SUN-LIKE STARS HOLD CLUES TO EARTH'S CLIMATE
CAMBRIDGE, Mass.--The Sun may dim slightly in the mid-21stcentury, which could lead to cooler temperatures on Earth,report scientists at the Harvard-Smithsonian Center forAstrophysics and colleagues. Their finding, announcedtoday at the American Astronomical Society meeting inPittsburgh, PA, results from studying regions of intensemagnetic activity (referred to as sunspots, or starspots)on nearly 1,000 stars similar to our Sun. The Sun andother stars brighten during times of heightened magneticactivity, and dim when magnetic activity wanes.
"In addition to helping us predict the Earth's climate,surveys of Sun-like stars may also help us pinpoint starsthat might have planetary systems at the same stage of evolution as our own," says CfA astrophysicist SallieBaliunas, who led the study. If so, those stars might belikely candidates in the search for extraterrestrialintelligence, she adds.
Baliunas and CfA colleagues Willie Soon and RobertDonahue, with scientists from Tennessee State Universityand the Space Telescope Science Institute, used telescopesat Mount Wilson Observatory in California and Kitt PeakNational Observatory in Arizona to study stars locatedbetween 10 to 100 light-years away. Although it's notpossible to view starspots directly, they can be detectedindirectly by examining a star's spectra. One particularregion of the spectra--the calcium emissionline--indicates the strength of the star's magnetic field.
"Together as a group, these stars are like timemachines, because they help us understand the Sun's past,present, and future," says Baliunas. The research, whichconfirms links between the age, magnetic activity, andbrightness of stars, suggests that the Sun may decrease inbrightness by about 0.4 percent within the next 50 years. That dip could cool the Earth as much as 1 degree C (2 degrees F), says Baliunas, who notes that a similar "cold spell" occurredin the 17th century.
The researchers obtained a snapshot of the potentialfluctuations in the Sun's magnetism by sampling a largenumber of stars that are roughly the same age and size asthe Sun. Scientists have long known the Sun's magneticactivity is defined by sunspots, which appear as darkpatches on the Sun when viewed thru special filters. Thespots wax and wane in roughly an 11-year cycle. But thenew research suggests the Sun also undergoes much longercycles--on the scale of centuries.
Tree rings provide an additional record of the Sun'smagnetism over the past 70 centuries. By studying therings of 7,000-year-old bristlecone pines, otherresearchers have measured levels of C14, a form of carbon. During times of low magnetic activity on the Sun, cosmicrays from deep space create more C14 in the upperatmosphere, which is then incorporated (via carbondioxide) into trees.
"Combined with the star survey, that information suggeststhat the Sun's magnetic activity dwindles roughly onecentury out of every three," says Baliunas. The lasttime it happened was between 1645 and 1715, whichcorresponds to a period known as the "Little Ice Age." Europe's climate was about 1 degree C lower than today, probablydue to the decreased brightness of the Sun, according toBaliunas.
The scientists also found younger stars to have higherlevels of magnetism, while older stars have weaker levels,giving them a simple way to determine a star's age. Todate, they have measured the ages of 800 of the Sun'sneighbors. Some of these stars--including ones in theconstellations of Cetus, Hydra, and Pegasus--are nearlythe same age as the Sun.
"Those stars, if they have planetary systems, may haveevolved intelligent life similar to humans, assuming thatevolution proceeds at similar paces in both places," saysBaliunas. Life on planets around younger stars mightstill be one-celled organisms, while that on planetsaround older stars might be so far advanced that we cannotcommunicate with it, she notes.
For additional information, contact Sallie Baliunas (617) 495-7415.