ALCHIBA (Alpha Corvi). It is standard "knowledge" in astronomy that "Alpha" represents the brightest star in a constellation, "Beta" the second brightest, and so on. While such is often true, the rule is as much broken as held to, sometimes dramatically. Alchiba, the Alpha star of Corvus, the Crow, is a fine example. One wonders what Johannes Bayer, who lettered the stars in his great "Uranographia" of 1603, had in mind. The name, which from Arabic refers to a "tent" and is meant to describe the four fairly bright stars that make the distorted box of Corvus, is now erroneously applied to dim Alpha, which drops down from the right-hand side of the box and is outstripped by Beta, Gamma (Gienah), Delta (Algorab), and even Epsilon. Of the Alpha stars of the classical constellations, Alchiba, at mid-fourth magnitude (4.02), ranks number 3 from the bottom, beaten out (if that is the word) only by Alpha Crateris (Alkes) and Alpha Coronae Australis (Alfecca Merdiana). (Of all Alpha stars, including the modern constellations, the dimness record goes to fifth magnitude and un-named Alpha Octantis, in Octans, the Octant, the constellation that contains the South Celestial Pole.) Classification of Alchiba has been a bit confused. Once considered a giant and now given as a class F (F0) dwarf or subdwarf, its luminosity (only four times that of the Sun) and temperature (7000 Kelvin) strongly suggest an ordinary hydrogen-fusing dwarf, in fact a near-subdwarf that shines less brightly than other stars of its temperature class. Quite close to us, at a distance of only 48 light years, if the star were only 3 times farther away, it would be invisible to the naked eye. It is more similar to the Sun than it actually appears, its mass only about 1.2 times solar, just younger and hotter. Subdwarfs are not really too faint for their temperatures, but too hot for their luminosities, the result of low metal contents in their atmospheres. While clearly not one of the classical subdwarfs (which have quite low metal contents), the temperature-luminosity status of Alchiba is consistent with a fairly low iron abundance (relative to hydrogen, which makes 90 percent of the outer layers of nearly all stars) of 25 percent that of the Sun. There is some evidence from its spectrum that the star has a close binary companion, though nothing at all is known about it. Alchiba will in 10 billion or so years die as a common, relatively low-mass white dwarf shrunken to the size of Earth.
Written by Jim Kaler. Return to STARS.