See the Surface of Polaris, the North Star

Astronomers have used an array of telescopes to reconstruct images of the surface of Polaris, the North Star. It's the first time that astronomers have glimpsed the surface of a Cepheid variable.

Polaris is one of the most famous stars in the night sky. Sitting almost directly at the celestial north pole, the star’s position on the sky barely varies throughout the year. However, Polaris does change in other ways. It is the nearest Cepheid variable star, a type of giant star that brightens and dims in a regular, repeating pattern. What’s more, it’s a binary, with a companion star on a wide orbit.

To get a closer look at Polaris, a team of astronomers led by Nancy Evans (Smithsonian Astrophysical Observatory) set out to map the orbit of a companion star that swings around Polaris once every 30 years. The results are published in the August 20th Astrophysical Journal.

“The small separation and large contrast in brightness between the two stars makes it extremely challenging to resolve the binary system during their closest approach,” Evans says. To pull it off, Evans turned to the Center for High Angular Resolution Astronomy (CHARA) array, a set of six 1-meter telescopes on the summit of Mount Wilson in California. The combination of all six instruments acts like a single 330-meter telescope.

By adding new data to older measurements, including those from the Hubble Space Telescope, the team has now covered three-quarters of the two stars’ mutual orbit, and upped their estimate of Polaris's mass from a previous estimate of 3.5 solar masses to 5.1 solar masses.

This throws up a curious conundrum: Even with its increased mass, Polaris is brighter than it should be for where it is on its evolutionary journey. That isn’t surprising, though — it’s part of a long-standing issue known as the “Cepheid mass problem.” Astronomers can either infer a Cepheid’s mass using stellar evolution or the Cepheid’s pulsation period, but the two methods disagree by about 10%.. Solving this problem might affect how astronomers utilize Cepheid variables as standard candles, vital yardsticks for measuring distances in the universe.

The team also used a camera attached to the CHARA array to glimpse Polaris’s surface. The images show that Polaris is some 46 times the diameter of the Sun. Remarkably, they also show surface details. “The CHARA images revealed large bright and dark spots on the surface of Polaris that changed over time,” says team member Gail Schaefer (Georgia State University).

The existence of these starspots fits well with some of Polaris’s other behavior. For example, it is known to have a very low pulsation amplitude, meaning the difference between its dimmest and brightest points is smaller than for other Cepheids. Its atmosphere may have a lot in common with non-variable supergiants that have similar surface activity.

Starspots also open the door to the future possibility of measuring Polaris’s spin, just as Galileo used sunspots to estimate the rotation period of the Sun. Astronomers have already noted a 120-day variation in Polaris's pulsations and Evans speculates that this time interval could mark the star's spin.

“We plan to continue imaging Polaris in the future,” says team member John Monnier (University of Michigan). “We hope to better understand the mechanism that generates the spots on the surface of Polaris.”

“I believe the significance of discovering the variation of surface features on Polaris is important,” says Siobahn Morgan (University of Northern Iowa), who was not involved in the research. "A large, relatively nearby object like Polaris can provide direct observations of features and behavior that will improve our understanding of evolved stars.”

Polaris has been helping us to navigate on Earth for millennia. After these results, it could also point the way to a more comprehensive picture of the cosmos.