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.
Written by Jim Kaler. Return to STARS.