The North Star, Polaris, may be a constant in the night sky, but the star is far from consistent. A set of observations has revealed it has a spotted surface.
Polaris is well-known as the 'North Star', meaning you can use it to locate where 'north' is in the Northern Hemisphere night sky.
The pointer stars of the Plough asterism, Dubhe and Merak, form a line that points to the North Star, and this is a well-known way of locating it.
You may have seen Polaris with the naked eye, but what about observing it through a powerful telescope?
A group of astronomers decided to do just that, and get a closer look at the star's surface.
Polaris close-up
Located 448 lightyears away, Polaris is the closest known Cepheid variable star to Earth.
The rate at which a Cepheid varies is strongly linked to its luminosity, with fainter stars pulsing faster than bright ones.
How bright a star appears in the sky depends on both its intrinsic brightness and how far away it is.
This means astronomers can use the variability to measure a Cephid’s true brightness and work out its distance from us across space.
A team of astronomers used the CHARA Array at Mount Wilson, California, to take advantage of Polaris’s proximity and observe the fluctuating brightness of the star, finding it is 46 times brighter than the Sun.
The team also tracked the orbit of a companion star that orbits once every 30 years, allowing them to measure the mass of Polaris at 5.1 solar masses (previous estimates vary between 3.5 and 5 solar masses).
They also imaged the surface of the star, revealing something unexpected.
"The CHARA images revealed large bright and dark spots on the surface of Polaris that changed over time," says Gail Schaefer, director of the CHARA Array.
"We plan to continue imaging Polaris in the future," says John Monnier, from University of Michigan, who took part in the study.
"We hope to better understand the mechanism that generates the spots."
