Theta Aurigae

Lore & Background

The system's Bayer designation is θ Aurigae, Latinised to Theta Aurigae. The two components are designated Theta Aurigae A and B, following the convention of the Washington Multiplicity Catalog. The primary, Mahasim, derives its name from the Arabic al-miʽşam, meaning 'the wrist' (of the charioteer), a name also used with various spellings for Eta Aurigae and Lambda Herculis. In Chinese astronomy, it is known as 五車四, the Fourth Star of the Five Chariots.

The primary is a large A-type star with a stellar classification of A0pSi, indicating an abnormal abundance of silicon. It is an Alpha2 Canum Venaticorum type variable star with a surface magnetic field of about 1 kG, completing a rotation in 3.6 days. The secondary, Theta Aurigae B, is an F-type main sequence star (F2-5 V) with magnitude +7.2, located at an angular separation of 3.91 arcseconds along a position angle of 304.9° as of 2002. The system is an X-ray source.

Reader's Guide

Theta Aurigae holds significance as a well-studied example of an Alpha2 Canum Venaticorum variable, a class of chemically peculiar stars with strong magnetic fields and rotational modulation of their light curves. Its primary, Mahasim, is one of the brighter members of this class, with a short rotation period of 3.6 days that drives its 1.37-day brightness variation. The system's binary nature, with a fainter F-type companion at a moderate separation, allows for detailed study of stellar properties and evolution in a multiple-star context. The star's proper name, Mahasim, was formally recognized by the International Astronomical Union in 2017, linking modern astronomy to historical Arabic nomenclature. Its inclusion in Chinese star lore as the Fourth Star of the Five Chariots reflects its cultural significance across traditions. The system's X-ray emission adds to its astrophysical interest, and its relatively close distance of about 166 light-years makes it accessible for observational studies of magnetic activity and chemical peculiarity in A-type stars.

Did You Know?

• The primary component, Mahasim, has a rotation period of only 3.6 days.
• The system's brightness varies from magnitude +2.62 to +2.70 with a period of 1.37 days.
• Theta Aurigae is an X-ray source.
• The secondary star is an F-type main sequence star with a classification in the range F2-5 V.