DENVER, COLORADO--Polaris, the famed North Star that most students can identify after their first astronomy lesson, is proving to be an unreliable beacon. Despite its fabled constancy, the star has brightened by a startling 150% in the last 2000 years, according to research presented here 31 May at a meeting of the American Astronomical Society. Moreover, although Polaris pulsates every 4 days, the fluctuations have become weak and unpredictable.
Polaris has an undeserved reputation for steadiness. Because Earth's axis of rotation points very close to Polaris, all other stars in the Northern Hemisphere appear to revolve around it. Shakespeare referred to this celestial signpost in his classic line for Julius Caesar, "But I am as constant as the Northern Star," but astronomers know that's misleading. Polaris is a "Cepheid variable," a giant star near the end of its life that varies in brightness as it shrinks and swells. Studies of Polaris through the 1900s revealed two striking patterns: Its variations gradually became less pronounced, and the star's overall brightness increased by at least 15%.
New analyses of these variations hold more surprises. A team led by recent graduate Scott Engle of Villanova University in Pennsylvania recalibrated historic measurements of Polaris by Ptolemy in 137 C.E., by the Persian astronomer Al-Sufi in 964 C.E., and others. The team deduced that Polaris is 2.5 times brighter today than in Ptolemy's time, a remarkable rate of change. "If they are real, these changes are 100 times larger than predicted by current theories of stellar evolution," says Villanova astronomer Edward Guinan. The team's data also hint that the star's cyclic 4-day variation in brightness, although still weak, is once again growing more robust--but no one knows what's driving these flutterings or how long they will last.
Polaris may be experiencing a rare and rapidly changing period in its evolution, says astronomer David Turner of Saint Mary's University in Halifax, Nova Scotia. As stars run out of hydrogen fuel and burn helium instead, their luminosity waxes and wanes during several phases of pulsation, interspersed with times of relative calm. The first variable phase is by far the shortest, Turner notes, and it appears that Polaris is about to exit this turbulent interval. "Its atmospheric properties already are similar to those of a stable star," says Turner. "It may become stable 100 years from now."