The Stellar Dynamo - Scientific American Digital

Intensity of Calcium Emission Lines
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Young star, HD 20686, variable cycle
Mature star, HD 4628, 8.6-year cycle
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Mature star, HD 14376, flat
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1966 1970 1975 1980
1985 1990 1995 2000
INTERANNUAL MAGNETIC VARIABILITY of stars over the years is detected by way of violet calcium
emission lines. Here activity from three nearby stars reveals the likely states of our own sun: the
variable magnetic cycles of a young star (top); the steady cycles of a star at an age comparable to
our sun’s (middle); and the subsidence of a sunlike star into a Maunder-type minimum phase
(bottom). The magnetic, and therefore sunspot, activity of other stars indicates that our sun is
capable of far greater variability than it has shown in the past century.
Year
over a cycle: the bright plages overwhelm
the dark sunspots. (Presumably,
as the sun brightens and darkens, its total
energy is temporarily channeled into
different reservoirs—kinetic, magnetic,
thermal or potential.) During the past 24
years of satellite observations, the sun’s
total energy output has varied roughly
0.1 percent between a brighter, magnetically
active phase and a fainter, quiet one.
Because of the brevity of the satellite
records, we do not know the variability
of the sun’s brightness over decades.
Richard Willson and his colleagues at
Columbia University’s Center for Climate
Systems Research recently found a
slight, 0.05 percent increase in brightness
at the observed solar minima in 1986
and 1996. Finding a longer-term value
for brightness variability, however, is vital
to evaluating the sun’s influence on
Earth. One possible way to answer this
question is to examine “star spot” cycles
on other stars.
It is not easy to map the features on
the surface of stars. But as magnetic
fields heat the outer layers of a star’s atmosphere,
they radiate the energy in certain
spectral lines. For example, on our
sun, the intensity of the two violet emission
lines of calcium (having wavelengths
of 396.7 and 393.4 nanometers)
closely follows the strength and extent
of the magnetic fields. Variations in
these lines thus give us a measure of the
changing surface magnetism of a star.
At Mount Wilson Observatory in
1966, Olin C. Wilson began a program
of measuring the magnetic activity of
roughly 100 so-called main-sequence
stars—those that, like the sun, are burning
hydrogen. (When the hydrogen runs
out, a star expands into a red giant.)
Most of these stars show obvious signs
of magnetic activity, by way of variations
in their violet calcium emission
THE AUTHORS
lines. The fluctuations vary greatly in
amplitude and duration, depending primarily
on the age and mass of the star.
All these stars have a dynamo number,
D, higher than the critical value required
for sustaining magnetic fields. For
a young star of one or two billion years,
the rotation period is fast, roughly 10
to 15 days. The resulting high value of
D means that these young stars have erratic
fluctuations in magnetic activity
over intervals as short as two years and
no well-defined cycles. The fluctuations
sometimes repeat, however, having periods
between two and 20 years or so
that lengthen with age.
But as a star ages, it slows down—
because its angular momentum is carried
off by the magnetic wind—and D falls.
Then a consistent dynamo cycle begins
to appear, with a period of about six to
seven years and sometimes even with
two independent periods. Later on—for
an even lower D—one period starts to
dominate, lengthening with age from
eight to 14 years. In addition, there are
occasional Maunder minima. If rotation
were to slow further, in the very oldest
stars, we predict that the magnetic field
should be steady. The Wilson sample
contains a few very old stars, but they
still show cycles, indicating that the
steady dynamo would not be reached in
10 billion years—soon after which they
will expand into red giants.
To focus on the solar dynamo, Baliunas
and her collaborators restricted
Wilson’s broad sample of stars to those
similar to our sun in mass and age. That
group currently comprises 10- to 20-year
records of 150 stars, depending on the
criteria defining similarity to the sun.
Many of these stars show prominent cycles
similar in amplitude and period to
those of the sun. About one quarter of
ELIZABETH NESME-RIBES, SALLIE L. BALIUNAS and DMITRY SOKOLOFF all have been active
in unraveling connections between the sun’s variations and Earth’s climate. Nesme-
Ribes, who recently passed away, was an astronomer at the Paris Observatory and the National
Center for Scientific Research in France. Apart from studying the solar dynamo, she
conducted extensive searches into the 17th-century archives on sunspots at her home institution.
Baliunas is a scientist at the Harvard-Smithsonian Center for Astrophysics in Cambridge,
Mass. She observes the variations of sunlike stars at the Mount Wilson Observatory
in Pasadena, Calif. Sokoloff is professor of mathematics in the department of physics at
Moscow State University in Russia.
MOUNT WILSON OBSERVATORY HK PROJECT; PETER SAMEK Slim Films
40 SCIENTIFIC AMERICAN THE SECRET LIVES OF STARS
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