A nearby star could help explain the sunspot mystery that has baffled scientists for 300 years

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A new study has identified a nearby star whose sunspot cycles appear to have stopped. Studying this star could help explain the period from the mid-16th to early 18th centuries when our Sun broke its sunspot cycle. This image shows a typical 11-year cycle on the Sun, with the fewest sunspots appearing at the minimum (top left and top right) and most at the maximum (middle). Photo credit: NASA

The number of sunspots on our Sun normally ebbs and flows on a predictable 11-year cycle, but an unusual 70-year period in which sunspots were incredibly rare has puzzled scientists for 300 years. Now, a nearby Sun-like star appears to have halted its own cycles and entered a similar period of rare star spots, according to a Penn State team of researchers. Continued observation of this star could help explain what happened to our own Sun during this “Maunder Minimum” and provide insight into the Sun’s stellar magnetic activity, which is disrupting satellites and global communications, and possibly even the climate on the planet can affect the earth.

The star — and a catalog of 5 decades of starspot activity from 58 other Sun-like stars — is described in a new article to be published online at Astronomical Journal.

Starspots appear as a dark spot on a star’s surface due to temporarily lower temperatures in the area resulting from the star’s dynamo — the process that creates its magnetic field. Astronomers have documented changes in the frequency of starspots on our Sun since they were first observed by Galileo and other astronomers in the 17th century, so there is a good record of their 11-year cycle. The exception is the Maunder Minimum, which lasted from the mid-16th to early 17th centuries and has puzzled astronomers ever since.

“We don’t really know what caused the Maunder Minimum, and we’ve been looking for other Sun-like stars to see if they can provide insight,” said Anna Baum, a graduate student at Penn State at the time of the research and First author of the paper. “We have identified a star that we believe has entered a state similar to the Maunder Minimum. It will be really exciting to continue watching this star during this minimum and hopefully when it emerges from this minimum, which could be extremely revealing about the Sun’s activity 300 years ago.”

The research team pulled together data from multiple sources to stitch together 50 to 60 years of starspot data for 59 stars. This included data from the Mount Wilson Observatory’s HK project – which was developed to study stellar surface activity and ran from 1966 to 1996 – and from planet searches at the Keck Observatory, which collected this type of data as part of their ongoing search for exoplanets from 1996 to 1996 included 2020. The researchers compiled a database of stars that appeared in both sources and that contained other readily available information that might help explain starspot activity. The team also made a significant effort to standardize the measurements from the different telescopes so that they could be directly compared and otherwise cleaned up the data.

The team identified or confirmed that 29 of these stars exhibit starspot cycles by observing at least two complete periods of cycles, often lasting more than a decade. Some stars appeared to have no cycles at all, which could be because they are spinning too slowly to have a dynamo and are magnetically “dead,” or because they are nearing the end of their lives. Some of the stars need further study to confirm if they cycle.

“This continuous, more than 50-year time series allows us to see things that we would never have noticed in the 10-year snapshots we took previously,” said Jason Wright, professor of astronomy and astrophysics at the Penn State and author of the paper. “Excitingly, Anna has found a promising star who has been cycling for decades but has apparently given up.”

According to the researchers, the star, designated HD 166620, had a cycle of about 17 years but has now entered a period of low activity and has not shown any signs of starspots since 2003.

Christopher Scheiner Illustration

Sunspots were first observed in the 16th century using a modified telescope called a helioscope. The instrument projects an image of the Sun onto a surface where dark sunspots can be observed. This illustration was made by Christoph Scheiner in the 17th century for his book Rosa Ursina sive Sol. Photo credit: Christoph Scheiner

“When we first saw this data, we thought there must have been a mistake, that we were collating data from two different stars, or that there was a typo in the catalog, or that the star was misidentified,” said Jacob Luhn, a graduate student at Penn State when the project began who is now at the University of California, Irvine. “But we double and triple checked everything. The observation times matched the coordinates we expected for the star. And there aren’t as many bright stars in the sky as Mount Wilson observed. No matter how many times we’ve checked, we always come to the conclusion that this star just quit cycling.”

Researchers hope to continue studying this star during its minimum period and possibly when it leaves its minimum and begins orbiting again. This continued observation could provide important information about how the Sun and similar stars generate their magnetic dynamos.

“There’s a lot of debate about what the Maunder minimum was,” said Baum, who is now a graduate student Lehigh University Study of star astronomy and asteroseismology. “Did the sun’s magnetic field basically shut down? Has it lost its dynamo? Or was it still cycling but at a very low level that wasn’t producing a lot of sunspots? We can’t go back in time to measure what it was like, but if we can characterize the magnetic structure and magnetic field strength of this star, we might get some answers.”

A better understanding of the Sun’s surface activity and magnetic field could have several important implications. For example, strong stellar activity can cripple satellites and global communications, and a particularly strong solar storm in 1989 shut down a Quebec power grid. It has also been suggested that sunspot cycles may have a connection to Earth’s climate. In addition, the researchers said that information from this star could influence our search for planets outside our solar system.

“Starspots and other forms of stellar surface magnetic activity affect our ability to see the planets around them,” said Howard Isaacson, a research scientist at University of California, Berkeley, and an author of the paper. “Improving our understanding of a star’s magnetic activity could help us improve our discovery efforts.”

The curated database of the 59 stars and their starspot activity from this research has been made available to researchers for further study.

“This research is a great example of intergenerational astronomy and how we continue to improve our understanding of the Universe by building on the many observations and dedicated research by astronomers before us,” said Wright. “As a graduate student, I looked at starspot data from Mount Wilson and Keck Observatory for my thesis, Howard looked at starspot data from the California Planet Survey for his master’s thesis, and now Anna has put all the data together for a more comprehensive look over the years. We all look forward to continuing to study this and other promising stars.”

Reference: “Five Decades of Chromospheric Activity in 59 Sun-like Stars and New Maunder Minimum Candidate HD 166620” by Anna C. Baum, Jason T. Wright, Jacob K. Luhn and Howard Isaacson, March 22, 2022 Astronomical Journal.
DOI: 10.3847/1538-3881/ac5683

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