EPS LEP (Epsilon Leporis). Were it not for its brilliant
surroundings, Lepus, the Hare (just
south of mighty Orion), would surely be
more renown. But it is overwhelmed by Orion to the north and Sirius and Canis
Major to the east, not to mention several other bright constellations. The luminary Arneb (Alpha Lep) is almost second magnitude
(2.58), Nihal (Beta) at 2.84 not following
far behind. Coming in third and fourth are third magnitude Epsilon
(3.19) and Mu (3.31), while the star given
the third letter of the Greek alphabet, Gamma, ranks (at 3.60) number 6. Bright
as it is, Epsilon is left out of the early Arabs' "Chair of the
Giant" or the "Throne of al Jauza" (referring to the figure of our
Orion to the north), which are made of Alpha through Delta (see
Allen). Neglected Epsilon (both by the Arabs and to an extent by
modern researchers) is, explaining the latter, yet another class K
(K5) giant of the sort the dot
the skies. But it is bright enough to merit at least some
attention. At a precisely-known distance of 213 light years (give
or take just 3), the star, with a well-determined temperature of
4100 Kelvin (just as expected for a K5 giant) shines (after
addition of infrared
radiation) at a luminosity of 445 times that of the Sun, which leads to a radius of 41.9 times
solar (0.19 AU, about half the size of Mercury's orbit).
Interferometer measures give 41.5 solar, right on the mark, showing
that all the parameters are quite accurately known. A projected
equatorial rotation speed of 4.3 kilometers per second gives a
rotation period that could be as long as 1.3 years. There is some
indication of variability, but no confirmation except for possibly
some in the infrared. The mass is indeterminate, since stars of
differing masses in Epsilon's evolutionary state have rather
similar temperatures and luminosities. The best estimate seems to
be around 2.5 times the mass of the Sun. The actual evolutionary
status, like so many other similar stars, is also confused.
Epsilon Lep could be on its "first ascent" as a brightening giant
with a dead helium core about ready to fire up to fuse to a mixture
of carbon and oxygen (and may have done so already). Just as
likely, the star could be at the end of "helium burning," and
brightening for the second time with a dead carbon/oxygen core. If
so, Eps Lep is some 755 million years old, having given up core
hydrogen fusion 170 million years ago. Though appearing common,
class K giants fill the sky all out of proportion to their actual
numbers per unit volume of space as a result of their brightness,
a clear "selection effect" in which nature shows us what she wants,
not what is actually there. The most common kinds of stars are
actually dim red class M dwarfs like Proxima Centauri, not one of which is bright enough
to be visible to the naked eye.
Written by Jim Kaler 3/30/12. Return to STARS.