sciencemag.org SCIENCEPHOTO: GREG PIEPOL/SCIENCE SOURCEBy Ângela R. G. Santo s^1 andS avita Mathur2,3T
he Sun and other similar (solar-like)
stars have an internal magnetic field
that can emerge on the surface, form-
ing features such as dark spots, which
drive the so-called magnetic activ-
ity. For both the Sun and solar-like
stars, magnetic activity varies with time and
strongly correlates with photometric (bright-
ness) variability. In the solar and stellar scien-
tific communities, a debate rages about how
the magnetic activity of the Sun compares
with that of similar stars and whether the
Sun will reach very high activity levels or is
in a period of transition to a state of reduced
magnetic activity. On page 518 of this issue,
Re inhold et al. ( 1 ) compare the Sun’s photo-
metric variability with that of solar-like stars
and find the latter to be more active.
The appearance and disappearance of
magnetic features on the Sun’s surface cycle
over an 11-year period called the solar cycle( 2 ). As the magnetic fields emerge, they form
sunspots at mid- to low latitudes, which ap-
pear darker than the surrounding quiet sur-
face. The number of sunspots varies over
the activity cycle: Near activity maximum,
the Sun exhibits hundreds of sunspots, and
during an activity minimum, sunspots are
almost absent. The number of sunspots on
the Sun’s surface is the most regularly used
solar-activity proxy. When the magnetic field
lines are disturbed or during an activity
maximum, solar flares might occur. Coronal
mass ejections—release of magnetized solar
plasma—often follow the flares and represent
what is commonly called a solar storm.
The detailed mechanisms that control
solar magnetic activity are not yet fully un-
derstood. Studying stars that roughly share
properties with the Sun (similar tempera-
ture, mass, age, rotation period, and chemi-
cal composition) allows scientists to place the
Sun in the context of other stars, investigate
changes in the Sun over time, and predict its
future. Such comparisons often place the Sun
in a special category: an outlier in the context
of other stars.
Contrary to sunspots, which can be ob-
served with the naked eye, starspots on thesurface of distant stars cannot be resolved
in this way. Instead, scientists seek to de-
tect the indirect effects of starspots on, for
example, stellar brightness. The advent
of planet-hunting missions in space (that
is, avoiding Earth’s atmosphere), particu-
larly that of the Kepler space telescope,
has yielded unprecedented, high-precision,
long-term observations of stellar light. Such
information is exceptional not only for exo-
planet research but also for stellar physics.
Because stars rotate, dark spots move in
and out of view and modulate the observed
st ellar light (see the figure). Whereas the
periodicity of the stellar light modulation
is related to surface rotation ( 3 ), the ampli-
tude is related to the percentage of stellar
surface covered by spots, thus representing
a proxy for stellar magnetic activity ( 4 ).
The Ke pler space telescope observed a
small patch of the sky in the Cygnus and Lyra
constellations for almost four continuous
years. Kepler’s long-term and high-quality
observations are ideal to characterize stel-
lar activity. Combining Kepler data with thePERSPECTIVES
There is debate on whether the Sun is in transition
to a less active state.INSIGHTS(^1) Space Science Institute, 4765 Walnut Street, Suite B,
Boulder, CO 80301, USA.^2 Instituto de Astrofísica de
Canarias, Santa Cruz de Tenerife, Spain.^3 Department of
Astrophysics, Universidad de La Laguna, Santa Cruz de
Tenerife, Spain. Email: [email protected]
ASTRONOMY
What future awaits the Sun?
Stellar data question the notion that the Sun is stemming its magnetic activity cycle
466 1 MAY 2020 • VOL 368 ISSUE 6490