North Star is shrinking
Almost an Earth's worth of gas lost each year
According to new analysis of more than 160 years worth of data, the North Star may be wasting away, shedding an amount of gas equivalent to up to the mass of the Earth each year.
The North Star, also known as Polaris, has been used by navigators for centuries, as its alignment with the Earth’s rotational axis means it always sits directly above the North Pole. It is also a ‘Cepheid Variable’ star, meaning it pulsates, getting dimmer and brighter as the conflicting forces of gravity inwards and radiation pressure outwards constantly tug the outer layers back and forth from its dense, burning core.
This ‘pulse’ of the star, with a period of around 4 days, has been observed by ancient astronomers for hundreds of years. In 2004, David Turner, of St. Mary’s University in Halifax, Canada, compiled an archive of these measurements. His analysis, including data stretching back as long ago as 1844, showed that the North Star’s pulse was slowing down. Turner justified this increase in the star’s time period of around 4.5 seconds per year by concluding that the star was in the early stages of its evolution on its first crossing of the ‘Cepheid instability strip’, and still burning only hydrogen – a trademark of a young star.
However, Hilding Neilson of the University of Bonn, Germany, was not so sure. Combining his own recent observations with Turner’s archived data, Neilson and colleagues compared the star’s measured behaviour with state of the art models of stellar evolution. They didn’t match up.
“Only if the star is losing a lot of mass can that [discrepancy] be resolved,” says Neilson, referring to the North Star’s mysteriously decreasing pulse. In a paper published recently in the Astrophysical Journal Letters, he explains how the time period data is inconsistent with all regular phases of stellar evolution, and, in contradiction to Turner, he asserts that the star is old, having passed through the red giant stage and already burning Helium. In Neilson’s model, as the star pulses, swathes of gas are pushed out from it in waves, as the pent-up light bursts through the star’s outer layers. He concludes that the North Star must be losing mass, at the rate of around one Earth mass each year – equivalent to roughly a millionth of the mass of the star itself.
The result is particularly exciting because it’s the first truly strong argument for such extreme mass loss in a Cepheid star. While most suspected examples of Cepheid stars losing mass are uncertain, as they are based on shaky observations of the surrounding stellar material, this novel method employs the wealth of data already gathered about the North Star, to make actual quantifiable measurements of the rate at which it is wasting away.
The quest for understanding the mechanisms of the North Star is not over, however, and Turner is the first to admit that “there are many mysteries about Polaris that defy simple explanation.” In the coming months and years, new data from the Hubble Space Telescope about its distance from Earth, and its luminosity, will hopefully help solve the mystery once and for all of what exactly is going on deep inside the star we thought we knew so well.
