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.