Other designations
Bogardus, Maha-Sim, Mahasim, 37 Aurigae,BD+37 1380, HD 40312, HIP 28380,HR 2095, SAO 58636.

Theta Aurigae (θ Aur, θ Aurigae) is a binary star in the constellation Auriga. Rarely used proper names for this star include Mahasim, the latter from the Arabic المِعْصَم al-micşam "wrist" (of the charioteer), which was also used for Eta Aurigae.[8] It is known as 五車四 (the Fourth Star of the Five Chariots) in Chinese. Based upon parallax measurements, the distance to this star is about 166 light-years (51 parsecs).[1]

The primary component is a large star with more than three[5] times the mass of the Sun and over five[6] times the Sun's radius. It is radiating 263[5] times the Sun's luminosity from its outer atmosphere at an effective temperature of 10,400 K,[6] giving it the white hue of an A-type star. The star has a stellar classification of A0pSi,[3] with the 'pSi' suffix indicating it is a chemically peculiar star with an abnormal abundance of silicon.

The primary is classified an Alpha2 Canum Venaticorum type variable star and has a surface magnetic field of about 1 kG.[6] Its projected rotational velocity is 55 km s–1,[6] with the star completing a rotation in only 3.6 days.[7] The axis of rotation is inclined by an angle of 51◦ ± 6 to the line of sight from the Earth.[6]

There is a +7.2 magnitude companion, 4.5[3] magnitudes fainter than the primary, located at an angular separation of 3.91 arcseconds along a position angle of 304.9° as of 2002.[9] This is an F-type main sequence star with a stellar classification in the range F2-5 V.[3] The mean combined apparent magnitude of the system is +2.65 but the variation of the primary causes the system's brightness ranges from magnitude +2.62 to +2.70 with a period of 1.37 days. The system is an X-ray source with a luminosity of 9.49 × 1026 erg s–1.[3]