XI CYG (Xi Cygni). Xi Cygni (Xi the 14th letter of the Greek alphabet) is one whopper of a star, moreover one with an unusual spectral class. Back to basics first. Xi shines at the bright end of fourth magnitude (3.72) within Cygnus, the Swan, a few degrees to the east of much brighter Deneb, the two stars bookending the famed North America Nebula. Though orange class K giants are quite common, cooler class K supergiants (this one K4.5) are not. The relative faintness to the eye comes from the
First magnitude Deneb (Alpha Cygni) shines at far right, while fourth magnitude Xi Cygni lies at the far left edge just below center. Note the color contrast: Deneb is a white class A supergiant, Xi Cyg a much cooler mid-class K orange supergiant. In between (and closer to Xi) is the faint reddish glow of the North America Nebula, NGC 7000; the fainter Pelican Nebula is down and to the right. The two stars between Deneb and Xi (from right to left 56 and 57 Cygni) point upward to a fuzzy patch within the North America Nebula, a distant star cluster called NGC 6997. See the labelled version.
star's large (and somewhat uncertain) distance, measured at 1200 light years. There are two quite diverse temperature determinations, 4007 and 3490 Kelvin. Since the warmer value fits better with that expected for the class, it will be adopted here. Using the temperature to estimate the amount of infrared radiation, and of course the distance, the star is found to shine with the luminosity of 9400 Suns, which gives a radius of 200 solar, or 0.94 Astronomical Units. Direct measure of angular diameter coupled with distance gives 1.04 AU, essentially the same value, both showing that the star is as big as the orbit of the Earth. An extended atmosphere makes the star 30 percent bigger. As a stable supergiant, Xi Cyg has to be supported through the internal fusion of helium into carbon and oxygen, its mass between 8 and 10 times that of the Sun depending on the exact state of evolution. Three other characteristics vie for attention. Though clearly a star that belongs to our part of the Galaxy, Xi Cyg is metal- deficient, having only 35 percent the iron content (relative to hydrogen) of the Sun. And despite its great distance, there is no measurable interstellar dust absorption along the line of sight, the star appearing in a sort of clear Galactic "window." The equatorial rotation speed has also been measured at 3.4 kilometers per second, though that is a lower limit as the axial tilt is not known. If the star's axis is perpendicular to the line of sight, Xi Cyg takes eight years to make a full rotation! Having begun life as a hot class B dwarf (around class B1) only 30 or so million years ago, Xi Cyg is clearly on its way out. Right at the boundary of masses that will become either massive white dwarfs (below 8 or 10 solar) or blow up as "core-collapse" supernovae (above that limit), its fate is uncertain. However it ends, since the star is still fusing its internal helium, we still have some time left to admire it.
Written by Jim Kaler. Return to STARS.