The North Star is much heavier than previously thought

The star that marks true north is much heavier than we thought.

The North Star is 5.1 times the size of the sun, astronomers report in work submitted July 12 to arXiv.org. This value, calculated from the motion of a much fainter star orbiting the light, is nearly 50 percent heavier than a recent estimate of 3.45 solar masses.

Mass profoundly affects stellar life: The more mass a star has, the faster it burns its fuel and the faster it dies. The previous mass estimate had suggested that the North Star, also known as Polaris, is approximately 100 million years old (SN/12/2/16). The new estimate means the star formed later than that, but no one has yet calculated a revised age.

The companion star is so faint that it was not seen until 2005, when astronomer Nancy Evans and her colleagues spotted it with the Hubble Space Telescope. When the companion moved closer to the North Star in 2016, Evans and others began tracking it with the CHARA array, an observatory that combines views from telescopes atop Mount Wilson in California.

Because the close companion star takes three decades to orbit the host star, most of the orbit has now been observed, strengthening the reliability of the mass estimate. “These things take a long time,” says Evans, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Lying 447 light-years from Earth, Polaris is the closest member of a class of stars called Cepheids, which are crucial for measuring distances to other galaxies.SN: 21/7/21). The stars are big and bright – Polaris is 46 times wider than the sun – and are nearing the end of their lives. In particular, they expand and contract, which makes their luster wax and fade. The longer a Cepheid blinks, the more light it emits. Therefore, measuring the pulsation period indicates the Cepheid’s intrinsic luminosity. Comparing this to the star’s apparent luminosity gives the distance to the star and thus to its host galaxy.

“It’s extremely important to know the mass,” says Ed Guinan, an astronomer at Villanova University in Pennsylvania, who was not involved in the new work. This allows astronomers to check their models of how Cepheids evolve and better understand these cosmic measuring rods. But “there are only a few Cepheids that have fixed masses.”