Mesarthim, or γ Arietis, shines third-brightest among the triangle of stars that make the classical figure of Aries, the Ram (although it actually takes fourth place in the constellation after 41 Arietis). The name Mesarthim, derived from Arabic, originally came from the same root as that for Sheratan or β Ari (meaning "the 2"), but was corrupted by mistranslations into its current form.
Mesarthim is also called "the first star of Aries," since during the ancient times, it was the closest of the Ram's stars to the vernal equinox. Precession, the 26,000 year wobble of the Earth's axis, has since shifted the equinox westward to Pisces.
Properties
Shining from a distance of 204 light years, Mesarthim is one of the classic double stars of the sky, known since 1664. Its two components, of nearly equal brightness, are an easily-separable 8" apart. The slightly fainter, westerly component (γ1 Ari) has been called "pale grey," which is a visual contrast effect; both are actually white. Both components are fifth magnitude (γ1 at 4.83, γ2 at 4.75), and have a combined magnitude of 3.9.
γ1 Ari, a class B9 dwarf, is hotter, with a temperature of 11,000 K. γ2 has been classed as both an A1 dwarf and a B9.5 subgiant, with a temperature between 9200 and 9800 K. Though γ1 is a bit dimmer to the eye, it is actually the more luminous, as its higher temperature causes more of its light to shine in the invisible ultraviolet. γ1, around 2.8 solar masses, has a luminosity of 56 times solar, whereas γ2, around 2.5 solar masses, is between 43 and 52 solar luminosities.
γ2 Ari is an "Ap" star, the "p" standing for spectrally "peculiar." Such stars are actually highly magnetized; γ2 Ari's magnetic field is roughly 1000 times the strength of Earth's. The magnetism is concentrated into zones which aid in the separation of chemical elements (γ2 is notably high in silicon). As the star rotates, these concentrations swing in and out of view, allowing its rotation period to be found - in this case, 1.609 days - and causing subtle visual variations.
Sophisticated spectroscopic examination and measures of Doppler shifts allow astronomers to create "pictures" of the surfaces of such stars. γ2 Ari has the distinction of being the first to be so treated. The stars of the pair are separated by at least 500 AU, and take at least 5000 years to orbit each other.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]