The Stellar Dynamo - Scientific American Digital

The
Stellar
Dynamo
Sunspot cycles—on other stars—are helping
astronomers study the sun’s variations and the
ways they might affect Earth
By Elizabeth Nesme-Ribes, Sallie L. Baliunas
and Dmitry Sokoloff
MAGNETIC FIELDS on the
sun are rendered visible in
this x-ray photograph by the
curving contours of solar flares.
The lines of magnetic fields erupt
from the sun’s surface and heat the
gases of the surrounding corona to up to
25 million degrees C, causing them to glow.
Flares are more frequent during sunspot maxima.
In 1801, musing on the vagaries of English weather, astronomer William Herschel
observed that the price of wheat correlated with the disappearance of sunspots. But
the pattern soon vanished, joining what scientists at large took to be the mythology
connecting earthly events with solar ones. That the sun’s brightness might possibly
vary, and thereby affect Earth’s weather, remained speculative.
Thus, in the mid-1980s, when three solar satellites—Solar Maximum Mission,
Nimbus 7 and Earth Radiation Budget—reported that the sun’s radiance
was declining, astronomers assumed that all three instruments were failing. But
the readings then perked up in unison, an occurrence that could not be attributed
to chance. The sun was cooling off and heating up; furthermore, the variation
was connected with the number of spots on its face.
In recent years one of us (Baliunas) has observed that other stars undergo rhythmic
changes much like those of our sun. Such studies are helping refine our understanding
of the “dynamo” that drives the sun and other stars. Moreover, they
have revealed a strong link between “star spots” and luminosity, confirming the
patterns discovered in our sun. And yet astrophysicists, including the three of us,
are still debating the significance of the sun’s cycles and the extent to which they
might influence Earth’s climate.
Updated from the August 1996 issue 35
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