CHI AUR (Chi Aurigae). Located in a complex region in central Auriga four degrees north-northeast of Elnath (Beta Tauri, historically also known as Gamma Aurigae) and about the same angular distance north-northwest of the galactic anticenter (opposite the direction to the central black hole in Sagittarius), this fifth magnitude (4.76) class B (B5) mid-level supergiant (some say a B4 lesser supergiant) presents something of a mystery. The problem is that we have little idea of the star's distance. About all we know is that it's massive enough to blow up someday as a supernova. There is no measured parallax; what value exists is dwarfed by the statistical error, so it's useless. The distance is, however, great enough so that interstellar dust considerably reddens the star (dust absorption being more efficient at shorter optical wavelengths) to the point that instead of a gleaming blue white it takes on the more yellow-white color of a class F star, from which we estimate that the dust dims Chi Aur visually by as much as 1.7 magnitudes, a factor of five! Were the line of sight clear, Chi would shine at third magnitude (3.0), probably be part of the constellation outline, and maybe even have a proper name. Though the exact amount of dimming is controversial, it's enough that the star presents a good background with which to study the spectrum of the intervening interstellar medium, which includes the "diffuse interstellar bands" that have intrigued astronomers for decades. Even the temperature, which hovers around 14,100 Kelvin, is insecure. Chi Aurigae is listed as a member of the Auriga OB1 association of hot O and B stars, whose central distance is given as 4300 light years and which we might apply to the star, a dangerous procedure as OB associations are such vast assemblies. Straightforward calculations (which include the addition of ultraviolet radiation) get us a whopping luminosity of 230,000 times that of the Sun, which leads to a radius of 80 times solar, a rotation period (based on a projected equatorial speed of 40 kilometers per second) of under 100 days, a huge mass 25 times that of the Sun, and an age of 6.4 million years. There is a suggestion that the open cluster Messier 38 belongs to the association. If its distance of 3500 light years is used along with the minimum temperature and dust-dimming, we get a lesser luminosity of 90,000 Suns, 60 solar radii, a rotation period of under 75 days, and a mass of 18 Suns, still plenty enough for the star to blow. Playing around with the details does not buy us much. At the most extreme, we can drop the luminosity to 41,000 Suns and the mass to 14 Suns, making it still a supernova candidate. But the calculated age never comes up to that estimated for M 38, suggesting that the star and cluster are not part of the same set. For all the uncertainties, Chi Aur does have a well-observed spectroscopic companion with an orbital period of 1.79 years, which from Kepler's Laws gives an orbital radius in the neighborhood of four Astronomical Units. Even with all the interstellar obscuration, were the star to explode, it would be shine some 10 magnitudes (a factor of 10,000) brighter and could rival the light of a crescent Moon.
Written byJim Kaler 1/15/16. Return to STARS.