Chi Cygni is a Mira-type variable star 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, χ Cyg has a luminosity of roughly 3000 suns. At maximum, its cool, reddish surface, at 3000 K or below, has expanded to a diameter 300 times the Sun's - about the size of the orbit of Mars!
Evolution and Chemistry
Mira variables are all "second ascent giant stars." Having finished fusing helium into carbon and oxygen, they are slowly increasing in size and luminosity. When they get large enough, they begin to pulsate, massively change in brightnesses, and drive powerful winds from their surfaces. The expelled layers will eventually become luminous shells of gas called "planetary nebulae", while their old nuclear-fusing cores will become dead carbon-oxygen white dwarfs.
χ Cyg is a rather rare class 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. This results in class C carbon stars like Y Canum Venaticorum. In S stars, the carbon content about equals the oxygen content. S stars are also highly enriched in zirconium, the result of nuclear processing, which gives them their special spectral characteristic of strong absorptions of zirconium oxide. χ Cyg is thus a nascent carbon star.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]