At magnitude 2.00, Hamal dominates the northern autumn constellation of Aries, the Ram. Its name, directly from Arabic, means "the lamb".
More than 2000 years ago, the vernal equinox - the point where the Sun's path crosses the celestial equator - was in Aries. It is currently in Pisces, having shifted westward because of precession (the 26,000 year wobble of the Earth's axis). Around the time of Homer, when the Iliad and Odyssey were written, the equinox was situated only nine degrees south of Hamal.
Properties
Hamal is a warmish class K orange giant star. At a distance of 66 light years, it shines 90 times more luminously than our Sun. From this figure, and an accurate temperature of 4590 K, the star's diameter is calculated to be 15.0 times the Sun's. Hamal is a very normal giant with about twice the Sun's mass. Very little about it is unusual, except for a mild deficiency in metals compared with the Sun.
Because it is so "normal", α Ari is rather valuable as a comparison for other stars. It has the most accurately-measured angular diameter of any star, 0.00680" - the apparent size of a penny 60 kilometers away. From this we find another value for its true diameter: 14.7 times solar, beautifully in accord with that found from its temperature and luminosity.
This precise measurement of Hamal's disk allowed the detection of its "limb darkening." As a gaseous sphere, the star is slightly darker at the edge or "limb" than at its center, the result of our not looking as deeply into the stellar gases. Limb darkening can be detected in the components of eclipsing binary stars as they pass in front of each other, but Hamal is one of the few single stars for which limb darkening has been seen directly.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]