MESARTHIM (Gamma Arietis). Shining third among the stars of the flat triangle that make the classical figure of Aries, the Ram (for that reason gaining the Gamma designation), Mesarthim actually takes fourth place in the constellation after non-named 41 Arietis, which glows softly off to the northeast. The name (derived from Arabic) originally came from the same root as that for the Beta star, Sheratan (meaning "the two"), but was corrupted by mistranslations into its current form. Famed from history, Mesarthim is also called "the first star of Aries," as during the ancient times when the stars were being systematically organized, 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.) Shining at us from a distance of 204 light years, Mesarthim is one of the classic double stars of the sky, its two components, of nearly equal brightness, an easily separable 8 seconds of arc apart and known since 1664. Both actually white, the (just slightly) fainter one (Gamma-1, since it is the more westerly) has been called "pale grey," which is a visual contrast effect. Both fifth magnitude (Gamma-1 4.83, Gamma-2 4.75), they combine to make a mid fourth magnitude (3.9) star. Gamma-1, a class B (B9) dwarf, is the hotter, its temperature 11,000 Kelvin. Gamma-2, a bit controversial, has been classed both as an A (A1) (probably) dwarf and as a B (B9.5) subgiant (meaning that it may be starting to evolve), the temperature between 9200 and 9800 Kelvin. Though Gamma-1 is a bit dimmer to the eye, it is actually the more luminous, as the higher temperature causes more of its light to shine in the invisible ultraviolet. Gamma 1, around 2.8 solar masses, radiates 56 or so solar luminosities into space, whereas Gamma-2 (around 2.5 solar masses) releases somewhere between 43 and 52 times the power of the Sun. While Gamma- 1 is relatively ordinary, Gamma-2 is an "Ap" star,the "p" standing for (spectrally) "peculiar." It is now known that such stars are actually highly magnetized, Mesarthim-2's magnetic field roughly 1000 times the strength of Earth's. Like the prototype, Cor Caroli-2, the magnetism is concentrated into zones in which it aids in the separation of chemical elements (Gamma-2 notably high in silicon). As the star rotates, these concentrations swing in and out of view, allowing the 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, Gamma-2 having the distinction of being the first to be so treated. The stars of the pair are separated by at least 500 astronomical units (over a dozen times Pluto's distance from the Sun), and take at least 5000 years to orbit each other.
Written by Jim Kaler. Return to STARS.