CHI CYG (Chi Cygni). Sometimes part of the neck of the giant bird,
Cygnus the Swan, sometimes not, Chi Cygni
is a giant-star Mira variable that can reach magnitude 3.5 for some
time during its 400-day variation period, and then plummet to
magnitude 14, some 1500 times fainter than can be seen with the
naked eye. About 300 light years away, Chi Cyg radiates the power
of roughly 3000 Suns from a cool, reddish surface (3000 Kelvin or
below) that has expanded to a diameter 300 times that of the
Sun,
or the about the size of the orbit of Mars! Mira variables are all
"second ascent giant stars." Having given up the helium fusion
(which transforms helium into carbon and oxygen) that powers more
ordinary giants, they are slowly increasing their sizes and
brightnesses with dead carbon-oxygen cores. When they get large
enough, they begin to pulsate and massively change their
brightnesses as well as to drive powerful winds from their
surfaces. The expelled layers will eventually become luminous
rings and shells of illuminated gas called "planetary nebulae",
while the old nuclear-fusing core will become a dead carbon-oxygen
white dwarf like Sirius B. What makes Chi Cyg so fascinating is
that it is a rather rare class S (S6) star. As Mira variables
develop, they can dredge freshly made carbon from their interiors
to their surfaces, which makes carbon more abundant than oxygen
(the reverse of what we see in normal stars like the Sun and class
M Miras like Mira itself), resulting in class C carbon stars like
Y Canum Venaticorum. S stars are in the middle, at the point where
the carbon content about equals the oxygen content. They are also
highly enriched in zirconium, the result of nuclear processing,
which gives them their special spectral characteristic of strong
absorptions of zirconium oxide. Chi Cyg, one of the "Hundred
Greatest Stars," is thus a nascent carbon star.