The amount of sunspots on our Sun generally ebbs and flows in an 11-year cycle, but one extraordinary 70-year period when sunspots were very uncommon has perplexed astronomers for three centuries. According to a team of Penn State researchers, a neighboring Sun-like star seems to have interrupted its own cycles and entered a comparable phase of unusual starspots. Continued observations of this star might help explain what happened to our own Sun during the “Maunder Minimum,” as well as provide insight into the Sun’s stellar magnetic activity, which can interfere with satellites and global communications, and perhaps even alter Earth’s climate.
The star, as well as a database of 58 other Sun-like stars’ starspot activity across 5 decades, is reported in a new research published online in the Astronomical Journal.
Starspots emerge as a black patch on the surface of a star as a consequence of transient lower temperatures in the region caused by the star’s dynamo – the mechanism that generates its magnetic field. Changes in the frequency of our Sun’s starspots have been documented by astronomers since they were first discovered by Galileo and other astronomers in the 1600s, thus there is a reliable record of its 11-year cycle. The Maunder Minimum, which lasted from the mid 1600s to the early 1700s and has confused astronomers ever since, is an exception.
“We don’t really know what caused the Maunder Minimum, and we’ve been turning at other Sun-like stars to see if they may give any insight,” said Anna Baum, the paper’s first author and an undergraduate at Penn State at the time of the study. “We have discovered a star that seems to be in a condition akin to the Maunder Minimum. It will be thrilling to continue observing this star throughout, and hopefully after, this minimum, which might be tremendously illuminating about the Sun’s activity three hundred years ago.”
The researchers gathered information from several sources to piece together 50 to 60 years of starspot data for 59 stars. This includes data from the Mount Wilson Observatory HK Project, which operated from 1966 to 1996 and was meant to examine star surface activity, as well as planet searches at Keck Observatory, which contain this kind of data as part of their continuous search for exoplanets from 1996 to 2020. The researchers created a database of stars that featured in both sources and had additional publicly accessible data that might assist explain starspot activity. The team also worked hard to standardize measurements from various telescopes so that they could be compared directly and to clean up the data.
The scientists detected or verified that 29 of these stars have starspot cycles by watching at least two entire periods of cycles, which typically span more than a decade. Some stars did not seem to have any cycles, which might be because they rotate too slowly to have a dynamo and are therefore magnetically ‘dead,’ or because they are nearing the end of their lifetimes. Several of the stars need to be studied further to see if they have a cycle.
“This continuous, more than 50-year time series enables us to observe things that we would never have detected with the 10-year snapshots that we were taking previously,” said Jason Wright, a Penn State professor of astronomy and astrophysics and one of the paper’s authors. “Anna has discovered a promising star that has been cycling for decades but looks to have stopped.”
The star, known as HD 166620, was predicted to have a 17-year cycle but has now entered a phase of low activity and has displayed no traces of starspots since 2003, according to the researchers.
“When we first saw this data, we thought it had to be a mistake, that we had combined data from two different stars, or there was a typo in the catalog, or the star had been misidentified,” said Jacob Luhn, who was a graduate student at Penn State at the time the project began and is now at the University of California, Irvine. “We did, however, double and triple verify everything. The timings of observation corresponded to the predicted coordinates of the star. And, as Mount Wilson remarked, there aren’t many brilliant stars in the sky. We always arrive to the conclusion, no matter how many times we check, that this star has just ceased spinning.”
The astronomers plan to continue researching this star throughout its minimum phase and maybe when it emerges from its minimum and starts to cycle again. This ongoing study might provide critical details about how the Sun and other stars create magnetic dynamos.
“There’s a lot of disagreement over what the Maunder Minimum was,” said Baum, a doctorate student at Lehigh University researching stellar astronomy and asteroseismology. “Did the Sun’s magnetic field go completely dark? Is it missing a dynamo? Or was it still cycling, but at a very low level, producing few sunspots? We can’t travel back in time and take measurements, but if we can describe the magnetic structure and magnetic field strength of this star, we may be able to obtain some answers.”
A greater knowledge of the Sun’s surface activity and magnetic field might have numerous significant effects. Strong star activity, for example, may disrupt satellites and worldwide communications, and a particularly powerful solar storm in 1989 knocked down a power infrastructure in Quebec. It has also been proposed that sunspot cycles are related to Earth’s climate. Furthermore, the researchers said that information from this star might influence our hunt for planets outside of our solar system.
“Starspots and other types of surface magnetic activity of stars interfere with our capacity to discover planets orbiting them,” said Howard Isaacson, a research scientist at the University of California, Berkeley and one of the paper’s authors. “By better understanding a star’s magnetic activity, we may be able to boost our detection attempts.”
The curated database of the 59 stars and associated starspot activity from this study has been made accessible for further investigation by scholars.
“This discovery is an excellent example of cross-generational astronomy, and how we may continue to advance our knowledge of the cosmos by building on the numerous observations and diligent effort of astronomers who came before us,” Wright said. “When I was 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 stitched all the data together for a more thorough view over the years. We’re all enthusiastic to keep researching this and other bright stars.”