DENEB ALGEDI (Delta Capricorni). As the Sun leaves the winter solstice and
Sagittarius behind, it moves into its next zodiacal station,
Capricornus,
the "water goat, and in early February crosses between
the figure's head and tail, between
Algedi (Alpha Capricorni) and
our star Deneb Algedi, the Delta star. "Deneb" in Arabic refers to
"tail," the name found in several constellations including Deneb
itself in Cygnus, Denebola (the Lion's tail), Deneb Kaitos (the Sea
Monster's tail), and here the "Kid's tail." The star is a
fascinating, confusing wonder, and astronomers seem unsure of just
how to classify it. It is a white star with a temperature of 7700
Kelvin, placing it (like Vega) among the "A star" class. Deneb
Algedi is a mid-third magnitude (2.87) star 39 light years away, from
which we calculate a luminosity 8.5 times that of the
Sun. It is
not alone, however, but has a companion of unknown type that
eclipses it every 1.023 days, causing the apparent brightness to
drop by about 0.2 magnitude, just enough to be seen (with care)
with the naked eye. (There are two other very faint and more
distant companions one and two minutes of arc away.) It has been
variously called a "main sequence star" that, like the Sun, lives
off the ordinary fusion of hydrogen into helium, a giant star, one
that has ceased such fusion and has expanded to larger proportions,
and a "subgiant," an intermediate phase. Most likely it is in its
last stages of ordinary solar-type life. Its greatest claim to
distinction is that it is among the brightest of the "metallic A
stars," those hotter stars that seem to be highly enriched in most
metals yet have deficiencies in others like calcium. Theories for
the odd chemistry include contamination from a companion that, in
the process of dying, passed enriched matter to the surviving star.
More likely, some process has separated various chemical elements,
causing some to drift downward, others to rise upward. On top of
all this, Deneb Algedi is suspected of being slightly variable all
on its own, of a type called a "
Delta Scuti" star, one that begins
to pulsate in part as a result of the onset of its impending
demise. With all these features, the star has spawned a small
industry of research and publication, and a great deal remains to
be learned about it.