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HAMAL

(Alpha Arietis). Climbing the eastern sky in northern Autumn, Aries presents a small flat triangle below Andromeda dominated by mid-second-magnitude (2.00) Hamal. The name, directly from Arabic, means "the lamb," rather obviously standing in for the whole constellation of the Ram. The vernal equinox, the point, where the solar path crosses the celestial equator, is in Pisces south of the west edge of the Great Square of Pegasus. Over 2000 years ago, however, it was in Aries, having shifted westward into Pisces because of precession, the 26,000 year wobble of the Earth's axis. Around the time of Homer, the equinox was situated only nine degrees south of Hamal, allowing us to personalize the time that has passed since the Iliad and Odyssey were written. True to formal form, Bayer called Hamal the Alpha star and the second brightest (Sheratan) Beta, but then quite broke down as the third brightest (Flamsteed 46) has no Greek letter at all. Hamal is a warmish class K orange giant star. At a distance of 66 light years, it shines (allowing for infrared radiation) 90 times more brightly than our Sun. From this figure and an accurate temperature of 4590 Kelvin, the star is calculated to be 15.0 times the solar diameter. Hamal is characterized mostly be being so very normal, at least as far as a dying giant about twice the solar mass can be. Very little about it is unusual except for a mild deficiency in metals compared with the Sun. That in itself makes it rather valuable as a comparison for other stars. It has the honor of having (along with Shedar, Alpha Cassiopeiae) the most accurately-measured angular diameter, 0.00680 seconds of arc (a penny 60 kilometers away), from which we find another value for true diameter of 14.7 solar, beautifully in accord with that found from temperature and luminosity. This precise measurement allowed the detection of Hamal's "limb darkening." As a gaseous sphere, the Sun is slightly darker at the edge (its "limb") than at the center, the result of our not looking as deeply into the solar gases. Similar limb darkening can be detected in the members of eclipsing stars as they get in front of each other. Hamal is one of the select few single stars for which limb darkening has been seen, showing that we are not far from observing features on the surfaces of at least some of our stellar neighbors.