THETA AUR (Theta Aurigae). Within the confines of Orion, Gemini, Taurus, Auriga, and Perseus, there are so many bright stars that even those that would be singled out anywhere else in the sky get little respect at all. Third brightest in the constellation (fourth if you count the Taurus- linking star Elnath, which is both Beta Tauri and Gamma Aurigae), Theta Aurigae, which has no proper name, is still rather far down the Greek letter list, Bayer preferring the three "Kids" near Capella. The star is the southernmost of the trio that outlines the Solstitial Colure, the full story told by Delta Aurigae. Theta Aur is a white class A (A0) star with a surface temperature of 10,200 Kelvin. From its distance of 173 light years, it shines with a luminosity 285 times that of the Sun, which gives it a radius of 5.4 solar. It is one of the sky's "Ap" or "A-peculiar" stars, which in the jargon of astronomy means that it is strongly magnetic and has very odd chemical abundances. Theta Aur has a variable magnetic field that can be as much as a thousand times or so stronger than Earth's. The magnetic field is not aligned with the rotation axis, but wobbles with the 3.61-day rotation period of the star. Slowly rotating dwarf stars (those fusing hydrogen into helium in their cores) have quiet atmospheres in which some chemical elements settle under the force of gravity, whereas others are pushed upward as a result of the pressure of radiation. Theta Aur is especially strong in the elements silicon, which can be as much as 10 times more abundant than in the Sun, and chromium, which can be up by 100, iron overabundant as well. The odd overabundances of the elements are concentrated into patches that involve the magnetic fields (which seem to help still the stellar gases in stars that are still rather rapidly rotating) and wobble in and out of sight. Theta Aur is blessed with a small solar-class (G2 dwarf) companion that lies at least 185 Astronomical Units from Theta Aur proper (over 4 times Pluto's distance from the Sun). Given a mass of 3.5 solar and a mass of one solar for the companion, the two take at least 1200 years to orbit. Both stars are too bright for their classes. The brighter component has the luminosity of a somewhat advanced subgiant (a star that has given up core hydrogen fusion or is about to do so). The companion, however, seems similarly advanced, which is not possible given the timescale of evolution for the brighter star. Quite possibly the distance is overestimated.
Written by Jim Kaler. Return to STARS.