Just as earthquakes help scientists learn about the interior of our planet, the way a star’s surface oscillates yields clues to its internal structure and other key characteristics. Researchers with the Kepler Asteroseismic Investigation are putting that concept to work on a grand scale, monitoring the surface vibrations of thousands of stars to learn their ages, sizes, and inner dynamics.
The Kepler spacecraft’s primary mission is hunting for new planets by tracking the brightness of more than 100,000 stars, watching for the telltale dimming that occurs when an orbiting planet happens to pass in front of its sun. Since seismic oscillations alter the surface area of the star, measurably changing its brightness, the data Kepler collects also reveal these stellar vibrations.
Seismic waves form as a result of turbulent convection in the roiling stellar interior. Those waves eventually hit the star’s surface layer, like a clapper ringing a bell. The speed at which the waves propagate through the star depends on the structure of its interior. Stars shine by fusing hydrogen into helium in their cores, a process that gradually uses up the star’s hydrogen fuel. Observations of starquake vibrations allow researchers to deduce how much hydrogen remains, revealing a kind of time stamp. “If we know how much of the hydrogen has turned into helium, we know in a sense how old the star is,” says Ronald Gilliland, an astronomer at the Space Telescope Science Institute and a Kepler coinvestigator. Precise stellar ages will help theorists refine models of stellar evolution, says Sarbani Basu, an astronomer at Yale University and another Kepler researcher.
The new study may also brighten our understanding of our nearest star. “We know the sun at only one point in time—now,” Basu says. “Asteroseismology should give us confidence in the history and the future evolutionary path of our sun.” It may also help researchers understand current processes in the sun, such as sunspots and solar storms. In August European and American researchers used seismological data from the Corot telescope (similar in concept to Kepler) to study magnetic activity on a distant sunlike star, an achievement that could provide insights into the poorly understood dynamo that drives our sun’s 11-year activity cycle.