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.
Written by Jim Kaler. Return to STARS.