Can you find the North Star? Called Polaris, it’s in exactly the same position every night. It never moves or, at least, it never appears to. That’s because Earth’s northern axis points straight to it, so while all the other stars in the night sky are in different positions throughout the night and the seasons, the position of Polaris never changes.
That’s why it’s a useful navigational aid, but new research reveals it as a special star for other reasons.
‘North Star’ Has A ‘Spotty’ Surface
Is Polaris the brightest star in the night sky? No — this supergiant star about 323 light years from the solar system in the constellation Ursa Minor is only the 48th brightest, but it has, unusually, spots all over its surface, according to new research.
Published in The Astrophysical Journal using the CHARA Array of six optical telescopes on Mount Wilson, California, reveals Polaris to have spots on its surface. That may help explain why it waxes and wanes in brightness over 120 days.
“The CHARA images revealed large bright and dark spots on the surface of Polaris that changed over time,” said Gail Schaefer, director of the CHARA Array.
It’s the first-ever glimpse of the surface of what astronomers called a Cepheid variable star — a very special kind.
The ‘North Star’ As A ‘Standard Candle’
Polaris is a special kind of star for two reasons. As well as being positioned very close to Earth’s north celestial pole, Polaris is a Cepheid variable, a star that pulsates and changes in brightness in a highly predictable way.
That’s hugely useful to astronomers because it allows them to calculate distance. How bright a star appears in the sky depends on the star’s true brightness and the distance to the star.
If you know the true brightness of a Cepheid variable star like Polaris — which astronomers do because they know it pulsates every four days — you can use it to measure distances and, ultimately, even infer the universe’s expansion rate.
The ‘North Star’ Is Forty-Six Times The Size Of The Sun
Polaris pulsates over four days as its diameter expands and contracts. But it’s not actually one star. It’s the brightest member of a triple-star system. The research aimed 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,” said Nancy Evans, who led a team of astronomers from the Center for Astrophysics at the Harvard & Smithsonian in Cambridge, Massachusetts.
Combining the light from six separate telescopes on Mount Wilson, the CHARA Array acted like a giant 330-meter telescope, allowing the faint companion to be detected. They found that Polaris has a mass five times larger than the sun and a diameter 46 times the size of the sun. Unexpected surface details on Polaris were then discovered and imaged.
‘North Star’ Status Is Just A Phase
From the northern hemisphere, the night sky appears to revolve around Polaris, allowing anyone to navigate by the stars by making it possible to calculate latitude and north-south direction.
It won’t always be that way. Earth’s precessional motion causes the pole stars to change. As it spins on its axis, a slight wobble causes Earth’s axis to draw out a 47 degrees circle in the stars every 25,800 years. For now, it points to Polaris; in 2,000 years, it will point to Errai in the constellation Cetus; and in 23,000 years, it will point to Thuban in Draco, and so on, until it returns to the point at Polaris once again in about 25,800 years.
Wishing you clear skies and wide eyes.
This post was originally published on here
