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.