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.