An extended period of low activity has been found on a nearby star, and astronomers hope it will help explain why the same thing sometimes happens to our Sun.
The Sun has an 11 year activity cycle, with solar flares and sunspots currently in the rising phase. It's been like that for at least 400 years, and almost certainly much longer – but from 1645 to 1715, sunspots almost vanished, an era known as the Maunder Minimum.
Numerous people, with varying levels of credibility, have tried predicting the timing of the next one. These efforts have had little success, largely because astronomers don't really know what caused the Maunder Minimum (nor the much shorter Dalton minimum), which isn't all that surprising with such a limited sample.
Two students at Penn State University and their supervisors tried looking for counterparts on nearby stars. In the Astronomical Journal they report success with one previously obscure K-type star, HD 166620.
“There’s a big debate about what the Maunder Minimum was,” said first author Anna Baum in a statement. “Did the sun’s magnetic field basically turn off? Did it lose its dynamo? Or was it still cycling but at a very low level that didn’t produce many sunspots? We can’t go back in time to take measurements of what it was like, but if we can characterize the magnetic structure and magnetic field strength of this star, we might start to get some answers.”
The authors examined the data for 59 stars, most with masses quite similar to the Sun's, where between the Mount Wilson and Keck Observatories we have more than 50 years of observations.
Some stars showed no clear cycle, particularly those rotating too slowly to create strong magnetic fields or approaching the end of their life. On the other hand, 29 showed strong cycles of varying lengths. Among these, HD 166620 stood out, showing a 17 year cycle from 1966 to around 2003, and almost no starspots since. HD 166620 is among the oldest stars in the survey, and 24 percent less massive than the Sun.
“When we first saw this data, we thought it must have been a mistake,” Dr Jacob Luhn, who worked on the project as part of his PhD said. “No matter how many times we checked, we always come to the conclusion that this star has simply stopped cycling.”
The answers could have far more immediate implications for most people than the majority of astronomical puzzles.
Peaks in sunspots are associated with increasing Solar radiation. This has been shown to produce a small but detectable impact on global temperatures.
Climate change deniers have attempted to use this to argue the current increase in global heat is caused by increased activity on the Sun, rather than fossil fuel release on Earth. This has been refuted not only by the fact these changes are at least 10 times too small to produce the current warming, but also because they are in the wrong direction – solar activity peaked in the 1950s and has been on a slightly downward path since.
The Maunder Minimum fell within the Little Ice Age, when temperatures dipped around the North Atlantic. When the Little Ice Age was thought to be a global phenomenon it was widely attributed to reduced output from the Sun. More recently, evidence from other parts of the world has cast doubt on whether the planet as a whole really experienced much temperature drop then, leaving open the question of how much effect the Maunder Minimum had.
HD 166620 could help improve our knowledge of the drivers of Earth's climate. It could also tell us how much danger satellites can expect to face in the near future from solar activity.