ZETA CAP (Zeta Capricorni). Dipping into the eastern lower half of
mostly faint Capricornus, find the modest
fourth magnitude (3.74) "yellow" class G (G4) supergiant ("yellow" referring
not so much to actual color but to the part of the spectrum where
much of its light emerges) Zeta Capricorni. The "supergiant" tag
implies that the star must be fairly distant to shine so faintly,
and sure enough, it measures 122 light years away. The temperature
of 5050 Kelvin (not that much cooler than our Sun) allows for a
small correction for some infrared light, which with distance
yields a luminosity 490 times that of the Sun, and a resulting
radius 29 times solar. These are distinctly smallish values for a
star called "supergiant," and more like those of a "bright giant."
Since the term comes from simple examination of the spectrum, however, it remains in use,
showing that some stars are not quite what they are at first made
out to be. Combination of temperature and luminosity suggest a
star of about four solar masses, which makes it under 200 million
years old, one that is now most likely fusing helium into carbon in
its deep core and will "soon" (on an astronomical timescale) begin
to brighten once again to close to real supergiant status. The
interest in Zeta Cap lies not in these characteristics, however,
but in its chemical composition and in its classification as a
"mild barium star," one rich in that heavy element, and rich in
other elements as well that are created under high temperature
conditions through capture of neutrons (Zeta Cap enriched in such
elements as zirconium and the "rare earths" by a factor of 10). In
some stars, chemical enrichment comes from interior nuclear
reactions, the newly-formed elements raised upward by convection.
In others, however, they come from contamination from a companion
that has since died and left a legacy of its own nuclear reactions
brought to the current giant star through mass exchange. Zeta Cap
and all other barium stars (epitomized in the naked-eye sky by Alphard) fall into the latter category, the
earlier (once more massive and brighter) star now reduced to a dead
cinder, a white dwarf.
While the white dwarf is unseen, measures of Zeta Cap's spectrum
(through the Doppler effect) reveal a companion in a 6.5 year
period at a separation of around 6 Astronomical Units. Another
"companion," a 12th magnitude star at a separation of some 20
seconds of arc, is also a white dwarf, but the alignment is merely
a line-of-sight coincidence, again showing that not all things are
what they at first seem to be.