Polaris, the star marking the direction of Earth's North Pole, is renowned not only as a navigation beacon but also as a significant astronomical object. It is the brightest component of a triple-star system and is classified as a pulsating variable star, meaning its brightness fluctuates over a four-day cycle as the star expands and contracts.

As a Cepheid variable star, Polaris serves as a crucial "standard candle" for astronomers. These stars' known relationship between pulsation period and intrinsic brightness allows astronomers to determine distances to far-off galaxies and estimate the universe's expansion rate.

A group of astronomers, led by Nancy Evans from the Center for Astrophysics | Harvard and Smithsonian, utilized the CHARA Array's optical interferometry capabilities to study Polaris. Their primary goal was to map the orbit of Polaris's dim companion, which completes an orbit around the star every 30 years.

"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 explained.

The CHARA Array, with its six telescopes situated at Mount Wilson Observatory, effectively operates as a 330-meter telescope by merging the light collected from each telescope. This setup enabled the team to detect Polaris's faint companion during its close passage. The observations were carried out with the MIRC-X camera, a specialized instrument developed by astronomers from the University of Michigan and Exeter University, known for its ability to detail stellar surfaces.

Through this investigation, the team was able to track the companion's orbit and measure the size changes in Polaris as it pulsated. The orbital data revealed that Polaris has a mass approximately five times that of the Sun and a diameter 46 times greater.

The most unexpected discovery was the detailed view of Polaris's surface. The CHARA observations provided the first close-up images of a Cepheid variable's surface, revealing striking details.

"The CHARA images revealed large bright and dark spots on the surface of Polaris that changed over time," noted Gail Schaefer, director of the CHARA Array. The presence and evolution of these spots, along with the star's rotation, might be connected to a 120-day velocity variation observed in the star.

"We plan to continue imaging Polaris in the future," added John Monnier, an astronomy professor at the University of Michigan. "We hope to better understand the mechanism that generates the spots on the surface of Polaris."

Research Report:The Orbit and Dynamical Mass of Polaris: Observations with the CHARA Array

Related Links
Center for High Angular Resolution Astronomy (CHARA) Array
Stellar Chemistry, The Universe And All Within It