78 kilometers per second) a rotation period of less than three
days, which suggests it might be rotating in synchrony with its dim
companion. Theory then gives a mass of 9 times that of the Sun,
and shows clearly that the star is a dwarf with an age of 9.4
million years, about a third of its total hydrogen-fusing (dwarf)
lifetime. Nu Centauri possesses a couple of subtle characteristics
that have not been thoroughly researched. Though it's known to
vary slightly with a period if 0.17 days (putting it into the class
of Beta Cephei stars, those that chatter away like Alfirk), little is known of the character of
the variation. Emission in the spectrum also suggest that it's a
weak "B-emission" (like Zeta Tauri) star
that might have a circumstellar disk, at least off and on.
Assuming the companion to have low mass, it would have to be only
about 0.08 Astronomical Units from Nu Cen proper, just 20 percent
Mercury's distance from the Sun and just over three and a half
times the radius of Nu itself. At that distance, when the primary
star begins to die and expand, the little orbiting neighbor is in
deep trouble. Even if the companion's mass is much higher, say as
much as 5 Suns, it does not make much difference. Running all the
numbers lowers the luminosity of Nu Cen A to 4200 Suns and brings
the mass down to 8.5 solar masses, while increasing the separation
to 0.09 AU. Whatever the case, Nu Cen A, the bright primary, is
just at the edge of the limit above which stars blow up as supernovae. Given that it
will probably someday lose mass to its neighbor, it's more likely
that it will die as a massive white dwarf.
Written by Jim Kaler 8/07/09. Return to STARS.