ALGENUBI (Epsilon Leonis). Situated in the forward part of the
Sickle of Leo, the star's name should read
really read "AL RAS AL ASAD AL JANUBIYYAH," so named by the
Arabians, but that is a bit long for general use, and it is no
wonder it did not catch on. The phrase means (from Allen) "the
Southern Star in the Lion's Head," and a minor source of star names
simply reduced it to "Algenubi," the part of the name that refers
to "south." The name evokes "Zubenelgenubi," the "southern claw" of Scorpius (but in Libra), one of the most beloved of stars. Even
"Algenubi" is rare in use, so best to stick with "Epsilon," the
fifth letter of the Greek alphabet. And
indeed, at mid-third magnitude (2.98), the star ranks number five
in brightness, one of the few that actually follows the so-called
rule in which the Bayer Greek letters follow brightness. At a
distance of 250 light years, this yellow class G (G1) bright giant
radiates with a luminosity of 360 times that of the Sun from a surface with a temperature of 5300
Kelvin. From temperature and luminosity, we find that the star has
a radius 23 times solar. The angular diameter and distance give
the same thing, showing that the temperature is quite accurate, as
is the small adjustment for a bit of infrared radiation. Rotating
ponderously, with an equatorial speed of at least 5.7 kilometers
per second, the star takes (at most) some 200 days to make a
complete turn. With no companion to keep it company, this four-
solar-mass star is a relatively rare transition star, one with a
dead helium core that is making the quick jump between the
hydrogen-burning "main sequence" of stars and the true red giants.
It seems to have a "Cepheid-like"
variation of about a tenth of a magnitude over a few-day period.
Epsilon Leonis started as a hot class B (B4) dwarf some 165 million
years ago, and does not have much time left to it. For all the
touted rarity, there do seem to be a number of such stars,
including Capella-B and Muscida. That they seem to be relatively
common is an effect of "observational selection." Like ultra-rare
hot O stars and huge red supergiants, they can be seen over large
distances and thus seem more numerous than they really are. The
most numerous kinds of stars, the M dwarfs like Proxima Centauri, are so dim that none is visible to
the naked eye, and without telescopic study seem to be not there at
all. Thanks to Matthew Branham, who helped research this star.