In 1645, something strange happened with our sun. Galileo was one of the first astronomers to document temporary blemishes on the solar surface, called “sunspots.” But in the mid 17th century they seemed to all but disappear. This period of time, now called the Maunder Minimum after astronomer Edward Walter Maunder who reconstructed the sunspot cycle more than a century later, lasted for 75 years and has confused scientists ever since.
Now, researchers from Pennsylvania State University have found another star 36 light years away that appears to be undergoing its own Maunder Minimum. Their new study published to The Astronomical Journal could present an opportunity for astronomers to understand what happened during this mysterious period of our sun’s history.
“It’s probably the first really, absolutely clear evidence of a star that went into a Maunder-like episode,” said Steven Saar, a research astronomer at the Harvard-Smithsonian Center for Astrophysics who was not involved in the research.
Sunspots indicate areas of the solar surface with a stronger magnetic field than any other areas of the sun. Typically, the amount of sunspots on the solar surface increase and decrease on a cycle of 11 years. The Maunder Minimum represented an unprecedented decrease. Over a 28 year timespan during the period, only 50 sunspots were documented. Before that, between 40,000 and 50,000 could be seen in the same span.
Solving a centuries old celestial mystery didn’t start high up on the checklist of the researchers. The initial project’s main goal was to create a catalog of long-term observations of different stars and their magnetic starspot cycles.
“Finding a Maunder Minimum star really was a hope,” said Anna Baum, the lead author on the paper who began the project as an undergraduate student at Penn State. But because the idea of having such an outcome was so unexpected, “it was kind of on the back burner.”
Jason Wright, a professor of astronomy and astrophysics at Penn State who co-authored the research, said the team started combing through the data because they were just “curious about the sun.”
Baum, who is now pursuing her PhD in physics at Lehigh University, and colleagues analyzed data covering five decades of research on the starspot cycles of sun-like stars. They looked at data taken from the Mount Wilson Observatory HK Project, which started up in the 1960s to monitor a group of stars to see if they had magnetic cycles like our own sun, and the later observations continued with the California Planet Search at the Keck Observatory.
Out of 29 stars they analyzed with trackable starspot cycles, a star a bit cooler and smaller than our own sun called HD 166620 was a clear outlier. In 2003, its regular starspot cycle appeared to have stopped entirely.
“That we found such a clear example of a Maunder Minimum was really surprising,” said Wright. When Baum first showed the team the data plot showing the starspots disappear, he said everyone thought there might be a mistake.
The researchers double-checked all the data and went back through observation logs. Given that there was a gap in the data from 1995 to 2004, they checked to ensure that they were actually looking at the same star.
“After exhausting all of those options, it was pretty clear to us that this was actually what was going on,” said Baum of the Maunder Minimum candidate. “It was really exciting.”
A minimum of sunspots represents a period of time when the sun has low magnetic activity, including less solar flares, which can cause geomagnetic storms here on Earth. Understanding the time when the sun may have had no magnetic activity at all could help shed light on the overall affect of the sun’s activity on our own planet.
“The sun’s magnetic activity affects Earth profoundly,” said Wright. “And we don’t really understand how it’s generated.”
There’s still many questions surrounding the Maunder Minimum and uncertainty about what actually happened to the sun’s magnetic field during that time period. The researchers hope the observations of HD 166620 can be a jumping off point to gain a new understanding of the past event.
“We can’t go back to 1645 and take new observations,” said Baum. “The only way to really learn about what the Maunder Minimum was is by looking at other stars.”