MU AUR (Mu Aurigae)

MU AUR (Mu Aurigae). Fifth magnitude (4.86, almost fourth) Mu Aurigae sits in somewhat lonely splendor in western Auriga within the confines of the main pentagonal figure two or three degrees south-southeast of the Kids. Perhaps "splendor" is too strong a word for the star. It does dominate a relatively dim area of sky, however, and is thereby quite easy to locate, which is worthwhile as the star has a number of things that recommend it. A white, class A (A4?) hydrogen-fusing dwarf with one cited temperature of 7560 Kelvin, Mu Aur's major claim to fame lies in its surface chemistry. It's a "metallic-line" star of the sort within class A whose atmospheres are greatly enriched in copper and zinc, even more in the "rare earths" such as europium, but depleted in calcium. The cause is elemental separation, wherein some heavy atoms fall under the force of gravity, while other kinds re raised up by the powerful radiation emitted by the star. The phenomenon works only when the outer stellar gases are relatively quiet and unstirred by circulation caused by rotation. And, ignoring axial tilt, Mu Aur accommodates by spinning at its equator by only 89 kilometers per second. While this may seem high as compared with 2 km/s for the Sun, it's nothing for an A dwarf, whose spin rates can go much higher. The odd metallicities make it difficult to classify such stars, as the classes are based upon strengths of various absorption features in the spectra. Standard classes are tied to solar chemical abundances, and when they go awry, you don't get a unique class. Mu Aur has thus been classed anywhere between A1 and F1 depending on which absorption lines are used. The temperature, low for A4, suggests that a cooler class might be more appropriate. Though only 153 light years away (give or take 6), there seems to be a small amount of dimming by interstellar dust, which might be a result of an erroneous spectral class. Nevertheless we adopt 0.22 magnitudes of interstellar absorption. In a way, that would not be all that surprising, as the star is almost at the anticenter of the Galaxy, with a galactic latitude just 0.3 degrees south of the galactic equator (which is based on the adopted centerline of the Milky Way). The galactic equator then goes northwest right between the Kids' Zeta and Eta Aurigae. With virtually no correction for infrared or ultraviolet radiation, Mu Aur shines with the light of 23.8 Suns, which yields a radius of 2.85 solar. From the projected equatorial rotation velocity, the star spins in under 1.6 days. Application of theory yields a modest mass twice that of the Sun, and indeed shows the star to be a dwarf that is probably nearing the end of its hydrogen-fusing lifetime of 1.1 billion years (a tenth that of the Sun's, showing how sensitive lifetime is to mass.) In 1986 a tiny companion was found 0.1 seconds of arc away from the main star. If it exists, the true separation is at least 4.7 Astronomical Units and the period more than 7.2 years.

Written byJim Kaler 05/19/17. Return to STARS.