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-type long-period variable (LPV) that
can reach magnitude 3.5 for some time during its 407-day variation
period, and then plummet to magnitude 14.2, some 1500 times fainter
than can be seen with the naked eye. Within the Milky Way, the disk of our Galaxy, Chi Cyg lies at a distance
of 590 light years (with a large uncertainty of 120 light years; second
Hipparcos reduction). 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 are brightening with dead
carbon-oxygen cores at their centers. When they get large enough,
they begin to pulsate and vary in magnitude as they 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
cores will turn into a dead carbon-oxygen white dwarfs like Sirius B and Procyon
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 and R Leporis. S stars are in the middle, at the
point where the carbon content about equals that of oxygen. This
balance point in oxygen/carbon abundance along with their enrichment
in zirconium, the result of deep nuclear processing, gives S stars
their special spectral characteristic of strong absorptions of
zirconium oxide rather than the usual titanium oxide seen in the more
ordinary class M stars. That such nuclear processing is going on is
proven by absorptions of technetium, which has a very short half-life
(there is practically none on Earth). Chi Cyg is thus a nascent carbon
star that is on its way to its death, its surrounding ejected cloud
the source of a strong silicon monoxide maser (the microwave version
of a laser). Temperature measures of carbon and S stars, hence the
usual calculation of luminosity from temperature and distance, are
difficult. Besides, we don't know with any accuracy how much the star
is dimmed by interstellar dust
(though probably not by much). Interferometer measures give a
whopping average radius of 2.2 Astronomical Units, 470 times that of
the Sun, almost half again the size of the orbit
of Mars. Adoption of 3000 Kelvin or below indicates a luminosity of
at least 16,000 Suns. The mass has been estimated at between 2 and
3 times solar. The pulsation causes the star to vary over its cycle
from as "small" as 1.3 AU to as large as 3.1 AU, with concomitant
variations in luminosity and temperature, which causes much of the
optical variation as a result of radiation emitted in and out of the
infrared. Chi Cygni is being closely tracked by a 12th magnitude
(11.85) possible companion
separated by 2.5 minutes of arc. If it is really gravitationally bound,
the more or less sunlike star orbits at least 28,000 AU away and takes
more than 2.7 million years to make a circuit. (Thanks to S. Lacour
et al. in the Astrophysical Journal, 707, 632, 2009.)
Written byJim Kaler 9/27/03; revised 9/26/14.
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