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 41)
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 by
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.