'Starspots' could improve our understanding of habitable zones

Active regions on the Sun combined to look something like a jack-o-lantern’s face on Oct. 8, 2014. The active regions appear brighter because those are areas that emit more light and energy — markers of an intense and complex set of magnetic fields hovering in the Sun’s atmosphere, the corona.
(Image credit: NASA/GSFC/SDO)

Astronomers have developed a new technique for identifying "starspots" — cool, dark regions of stars akin to the sunspots on our star, the sun.

These regions are believed to form on the surfaces of stars when strong magnetic fields tangle and suppress the churning of plasma, thus impeding light from escaping from that region of the star. 

The research could shed light on why some stars are highly active, and could eventually help astronomers better define stars' habitable zones, the regions around stars where planets could sustain liquid water at their surfaces and thus have the potential to support life. 

Related: Superhabitable planets: Alien worlds that may be more habitable than Earth

"Our study is the first to precisely characterize the spottiness of stars and use it to directly test theories of stellar magnetism," Lyra Cao, an astronomer at The Ohio State University and lead author of the research, said in a statement. "This technique is so precise and broadly applicable that it can become a powerful new tool in the study of stellar physics."

The technique has allowed Cao and her colleagues to develop a catalog of starspot and magnetic field measurements for over 700,000 stars. The catalog, which will be released soon, will increase the amount of data available on star "spottiness" by a factor of thousands. 

Cao developed the new technique by analyzing high-resolution infrared spectra from the Sloan Digital Sky Survey and then using it to identify starspots for 240 stars from two open star clusters: The Pleiades and M67. This allowed them to get more precise measurements of starspots, yielding a powerful new class of data that could direct the study of stellar magnetic fields.

Since the discovery of sunspots in the 1600s, astronomers have mainly studied stellar magnetism indirectly by looking at stars through different filters or by detecting starspots in the light curve of a star. But now, the new technique allows them to study this phenomenon more directly.

Two major sunspot groups tracked across the surface of the sun between Dec. 2 and Dec. 27, 2022 in this mosaic created by Şenol Şanlı..   (Image credit: Şenol Şanlı)

"It was lurking in plain sight," Cao said. "Within the spectrum, there was a cooler component corresponding to the starspot which was only visible in the infrared."

Additionally, because younger stars can be covered much more extensively with starspots — sometimes up to 80%, making them "more spot than star" — Cao realized that these huge, cool regions could block enough light to have a measurable effect on the stars. And because the blocked light has to escape at some point, she reasoned, the extremely spotty stars would compensate by swelling and cooling, thus expanding the stellar surface area from which light can escape. 

The method pioneered by the team could help improve astronomers' use of stellar parameters to understand the neighborhoods around stars, including the stars' habitable zones, where temperatures are optimal to allow liquid water to exist. 

The new method could also lead to more accurate measurements of stars' sizes. Historically, astronomers have estimated the size of a star by measuring its temperature, and this measurement could be off by hundreds of degrees — meaning the radius of the star is calculated as smaller than it actually is. 

"When this happens, you start seeing large changes in the stars' structure, which can throw other important astronomical measurements off as well," Cao said.

The new findings could also help to explain a class of stars found in the Pleiades cluster, also called The Seven Sisters, located around 444 light-years from Earth. This cluster seems to be too active to be explained by existing stellar models. These stars are rife with starspots and highly magnetic, Cao said, but they are also bursting with high-energy ultraviolet and X-ray radiation

"You wouldn't want to live around these stars," Cao said. "But understanding why these stars are so active could change our models and criteria for exoplanetary habitability."

In addition to shedding light on these unusually active stars, the technique could help astronomers understand why low-mass stars are also highly active.

"We can directly study the evolution of stellar magnetism in hundreds of thousands of stars with this new dataset, so we expect this will help develop key insights in our understanding of stars and planets," Cao concluded. 

The team's research is published in the journal Monthly Notices of the Royal Astronomical Society.

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Robert Lea
Senior Writer

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.