In our solar system, scattered across one of Earth’s green mountains, six eggshell-white telescopes peer into the depths of the universe. Like a connected beehive, the domed structures collect cosmic light to guide modern astronomers as they explore space—and thanks to that hive, we now have a brilliant new perspective on the light that guided astronomers of the past: Polaris.
Our visual knowledge of the current North Star (due to the Earth’s axial wobble, the title goes to different stars over the eons) is deep. Artists, old and new, have depicted Polaris in their paintings, astrophotographers have photographed it from their backyards, and scientists have pointed their instruments at it for decades. But what’s special about these new Polaris views, provided to the CHARA Array on Mount Wilson in California, has to do with resolution. What’s special about CHARA, as mentioned, is that its telescopes work in tandem. Their light data is combined in a central facility to provide a whole, clear picture of a source. It’s as if the sextet of working telescopes forms a single ultimate telescope 330 meters (1,083 feet) across. And because of that, the project’s image resolution—more accurately, angular resolution—is excellent.
In fact, after reviewing some of these Polaris images compiled using CHARA observations from 2016 to 2021, scientists found some previously unknown features of the star. The most striking are the recognizable spots on the surface of the star, similar to the sunspots that we occasionally see on the sun.
“The CHARA images showed large bright and dark patches on the surface of Polaris that changed over time,” said Gail Schaefer, director of the CHARA array, in a opinion.
One of the main reasons this finding came as a surprise is the fact that Polaris is not just any star. It is a type of star known as a Cepheid star, which means it gets brighter and dimmer on a regular basis. Polaris in particular gets brighter and dimmer on a four-day cycle, and scientists love locating Cepheids because of their very predictable behavior. That’s because these stars can be used for cosmic distance measurements. Essentially, observing how a Cepheid changes in brightness over a cycle can reveal its true brightness.
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Without predictable periodic pulsations, on the other hand, a star would not be very reliable for such measurements. At the risk of oversimplifying, a dark star, for example, could either be far away or just small – or it could be strangely dark for some other reason. Or, It may simply be that it was dark at the time of observation.
Back to those spots, the CHARA team says these high-resolution images of Polaris offer the first “glimpse into the surface of a Cepheid.” So it was fascinating to find spots that were immediately recognizable. But those spots weren’t the only result of the team’s analysis.
Unlike our lonely sun, Polaris does not roam the universe alone. It is about 46 times the size of our parent star and over 400 light-years away from us. It is part of a triple star system. It happens to be the brightest of its siblings. In fact, the original goal of CHARA’s Polaris survey was to map the orbit of the star that orbits our North Star once every 30 years. It is much closer than the other, making the endeavor a complex feat. Not only does this star hover very close to Polaris, but its companion is also incredibly faint. Its existence was only confirmed around 2005, thanks to the Hubble Space Telescope.
“The close separation and large brightness contrast between the two stars make it extremely difficult to detect the binary system at their closest approach,” said team leader Nancy Evans of the Center for Astrophysics, Harvard & Smithsonian, who also helped identify the companion, in the statement. For this reason, the team also resorted to other astronomical tools, such as a speckle interferometer at the Apache Point Observatory in New Mexico.
This mission was indeed a success. It helped confirm things like the size of Polaris and suggested that the star might be about five times more massive than the Sun – which is heavier than previously thought. This discovery is important in itself because, as Evans said ScienceNewsThe mass of only a few Cepheids has been determined so far. This also raises some interesting questions that need to be investigated in the future. “The mass in combination with the distance shows that the Cepheid is more luminous than predicted for this mass based on the evolutionary traces,” write the authors of the study, for example.
Still, the spotty nature of the North Star seems to be in the spotlight. Consider how the areas of the star where spots are seen and the speed at which the star rotates suggest a 120-day radial velocity variation – which essentially refers to changes in the object’s speed as seen along an observer’s line of sight. This raises another puzzle.
“We plan to continue imaging Polaris in the future,” John Monnier, a professor of astronomy at the University of Michigan and co-author of the study, said in the statement. “We hope to better understand the mechanism that creates the spots on Polaris’ surface.”
The study was published on August 20 in the Astrophysical Journal.
