LAMBDA ERI (Lambda Eridani). Though at fourth magnitude (4.27)
comparable to many of the other stars of dim Eridanus, the River, Lambda Eridani is not usually
taken as part of the classic figure itself. Seen just to the south
of the River's origin at Cursa (Beta Eri),
the star does however serve a role as the southern anchor of a
small asterism called Orion's Footstool,
upon which the Hunter rests his left foot. (The others are Cursa
itself, plus Tau Orionis to the east and
Phi Eri to the west.) The apparent dimness of this blue-white hot
(23,100 Kelvin) class B2 subgiant is the result of a distance of
around 1750 light years (an uncertain value), more than twice as
far as neighboring Rigel. After allowing
for a lot of ultraviolet light (and 0.16 magnitudes of interstellar
dust absorption), Lambda Eri shines with the light of nearly 39,000
Suns, which is comparable to the energy
radiated by Rigel (which has less in the ultraviolet, that and a
closer distance making it brighter to the eye). From that figure
we deduce a radius of 12 times that of the Sun and a mass of 14
solar. Lambda's most notable point is that its rapid rotation has
(for reasons still not clear) turned it into a B-emission (Be) star
that has a radiating and rotating circumstellar disk much like that
belonging to Dschubba's or Gamma Cas. The disk is probably seen more
or less edge on, rendering Lambda Eri a "shell star," a special
category in which the disk appears more opaque than usual. The
phenomenon is probably related to the star's small periodic
variation of about a tenth of a magnitude over a quarter of a day,
with other periods superimposed. (For a time, the star was thought
to be a rapid "Beta Cephei" type of
variable, but that notion has been rejected.) Oddly, the projected
equatorial rotation speed is quite uncertain, the measures ranging
from 255 kilometers per second to 336, which respectively give
rotation periods of less than 2.4 and 1.8 days. The rotation
flattens the star at the poles, which makes the temperature change
across the stellar surface (hottest at the poles, coolest at the
equator), in turn making the various parameters deduced from it
uncertain. The star, which began as a hotter class B0 star only
half as bright, is now near the end of its core hydrogen-fusing
lifetime and will next turn into a red supergiant, after which it will
probably blow up as a supernova.
Written by Jim Kaler 12/26/08. Return to STARS.