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.