LAMBDA CEN (Lambda Centauri). Crux, the Southern Cross, was in the seventeenth century carved from the feet of Centaurus, the Centaur, the two constellations thus bearing an intimate relation with each other. Starting to the southeast (with Alpha and Beta Centauri), Centaurus wraps around the Cross to the north and then a bit to west, where we find the luminous mid-third magnitude (3.13) star Lambda Cen, a class B (B9) giant (some say bright giant) 420 light years (give or take 18) away. It's a part of the "Lower Centaurus Crux" association of O and B (and lesser) stars that are something of a family with members born more or less at the same time, the system now expanding and dissipating into space. "LCC" lies at an average distance of 385 light years, Lambda Cen thus being a bit on the back side of it. The star is dimmed by about six percent by interstellar dust, a surprisingly small amount given its position in the Milky Way. Separated from Lambda proper ("Aa") by under a second of arc lies Ab, which in infrared light shines at seventh magnitude. The assumption that it is the same in visual light (perhaps a bit rash) drops Aa to magnitude 3.17. Several temperature measures are all over the place, averaging 10,170 Kelvin. Lambda itself (Aa) shines with the light of 955 Suns, which leads to a radius of 10 times solar. Measures of projected equatorial rotation speed are odd, an early citation giving zero, two later ones averaging 185 kilometers per second, which is probably the more correct. If so, the star completes a rotation in under 2.7 days. It's fast enough (supporting the rapid rotation) that the chemical abundances are not weird, as they are in so many stars of its class. That said, the star does appear to be metal-rich with an iron abundance as much as two to three times normal, with silicon and carbon a bit down. As are so many stars within its association, Lambda is on the massive side, carrying about 4.5 times the mass of the Sun. Not yet a true giant, the star is now in the process of becoming one with a dead helium core, its age about 125 million years. The companion may be a mid-class A star that carries around double the solar mass. With a mean orbital size of at least 90 Astronomical Units, from Kepler's Laws Ab must take more than 335 years to make a full orbit. Around 16 seconds of arc distant we find 11.5 magnitude Lambda B, which if real is a solar-type star separated from the inner pair by at least 2000 AU in an orbit that must take at least 34,000 years to complete. From the outer star, the stars of the inner double would appear two or three degrees apart, their orbit making quite a sight, were there anyone there to see it. X-ray radiation may be associated with the lesser companion. Lambda Cen is associated with a small nebula, but it's probably just a line-of-sight coincidence.
Written by Jim Kaler 5/25/12. Return to STARS.