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.
Written by Jim Kaler. Return to STARS.