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.