AZHA (Eta Eridani). Eridanus, the great
celestial River, has its source at Cursa
near Rigel, then flows more or less to the
east. Suddenly, at Azha (to which Bayer gave the Greek letter Eta), Eridanus makes a sharp
bend toward the south toward its first and ancient end at Acamar (Theta Eridani), then at its more
modern finish at Achernar (Alpha
Eridani). One might think that the word "Azha" would have
something to do with the steep southerly plunge. Instead, the name
comes to us as a near hopeless corruption of the name for an Arabic
asterism that means "the Ostrich's Nest" and that includes a whole
gang of stars (Zeta, Rho-1, Rho-2, Eta, the various "Taus," and a
couple stars in nearby Cetus.) rather than just one. Azha is a
classic orange class K (K1) giant star whose characteristics fall
almost exactly between those of Arcturus and Pollux. At fourth magnitude (3.89) it is
not nearly so well known as these two only because it is
considerably farther away, 133 light years. Accounting for some
infrared radiation from its 4650 Kelvin surface, the star radiates
a power 59 times the Sun's, its radius 12
times solar, the mass between 1.7 and 2 times solar. Like all such
orange giants, Azha is dying, its core hydrogen fuel having run
out. As a "clump star," however, it is in a relatively stable
period in which it is now converting its core helium into carbon
and oxygen, the core surrounded by a shell of fusing hydrogen. The
chemical compositions of giant stars are of considerable
importance, as they tell both something of the star's birth history
(and hence how the Galaxy developed) and of any internal nuclear
processing whose by-products have been brought to the surface.
Here there is argument, some authors seeing Azha as slightly metal-
deficient, others as a bit metal-rich, the average being close to
solar. There is some slight evidence that even-numbered chemical
elements are a bit high relative to what is found in the Sun,
suggesting that Azha has a somewhat different heritage than does
the Sun (as do most of the nearby stars). Azha has also been
listed as a "mild" or "semi" barium star. Such stars have not
enriched themselves, but aeons ago were enriched in heavy elements
through mass transfer from a more massive companion that is now a
white dwarf. The seeming barium enrichment is so small, however,
that it is probably a false reading, and no companions at all are
detected, the star remaining a near-perfect paradigm of its class.