Alpha Arietis (Alpha Ari, α Ari, α Arietis), which also has the traditional name Hamal, is the brightest star in the constellation Aries. Its Flamsteed designation is 13 Arietis, but this is very rarely used because it has a Bayer designation.
α Arietis has spectral type K2 IIICa-1, which means that it is a K-type giant star, slightly cooler than the Sun and much larger. The Ca portion of the spectral type indicates that it shows calcium lines in its spectrum. It is estimated to be approximately twice as massive as the Sun and 15 times larger in diameter, and has around 90 times the Sun's bolometric luminosity. The Hipparcos satellite indicates that α Arietis is about 66 light-years from Earth. Combined with its intrinsic brightness, this relatively small distance makes it shine at an apparent visual magnitude of about 2.00; it is the 48th brightest star in the night sky. It has been reported to be slightly variable, by about 0.06 magnitude.
Hamal's orientation with relation to the Earth's orbit around the Sun gives it a certain importance not apparent from its modest brightness. Between 2000 and 100 BCE, the apparent path of the Sun through the Earth's sky placed it in Aries at the vernal equinox, the point in time marking the start of spring. This is why most astrology columns in modern newspapers begin with Aries. While the vernal equinox has moved to Pisces since then due to precession of the equinoxes, Hamal has remained in mind as a bright star near what was apparently an important place when people first studied the night sky.
Stellar expert James Kaler notes that Hamal:
"...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."
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