For 70 years, from 1645 until 1714, early astronomers reported almost no sunspot activity. The number of sunspots - cooler areas on the sun that appear dark against the brighter surroundings - dropped a thousandfold, according to some estimates. Though activity on the sun ebbs and flows today in an 11-year cycle, it has not been that quiet since.
Since 1976, when it was pointed out that this lengthy period of low sunspot activity, the so-called Maunder minimum, coincided with the coldest part of the Little Ice Age in Europe and North America, astronomers have been searching nearby sun-like stars for examples of stellar minima. They have hoped to determine how common such minima are and to predict the next solar minimum - and perhaps the next period of global cooling.
Now, data from a group of UC Berkeley astronomers cast doubt on the hundreds of stars thought to be examples of stellar minima analogous to the quiet period the sun experienced 300 years ago.
In a poster to be presented Monday, May 31, at the Denver meeting of the American Astronomical Society, UC Berkeley graduate student Jason Wright shows that nearly all the supposedly sun-like stars displaying minimal activity are, in fact, much brighter than and significantly different from the sun and therefore not examples of Maunder minima. The findings throw into question all studies using these stars to make inferences about the sun's own activity and future minima, Wright said.
"Star surveys typically find that 10 to 15 percent of all sun-like stars are in an inactive state like the Maunder minimum, which would indicate that the sun spends about 10 percent of its time in this state," Wright said. "But our study shows that the vast majority of stars identified as Maunder minimum stars are well above the main sequence, which means they're not sun-like at all, but are either evolved stars or stars rich in metals like iron and nickel. To date, we've found no star that is unambiguously a Maunder minimum star."
"We thought we knew how to detect Maunder minimum stars, but we don't," he said.
The main sequence is a region where normal, steady-burning stars cluster when plotted on a chart of color versus brightness. As stars age, however, they get redder and brighter - becoming what are called subgiant stars - and move upward off the main sequence. The sun has been on the main sequence for about 5 billion years, ever since it settled down after igniting hydrogen fusion in its core, and will remain there for another 5 billion years until it starts to swell and become a subgiant.
"The fact is, we still don't understand what's going on in our sun, how magnetic fields generate the 11-year solar cycle, or what caused the magnetic Maunder minimum," said Wright's advisor, Geoffrey Marcy, professor of astronomy at UC Berkeley. "In particular, we don't know how often a sun-like star falls into a Maunder minimum, or when the next minimum will occur. It could be tomorrow."
The drop in solar activity in the late 17th and early 18th centuries was drawn to the world's attention in 1893 by English astronomer Edward Walter Maunder, who also noted a dip during the same period in the intensity and frequency of the northern lights, which are caused by storms on the sun. Again, in 1976, astronomer John Eddy reviewed various pieces of evidence for the Maunder minimum and concluded not only that it was real, but cited a 1961 paper linking the minimum with a contemporaneous period of cooling throughout Europe, perhaps due to decreased energy output from the sun. The sun, and stars like the sun, are dimmer when inactive.
The idea of a Maunder minimum is controversial, however, because no one really knows how closely people were observing the sun in the mid-1600s, a mere 40 years after the invention of the telescope. No record of solar activity exists before the Maunder minimum, though a surge in activity signaled its end in 1714.
Uncertainty also surrounds the cause of the Little Ice Age, which began around 1300 A.D. and lasted for several hundred years. Characterized by colder than normal winters and cool summers throughout the Northern Hemisphere, it may have been caused by greenhouse gases and particulates spewed into the atmosphere by volcanoes, or by fluctuations in the sun's output.
Many climate experts take the Maunder minimum seriously, however, and astronomers have put together a long list of stars supposedly exhibiting the same dip in activity, as evidenced by decreased emission from the element calcium in the star's atmosphere. Solar activity is characterized by strong magnetic fields that heat the sun's upper atmosphere, or chromosphere, to some 8,000 to 10,000 degrees Kelvin, exciting calcium to emit blue light.
The question, Wright said, is whether the cause of decreased calcium emission is a stellar Maunder minimum or something else, like age - stars spin more slowly as they age, lose their magnetic dynamo and no longer produce magnetic fields or spots - or high metal content. "We've now found that it's not from Maunder minima," he said.
"What astronomers have assumed is that sun-like stars going through a stellar funk are actually very, very old stars whose magnetic fields have turned off forever. They are not in a temporary Maunder minimum, but a permanent one. They're dead," Marcy said. "The sun will be in that state in 4 billion years or so."
"This implies that if other stars do undergo Maunder minima of their own, then it is either a rare occurrence nearly undetected in activity surveys or it is not necessarily indicated by low calcium ... emission levels," Wright wrote. Therefore, he added, some other criterion is needed to discern those stars in a stellar downturn.
The problem with stars thought to be in a Maunder minimum went unnoticed because it wasn't until 1998 that the Hipparcos satellite was launched and began determining the precise distances to many nearby stars. It then became possible to calculate the absolute brightness of these stars, and to place them precisely on a color-brightness plot, known as a Hertzsprung-Russell diagram.
Wright decided to look systematically at Maunder minimum stars after he and Marcy noticed that many seemingly inactive nearby stars were actually brighter than main sequence stars. They have collected spectra of more than 1,000 nearby stars to look for evidence of planets.
In his analysis, Wright used Hipparcos data on distance to determine the absolute brightness of several thousand nearby stars surveyed not only by Marcy's California and Carnegie Planet Search Program but also by other projects, such as the Mount Wilson H-K Project and Project Phoenix. He noted that some of the stars previously identified as Maunder minimum stars may be metal-rich stars, which also burn brighter than our sun and show less activity. Further analysis of nearby stars is needed to characterize these quiet stars.
The findings, which have been submitted to Astrophysical Journal, resulted from work supported by Sun Microsystems, the National Aeronautics and Space Administration, and the National Science Foundation.