LAMBDA LEP (Lambda Leporis). Just above the pair of distorted
boxes that make the most prominent part of Lepus, the Hare, and more pointedly five degrees south
southeast of Orion's Rigel, lies an anonymous-appearing, but quite
spectacular blue-white hot class B (B0.5) "subgiant" (but see
below), fourth magnitude (4.29) Lambda Leporis. Placed by Bayer and his artist at the base of the
bunny's right ear, the star's relative faintness is the result of
a rather large distance of 850 light years (give or take 50 or so).
From that, a tenth of a magnitude correction for dimming by
interstellar dust, and a well-determined temperature of 29,300
Kelvin, from which we find the amount of ultraviolet light to be
added to the visual (which is a lot), we find a very respectable
total luminosity of 15,300 times that of the Sun (as compared with a strictly visual
luminosity of about 1200 Suns). That and temperature yield a
radius of 4.8 times solar. A relatively low projected rotation
speed of 46 kilometers per second gives a rotation period of under
5 1/4 days. Such a low spin speed for the class (which averages
about 175 km/s) suggests that the rotation pole may be more or less
pointed at us. Or maybe it's just a slowpoke. The abundance of
iron relative to hydrogen, though, is normal, about 90 percent the
solar value of 0.000032. More important, the theory of stellar
structure and evolution applied to luminosity and temperature give
a mass 13 times that of the Sun and show that, rather than being a
subgiant (which in theory implies the death or near death of the
hydrogen fusing core), the star is actually very young, a just
formed dwarf, with a dozen or so million years of core hydrogen
fusion in front of it. (Such variances between theory and spectral
taxonomy are common among these stars; they help lead us to truth.)
Lambda Lep's youth is in line with it belonging to the Orion OB1 association of hot, young,
massive stars, even though it's at about half the spread-out association's average distance.
Having a mass above the accepted dividing line of 8-10 (even 12)
Suns, Lambda Lep's fate would seem only to develop an iron core,
which through its collapse would generate enough heat to explode
the rest of the star outward as a grand supernova. Even if the birth
mass were somewhat less, Lambda Lep would still make a white dwarf near the upper
allowed limit of 1.4 solar masses. If a companion could pass enough
matter to the future white dwarf, it too could collapse to make a
different kind of supernova.
But there is no evidence for a neighbor, even a distant visual one
that an astronomer might see directly with a telescope. Alas,
Lambda Lep may have to go through its eventual evolution all alone.
Written by Jim Kaler 2/15/13. Return to STARS.