Mahasim (Theta Aurigae)

Mahasim (Theta Aurigae) is a star in the constellation Auriga, magnitude 2.6.

A magnetic star with a patchy, silicon-rich surface, at the Charioteer’s wrist.

Mahasim’s surface is a patchwork. This hot white star has a strong magnetic field and an atmosphere strangely rich in silicon, with the unusual elements gathered in patches. As it turns, they swing into and out of view, nudging its brightness up and down by less than a tenth of a magnitude — making it a variable star.

The star has more than three times the Sun’s mass, nearly five times its width and over 200 times its luminosity. A fainter, cooler F-type star keeps it company.

Its name is a late arrival. Some authors say the star had no traditional name at all; the IAU adopted Mahasim in 2017 from a passing mention by the star-name historian Richard Hinckley Allen. It is Arabic for “the wrist” — the Charioteer’s.

Mahasim (Theta Aurigae) is 170 light-years away: the light we see from it set off around 1855, 10 years after Neptune was discovered.

Did you know?

• Fourth of the Five Chariots. In Chinese astronomy Mahasim is the fourth star of the Five Chariots, a pattern in Auriga that also includes Hassaleh.
• One name, three stars. Versions of Mahasim, “the wrist”, were also used for Eta Aurigae and for Lambda Herculis, far away in Hercules.

A white star of magnitude 2.6 on the eastern side of Auriga’s pentagon, about 8 degrees south of Menkalinan. A fine test for a small telescope: at high power on a steady night, look for its 7th-magnitude companion about 4 arcseconds to the north-west.

What is a star?

When and where to see it

It is best placed in the evening sky around January, when it crosses the meridian at about 10 pm local time (around midnight in December). It can be seen from everywhere north of 53°S and never sets north of 53°N.

At its highest it stands 77° up from 50°N, 88° up from 35°N, 53° up from the equator, 18° up from 35°S.

Facts

Type

Star

Constellation

Auriga

Designations

Theta Aurigae, θ Aur, 37 Aur, HR 2095, HD 40312, HIP 28380

Magnitude

2.6

Position

RA 5h 59.7m, Dec +37° 13′ (J2000)

Spectral type

A0VpSi

Colour

white (B−V -0.08)

Distance

170 light-years

Discovered

Known since antiquity

Open AstroPilot to see when Mahasim (Theta Aurigae) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.