The Sun, a round orange orb, is observed through an ultraviolet filter that gives its surface a mottled deep orange look. There are a few prominences along the edge of the circular disk and one loop at around one o’clock, but there are no sunspots visible on the surface.
The Sun experienced a minimum in its 11-year sunspot cycle in 2009. Here the Sun in ultraviolet light on a sunspot-free day, 18 March 2009. Credit: NASA Solar and Heliospheric Observatory, Public Domain

Every 11 years, the number of spots dotting the surface of the Sun increases and decreases like clockwork. Astronomers have been tracking the 11-year sunspot cycle for more than 400 years, using it to better understand the chaotic magnetic field the Sun puts out. (The current solar cycle, number 25, started in 2019.) The timing of the solar cycle is remarkably consistent: Sunspot numbers rise and fall, rise and fall…except for that time that they disappeared and weren’t seen again for 70 years.

That period of time, from 1645 to 1715, is known as the Maunder Minimum, named after 19th century British astronomers Edward and Annie Maunder. Astronomers still don’t understand why the Sun ceased making sunspots for 70 years, but a new analysis of more than 5 decades of measurements of nearby stars has identified one that might be undergoing its own Maunder-like minimum. The star, HD 166620, could help scientists understand this mysteriously quiescent period of the Sun’s history and unlock clues about how the solar dynamo functions.

“We can’t build a time machine and go back to the Maunder Minimum and study the Sun as it was,” said Jason Wright, an astronomer at Pennsylvania State University in University Park and a coauthor on the new analysis. “But we can find an analog and study its Maunder minimum.”

Spotting Magnetic Activity

Sunspots are generated when the Sun’s magnetic field locally blocks a small bit of convecting plasma from sinking back into the Sun’s interior, leaving that patch to cool and become darker than the surrounding area. Anomalous sunspot behavior can be a clue that something strange might be happening with the Sun’s magnetic field at the solar surface.

“Seeing stellar activity cycles play out can tell us a lot about the environments that these planets might be forming and evolving in.”

Moreover, tracking spots and magnetic activity on Sun-like stars outside our solar system can guide searches for exoplanets. “Seeing stellar activity cycles play out can tell us a lot about the environments that these planets might be forming and evolving in,” said study coauthor Jacob Luhn, a postdoctoral researcher at the University of California, Irvine. “It can also tell us whether or not we should be looking at a particular star, because if it’s really active, it’s going to be a little trickier to find planets.”

Looking back at historical records, it’s clear that the Maunder Minimum qualifies as “something strange.” But at the time, Western astronomers had just started tracking sunspots with telescopes, so it took a while longer to realize what had happened. “For a long time, people thought this might just be poor historical record keeping,” Wright said. But the records were accurate; there were simply no or very few sunspots for several decades. “We’ve never seen the Sun do anything like that in the modern era. And it should be an important clue to the [solar] dynamo, but all we know is that there weren’t sunspots.”

“Too Good to Be True”

Finding a star undergoing a Maunder-like pause in its activity cycle is no easy task. First, you have to be able to measure how much star spot activity is happening. Spots on the Sun are easily trackable using an optical telescope, but spots on other stars are far too small to detect observationally (with a few, very large exceptions). Instead, astronomers track stellar activity using minute changes in a star’s spectrum, in particular, a well-characterized spectral feature produced by ionized calcium in a star’s chromosphere. A star’s magnetic field generates a lot of energy, and part of that energy is released in the ultraviolet by ionized calcium. The strength of this spectral feature is a good proxy for magnetic activity and, therefore, star spots.

Scientists have tracked the number of visible sunspots for more than 400 years. Here the oscillating blue and purple lines show the regular 11-year sunspot cycle. The red data points show the decline into the Maunder Minimum in 1645, the extended low/no sunspot period, and the gradual increase back to normal by 1715. The black line tracks the average sunspot number over a solar cycle. Credit: Robert A. Rohde, CC BY-SA 3.0

After knowing how to measure star spot activity, you next have to track that activity for decades. After all, some stars have activity cycles longer than the Sun’s 11 years, and a star has to have missed a cycle or two to be considered a Maunder Minimum candidate. Led by Lehigh University physics graduate student Anna Baum, the researchers who recently identified HD 166620 compiled archival records on Sun-like stars that go back more than 50 years.

They started with more than 100 Sun-like stars from a 1966–2001 survey conducted at the Mount Wilson Observatory in Los Angeles and cross-referenced those with more than 500 stars from the ongoing California Planet Search that started in 1996. “Making sure that they were all on a similar or the same baseline across the 50 years was…tedious, I would say,” Baum said. “We ended up with only 59 of those with good, precise parameters also measured from them,” parameters like mass, size, and temperature.

“All of us were very suspicious of what was going on with this star.”

Some of the stars had clearly defined stellar activity cycles, and some didn’t cycle at all, but one star stood out: In early data, HD 166620 appeared to have a regular 17-year activity cycle. But more recent data suggested that the star had little to no star spot activity at all. “All of us were very suspicious of what was going on with this star,” Baum recalled.

“It looked too good to be true,” Wright said, especially because there was a gap in their data that spanned when the stellar activity would have been declining toward its minimum.

“I can’t remember how many times we went back and forth on that,” Luhn said. “We would say, ‘Okay, I think it’s right,’ and then a month or two later we we’d looked at it again and we’d say, ‘There’s no way.’”

The Sun in Transition

Eventually, the team had conducted enough tests to believe that HD 166620 had missed at least one of its 17-year activity cycles and was indeed a candidate Maunder Minimum star. Although some low-activity stars are on Maunder Minimum watch lists, “Until now, there had never been a definitive identification of one,” Wright said. These results were published in the Astronomical Journal in March.

Luhn has continued to analyze more archival data on the new Maunder Minimum candidate, hoping that a new data cache will bridge the time gap between the two surveys and show the star’s decline in activity.

“Maunder-like magnetic minima may represent intermittency in the large-scale dynamo as the cycle gradually fades away.”

Travis Metcalfe, a research scientist at White Dwarf Research Corp. in Golden, Colo., who was not involved with this work, has looked at HD 166620’s activity recorded in other data sets. “Based on the complete data set, HD 166620 is a convincing candidate for a Maunder-like magnetic minimum,” he said.

Moreover, Metcalfe said, “there is growing observational evidence that the Sun may be in a transitional evolutionary phase, in which its large-scale magnetic dynamo is gradually shutting down.…In this context, Maunder-like magnetic minima may represent intermittency in the large-scale dynamo as the cycle gradually fades away.”

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

Correction 21 June 2022: A description of how the Sun’s magnetic field generates sunspots has been updated.

Citation: Cartier, K. M. S. (2022), Why did sunspots disappear for 70 Years? Nearby star holds clues, Eos, 103, https://doi.org/10.1029/2022EO220271. Published on 10 June 2022.
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