ANN ARBOR, Mich.—An international team of astronomers has taken close-up pictures of a nearby star that show starspots—sunspots outside our solar system.
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The researchers used a technique called interferometry to build the first time lapse of zeta Andromedae across one of its 18-day rotations. Zeta Andromedae is about 181 light-years away in the northern constellation Andromeda. The study was led by the University of Michigan using the world’s most powerful interferometer, Georgia State University’s CHARA Array on Mt. Wilson in California. Interferometry allows researchers to transform six telescopes into one super zoom lens.
The findings are published in the current issue of Nature in a paper titled “No Sun-like dynamo on the active star ζ Andromedae from starspot asymmetry.”
“While imaging sunspots was one of the first things that Galileo did when he started using the newly invented telescope, it has taken more than 400 years for us to make a powerful-enough telescope that can image spots on stars beyond the sun,” said John Monnier, professor of astronomy in Michigan’s College of Literature, Science, and the Arts.
Sunspots and starspots are cooler, darker areas of a star’s outer shell that form when stronger regions of the magnetic field block the flow of heat and energy in patches.
The pattern of spots astronomers saw on the star is different from how they’re typically arranged on the sun. The researchers say the findings challenge theories of how stars’ magnetic fields influence their evolution. At the same time, they give scientists a glimpse of how the sun likely behaved in its infancy, when the solar system was forming billions of years ago.
“Zeta Andromedae represents a critical first step in studying the magnetic storms on sun-like stars,” said Fabien Baron, assistant professor of astronomy at Georgia State. “The CHARA Array will soon create images of many other stars to document the diversity of their surfaces.”
“It’s important to understand the sun’s history because that dictates the Earth’s history—its formation and the development of life,” said Rachael Roettenbacher, a postdoctoral researcher in astronomy who conducted the research as part of her doctoral thesis at Michigan. “The better we can constrain the conditions of the solar environment when life formed, the better we can understand the requirements necessary for the formation of life. These are the highest quality images of a star we have other than the sun.”
To take full advantage of CHARA, Monnier developed the Michigan InfraRed Combiner to give dimension to astronomical objects like stars that had long been mere points of light, even to the most powerful telescopes.
On the sun, spots only form in bands just above and below its equator. Not so on zeta Andromedae. The new images showed one starspot in the star’s northern polar region and several additional spots that spread across lower latitudes. Both findings are important, researchers say. While previous studies using indirect approaches to find spots suggested stars with strong magnetic fields could, in fact, harbor them near their poles, that couldn’t be verified. Now, it has been.
And the additional, lower latitude spots are spread over such an extended cool region that scientists say they’ve found evidence that magnetic fields can suppress heat flow across a large part of the star’s surface, rather than just in spots. Astronomers use star temperatures to estimate their ages, so they need to know if anything, such as these extended cool regions, is throwing off those temperature measurements.
Others have speculated a similar mechanism generating the magnetic field of zeta Andromedae might be at work in young stars, which spin much faster than the sun. The new study connects the two.
Zeta Andromedae is a binary system—a bright giant star locked in orbit with a smaller unseen companion. The bright giant has a radius 15 times larger than the sun. Interactions with its companion have sped its spinning so much that it finishes a full rotation in less than three weeks (Earth time) despite its additional girth. It has unusually strong magnetic activity. New, growing stars also spin fast as they take in material from disks of dust and gas around them and contract in the process.
The sun rotates about once every 24 days, and the number of sunspots ebbs and flows along with the sun’s 11-year magnetic activity cycle. More sunspots signal more magnetic activity and, on Earth, greater potential for geomagnetic storms that could damage satellites and the electrical grid. Astronomers believe the sun spun a lot faster when it was younger.
The research team also included scientists from the Finnish Centre for Astronomy with ESO, the University of Copenhagen, Ohio Wesleyan University, the Hungarian Academy of Sciences, the University of Exeter, the Harvard-Smithsonian Center for Astrophysics and Pennsylvania State University. The research was funded by the National Science Foundation.
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Baron works on image reconstruction for optical and radio interferometry, making very high angular resolution images at GSU’s Center for High Angular Resolution Array. His main science interests are interacting binaries, evolved stars (supergiants, giants), young stellar objects with dusty environments, and Active Galactic Nuclei. He has taken active part in producing the very first images of interacting binaries, of Roche-lobe filling stars, and of convective and magnetic spots on the surface of stars.