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Astronomers using the Center for High Angular Resolution Astronomy (CHARA) Array have observed Polaris, a member of a triple system and the nearest and brightest classical Cepheid star.
This false-color image of Polaris, captured by the CHARA Array in April 2021, reveals large bright and dark spots on the star’s surface. Image credit: Evans et al., doi: 10.3847/1538-4357/ad5e7a.
Polaris, also known as the North Star, the Pole Star, α Ursae Minoris, HR 424 or HD 8890, is a kind of star known as a Cepheid variable.
Astronomers use these stars as ‘standard candles’ because their true brightness depends on their period of pulsation: brighter stars pulsate slower than fainter stars.
How bright a star appears in the sky depends on the star’s true brightness and the distance to the star.
Because they know the true brightness of a Cepheid star based on its pulsational period, astronomers can use them to measure the distances to their host galaxies and to infer the expansion rate of the Universe.
Dr. Nancy Evans from the Harvard & Smithsonian’s Center for Astrophysics and her colleagues observed Polaris using the CHARA optical interferometric array of six telescopes at Mount Wilson, California.
Their goal was to map the orbit of the close, faint companion that orbits Polaris 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,” Dr. Evans said.
The researchers successfully tracked the orbit of the close companion of Polaris and measured changes in the size of Polaris as it pulsated.
The orbital motion showed that Polaris has a mass five times larger than that of the Sun.
The images of Polaris showed that it has a diameter 46 times the size of the Sun.
The CHARA observations also provided the first glimpse of what the surface of a Cepheid variable looks like.
“The CHARA images revealed large bright and dark spots on the surface of Polaris that changed over time,” said Dr. Gail Schaefer, director of the CHARA Array.
“The presence of spots and the rotation of the star might be linked to a 120-day variation in measured velocity.”
“We plan to continue imaging Polaris in the future,” said University of Michigan’s Professor John Monnier.
“We hope to better understand the mechanism that generates the spots on the surface of Polaris.”
The team’s paper was published in the Astrophysical Journal.
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Nancy Remage Evans et al. 2024. The Orbit and Dynamical Mass of Polaris: Observations with the CHARA Array. ApJ 971, 190; doi: 10.3847/1538-4357/ad5e7a
