At magnitude 2.23, Eltanin (also spelled Etamin) is actually the brightest star in Draco, despite its Bayer designation of γ Dra. It outshines Rastaban (β Dra), the other bright star in the Dragon's head, by nearly half a magnitude. In fact, the name Rastaban was also formerly used for Eltanin, and the two terms share an Arabic root meaning "serpent" or "dragon."
Observations
Eltanin is the closest bright star to the "winter" side of the "solstitial colure," the great circle in the sky that passes through the poles and winter and summer solstices. Its proximity to the point directly overhead as seen from London also earned it the name "Zenith Star." As a result, the star was heavily studied. In 1728, while unsuccessfully attempting to measure the parallax of this star, James Bradley discovered the aberration of light, resulting from the motion of the Earth relative to the speed of the light coming from the star. Bradley's discovery proved Copernicus' theory that the Earth revolved around the Sun.
Properties
Physically, Eltanin is an orange giant of spectral class K5 III, with a cool (4000 K) surface, shining from 148 light years away with a luminosity of 600 suns. Calculations from temperature and luminosity, as well as from the measured angular diameter, agree that the star is 50 times the Sun's diameter. Beginning life as a star with a mass of about 1.7 suns, its days of core hydrogen fusion are long over. As a dying giant, it is now probably slowly increasing in brightness, as it prepares to fuse its internal helium. Its only marked spectral characteristic a slightly low iron abundance.
Eltanin is also moving toward us, and in 1.5 million years, will pass within 28 light years of Earth. At this point it will be the brightest star in the night sky, nearly as bright as Sirius.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]