ZETA CRU (Zeta Crucis). Among the more brilliant sights of the sky is four-star Crux, the Southern Cross, which contains two stars of first magnitude (Acrux and Mimosa, the Alpha-Beta pair) and precedes first magnitude Beta and (even brighter) Alpha Centauri across the sky. So prominent is Crux that it was actually cut out of Centaurus by the early southern explorers to stand on its own, its setting spectacular within the southern Milky Way (which more or less bottoms out here at its most southerly extension). The constellation perhaps bears more resemblance to a classic kite, with several stars in Crux and neighboring Centaurus making a tail as they come off the southern point (Acrux), notably Zeta, Eta, and Theta-1 and -2 Cru, extending to (lower case Roman) j and Lambda Centauri. Of the "tail stars," Zeta Cru, a mid-fourth magnitude (4.03), hot, blue-white class B (B2.5) star 358 light years away (give or take 13) is the closest to Acrux and second brightest (Lambda Cen, the most angularly distant, at magnitude 3.12 easily topping the set). Three temperature determinations, needed to account for severe spectra.html#emspectrum">ultraviolet radiation, average out to 21,900 Kelvin, which with distance and a 22 percent correction for dimming by intervening interstellar dust tells of a star with a luminosity 1950 times that of the Sun and a radius of 3.1 solar. For a hot star of this class, the measured projected equatorial rotation velocity of just 68 kilometers per second, is low, which would suggest at first that the spin-pole is pointed more or less at us. But Zeta Cru is definitely not an emission-line "Be" star (one with a surrounding disk) in the mold of Zeta Tauri or Dschubba (Delta Scorpii), which are related to HIGH rotation, so maybe the measured rotation speed is more or less close to the real value, which would give a rotation period of 2.2 days or a bit under. The real glory of this rather neglected star lies in it large mass, which from theory is 7.7 times that of the Sun, the star very young and apparently just starting on its 30-million-year hydrogen-fusing life. Just under the limit at which stars explode as supernovae, Zeta Cru will eventually shed its outer envelope and die as a fairly massive white dwarf of over a solar mass rather akin to Sirius B. Separated from the star by 34 seconds of arc is a 12th magnitude "companion" whose relative motion of nearly four seconds of arc over the past 150 years suggests that it is just there in the line of sight, as might be expected given such a crowded region of the Milky Way. While seemingly alone, though, Zeta Cru indeed has some relatives, as it is part of the 150 light-year-wide "Lower Centaurus-Crux" association of hot stars. Such assemblies are made of stars that were born more or less together (over a substantial period of time) but are not gravitationally bound together and are dissipating. The LCC's mean distance of 385 light years and age of just 10 million years fit well, Kappa's better-known mates being Pi, Lambda (of the Kite's tail) and Delta Centauri, as well as Mimosa, Beta Crucis.
Written by Jim Kaler 5/27/11. Return to STARS.