The North Star, a celestial beacon to navigators for centuries, may be slowly shrinking, according to a new analysis of more than 160 years of observations. The data suggest that the familiar fixture in the northern sky is shedding an Earth's mass worth of gas each year. Some researchers caution, however, that the conclusion depends on certain assumptions about exactly where the star is in its several-billion-year life cycle.
Also known as Polaris, the North Star always sits over the North Pole because it is aligned with Earth's axis. Find it in the night sky, at the end of the Little Dipper's handle, and you don't need a compass to orient yourself. To weigh Polaris, astrophysicist Hilding Neilson of the University of Bonn in Germany and colleagues essentially took its pulse. The star grows dimmer and brighter over a roughly 4-day cycle, and the team studied variation in the length of that cycle. Like all stars, Polaris is made of gas in layers around a core, where nuclear fusion occurs. As its gravity pulls the outermost gas inward, Polaris develops an opaque layer just under the surface that doesn't let light through easily, dimming its glow. Light then builds up beneath this layer and pushes on it like water vapor boiling up under the lid on a saucepan. That light heats the opaque layer, causing it to expand and making it more transparent. The star becomes bigger and brighter until those outer layers of gas fall inward again and the cycle begins anew.
Even that 4-day pulsation isn't constant: In 1844, it was about 12 minutes slower than it is now. Previously, astronomer David Turner of St. Mary's University in Halifax, Canada, who was not involved in the new analysis, and colleagues compiled an archive including historical measurements of the pulse. Their data set ended in 2004. Neilson and collaborators, including two citizen astronomers, have now added their own observations from the past decade. This long record, from 1844 to the present, shows that the pulse of Polaris runs about 4.5 seconds slower every year.
The changing rate suggests that the structure of the star is evolving. If, as Neilson and collaborators assume, Polaris is an older star that is fusing or "burning" helium nuclei in its core, then its pulse is decreasing too quickly to match the standard model for stellar evolution. "Only if the star is losing a lot of mass can that [discrepancy] be resolved," Neilson says. This mass may leave Polaris's surface in waves, pushed outward as the pent-up light bursts through the opaque layer, and the loss would slow down the star's pulse rate. To account for the relatively lethargic pulse, Polaris must be losing nearly the equivalent of Earth's mass—or a little under a millionth of its own mass—each year, the team reports in the 1 February issue of The Astrophysical Journal Letters. But never fear, Neilson doesn't think our beacon is hurtling toward oblivion. "Odds are [the mass loss] is episodic," he says.
Done deal? Maybe not, Turner says. The mass-loss argument hinges on the internal behavior of the pulsing star, and Neilson's team assumes that its layers are moving out of sync—when the outer layers are falling in, the inner layers are pushing out and vice versa. Turner suspects that Polaris is pulsing in a simpler way, with both inner and outer layers moving in the same direction, a hypothesis that his team revived in 2005. In this picture, he says, the North Star's changing pulse can fit the models without hemorrhaging mass because the star is in an earlier stage of its evolution—it's not yet burning helium but is instead preparing to blow up as red giant when the core runs out of hydrogen. On the other hand, if Polaris pulses the way Neilson's team describes it, then the North Star would be past the red giant stage and now burning helium in its core.
Polaris's distance from Earth is the key to figuring out which way it pulsates—and its place in stellar evolution. The more complicated pulse would mean that Polaris shines brighter in absolute terms, so to match its observed brightness in the sky, it would have to be farther away than if the pulse was simple. The Hubble telescope should be able to determine whether the North Star is closer to 325 light-years away, supporting Turner's case, or 425 light-years away, supporting Neilson's. "There are many mysteries about Polaris that defy simple explanation," Turner says. "I think I will sit on the fence in this case and await further observational results."