Known for guiding navigators with its position over Earth’s north pole, Polaris, also named the North Star or Pole Star, has intrigued astronomers for its unique characteristics. Recently, extensive observations by a team of astronomers have uncovered new insights about Polaris, shedding light on its properties and its role in understanding cosmic phenomena. These findings come from a 30-year study utilizing the CHARA Array, aiming to resolve ongoing questions about Polaris and its companions.
Polaris is a triple star system, with the primary star being a yellow supergiant called Polaris Aa, accompanied by Polaris Ab and an outer star named Polaris B. Historically, Polaris’ position as the North Star has changed due to Earth’s axial precession, and it will be replaced by Thuban around 20346. Earlier research on Polaris emphasized its importance as a Cepheid variable star, whose predictable brightness variations help measure cosmic distances and the Hubble constant. However, there has been a discrepancy in Hubble constant measurements, prompting further investigation into Polaris.
Advanced Observations with CHARA Array
The CHARA Array, an interferometer with six telescopes, has been crucial in the latest study. By combining images from these telescopes, astronomers achieved high resolution, enabling them to observe Polaris and its dim companion with greater accuracy.
“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,”
said the lead author Nancy Evans, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian. The researchers discovered that Polaris is five times as massive as the Sun and 46 times larger in diameter, although its mass measurement is influenced by its orbital eccentricity.
Challenges in Understanding Cepheid Variables
Polaris’ luminosity surpasses predictions for stars on its evolutionary track, highlighting the “Cepheid mass problem.” This discrepancy between inferred masses from evolutionary tracks and pulsation calculations is significant, as only a few Cepheid variables are in binary relationships, making Polaris a key target. Accurate mass determination of Cepheids, which involves binary relationships, is essential for refining the cosmic distance ladder and Hubble constant measurements. The study also revealed variable spots on Polaris’ surface, adding to its complexity.
“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. These spots, variable in nature, may explain the challenges astronomers faced in identifying additional periodicities in Polaris. The research team plans to continue imaging Polaris to understand these surface features better.
Continuing to explore Polaris’ surface phenomena will contribute to understanding not only this star but also the broader category of Cepheid variable stars. Such studies are vital for refining our cosmic measurements and resolving discrepancies in current models. Future observations may shed light on the mechanisms generating these spots, offering deeper insights into stellar behavior and evolution.
