High-resolution images unveil large spots on the surface of Polaris.
Researchers at Georgia State University’s Center for High Angular Resolution Astronomy (CHARA) Array have discovered new features regarding the size and appearance of Polaris, commonly called the North Star. This exciting research is featured in The Astrophysical Journal.
The North Star points to the Earth’s North Pole and serves as a crucial navigation guide, as well as being an interesting star in its own right. Polaris is the brightest star in a triple-star system and is classified as a pulsating variable star. It varies in brightness as its diameter expands and contracts in a four-day rhythm.
Polaris is categorized as a Cepheid variable star. Astronomers refer to these stars as “standard candles” because their intrinsic brightness correlates to their pulsation periods: brighter stars pulse less frequently than fainter ones. A star’s visibility from Earth is influenced by its intrinsic brightness and its distance from us. Given that we can determine a Cepheid’s true brightness from its pulsation period, astronomers use these stars to calculate distances to the galaxies they belong to, which also helps estimate the universe’s expansion rate.
Led by Nancy Evans at the Center for Astrophysics | Harvard & Smithsonian, a group of astronomers studied Polaris using the CHARA optical interferometric array composed of six telescopes located at Mount Wilson, California. The aim was to track the orbit of a faint companion star that orbits Polaris every 30 years.
“The small distance and significant brightness contrast between the two stars make it exceedingly difficult to distinguish the binary system at its closest approach,” Evans explained.
The CHARA Array merges the light from six telescopes dispersed across the summit of the historic Mount Wilson Observatory. By merging the light, the CHARA Array functioned like a 330-meter telescope, allowing for the detection of the dim companion as it passed near Polaris. The observations were captured by the MIRC-X camera, developed by astronomers from the University of Michigan and Exeter University in the UK, which excels at revealing details of stars’ surfaces.
The team successfully monitored the orbit of the nearby companion and noted changes in the pulsation size of Polaris. Their findings indicated that Polaris has a mass five times that of the Sun and a diameter 46 times larger than the Sun.
The most unexpected finding was the new look at Polaris’ surface from close-range images. The CHARA observations offered the first view of what a Cepheid variable’s surface resembles.
CHARA Array’s false-color image of Polaris from April 2021 reveals large bright and dark spots on its surface. In the sky, Polaris appears about 600,000 times smaller than a full Moon.
“The CHARA images showed significant bright and dark surface spots on Polaris that changed over time,” remarked Gail Schaefer, director of the CHARA Array. The existence of these spots and the star’s rotation might relate to a 120-day variability in observed velocity.
“We plan to continue imaging Polaris in the future,” stated John Monnier, an astronomy professor at the University of Michigan. “Our goal is to gain a better understanding of the processes that create the spots on Polaris’s surface.”
The new Polaris observations were captured and made available as part of the open access program at the CHARA Array, which allows astronomers globally to apply for observation time via the National Optical-Infrared Astronomy Research Laboratory (NOIRLab).
Situated at the Mount Wilson Observatory in California’s San Gabriel Mountains, the CHARA Array consists of six telescopes configured along three arms. Each telescope’s light is conveyed through vacuum pipes to a central beam combining laboratory, where all beams unite at the MIRC-X camera.
The CHARA Array’s open access program is supported by the National Science Foundation (grant AST-2034336), with institutional backing from Georgia State’s College of Arts ; Sciences and the Office of the Vice President for Research and Economic Development.
