The North Star has faithfully guided travelers for centuries, but its light is not constant. Instead, it’s brighter on some nights than others. Now, astronomers also know that the star, known as Polaris, is dotted with spots.
Polaris gets its nickname because of its proximity to the point in the sky where the Earth’s north rotation axis points. Because it’s so close to that point, it doesn’t seem to move much as the Earth spins. In other words, if you were looking up in the Northern Hemisphere, it would be there, in relatively the same location. Despite its constant position, Polaris is not stationary either. Classified as a Cepheid variable, its pulsing brightness is caused by periodic expansion and contraction of its surface. Because their pulses occur at regular intervals, their brightness can be used to measure the distance to their host galaxy.
Polaris is not alone. A much dimmer star orbits it every 30 years. To map the star’s orbit, a team of astronomers led by Nancy Evans of Harvard University and the Smithsonian Center for Astrophysics began observing Polaris using the Center for High Angular Resolution Astronomy (CHARA) telescope at Georgia State University.
The latest observation of Polaris in 2021 is the first close look at a Cepheid variable, showing a spotted star. In a paper published in the Astrophysical Journal, researchers say starspots moving around the Earth’s surface appear to be related to magnetic fields, much like sunspots on the Sun. However, there are some oddities, as Polaris’ magnetic polarization looks more like a supergiant than a Cepheid variable. The presence of starspots also opens up new avenues of research, and the astronomers write that they may help determine how quickly Polaris spins.
“We plan to continue imaging Polaris in the future,” John Monier, a professor of astronomy at the University of Michigan who participated in the study, said in a statement. “We hope to learn more about the mechanisms that create spots on Polaris’ surface.”
The spots aren’t the only new features the team discovered. By observing how the size of Polaris changes as it orbits its companion star, they were able to determine that Polaris is five times the mass of the Sun and 46 times the Sun’s diameter. Although Polaris would dwarf our Sun, it is 400 light-years away from Earth, a distance that means the star appears about 600,000 times smaller than the full Moon in the sky.
