ELTANIN (Gamma Draconis). Because it was the pole star during a
time of ancient Egypt, Thuban is the most
famed star of Draco, the Dragon, and justly
received Bayer's Alpha designation even though at bright fourth
magnitude it is hardly the constellation's brightest star. That
honor goes to bright second magnitude (2.23) Eltanin in the
Dragon's head, which nevertheless was called Gamma even though
notably brighter (by half a magnitude) than the other bright star
of the head,
Rastaban, which became Beta. The star's importance,
however, is seen in its name, which stands for the whole
constellation, from Arabic meaning "the serpent." The star is
prominent for several other reasons as well. It is the closest
bright star to the "winter" side of the "solstitial colure," the
circle in the sky that passes through the poles and winter and
summer solstices, the star lying almost exactly 75 degrees north of
the winter solstice in Sagittarius.
Eltanin's high northerly position also takes it nearly through the
zenith, the point overhead, as seen from London, causing it to
acquire the now obscure name "zenith star." As a result the star
was heavily studied. In attempting to find stellar parallax, the
annual shift in stellar position caused by the shifting position of
the orbiting Earth (from which we get stellar distance), in 1728
James Bradley discovered in "aberration of starlight," which is
caused by the velocity of the moving Earth relative to the speed of
the light coming from the star. The discovery once and for all
proved that Copernicus was right and that the Earth truly does move
around the central Sun. Eltanin is also moving toward us, and will
make a close pass at a distance of 28 light years 1.5 million years
from now, when it will be the brightest star in the sky and will
rival our current Sirius. Physically,
Eltanin is a cool (4000 Kelvin) class K (K5) orange giant shining from
148 light years away with a luminosity 600 times that of the Sun, its only marked characteristic a slightly
low iron abundance. Calculations from the temperature and
luminosity as well as from the measured angular diameter agree that
the star is 50 times the solar diameter, a bit over half the size
of Mercury's orbit. As a giant it is dying, its days of core
hydrogen fusion long over. Beginning life as a star with a mass
about 1.7 times that of the Sun, it is now probably slowly
increasing in brightness as it prepares to fire its internal
helium.