The sun might be a magnetic
slacker.

A census of stars similar to
the sun shows that our own star is less magnetically active than others of its kind, astrophysicists report in
the May 1 Science. The result could support the idea that the sun is in
a “midlife crisis,” transitioning into a quieter phase of life. Or,
alternatively, it could mean that the sun has capacity for much more magnetic
oomph than it’s shown in the past.

“Our sun could potentially
become [as] active” as those other stars in the future, says astrophysicist
Timo Reinhold of the Max Planck Institute for Solar System Research in Göttingen, Germany.

A star’s magnetism can drive
dramatic outbursts like flares and coronal mass ejections, which can cause chaos on orbiting planets (SN: 3/5/18). When these large ejections from
the sun hit Earth, they can knock out satellites, shut down power grids and
trigger beautiful auroras. Understanding the sun’s magnetic field is thought to
be the key to predicting such outbursts (SN: 6/30/19).

Magnetic fields also can create
dark sunspots and bright spots called faculae on a star’s surface. These
features change over time as magnetic activity changes, altering a star’s
brightness. 

Astronomers have been
observing the sun’s magnetism through those surface features since Galileo
turned a telescope toward the sun in 1610. While the sun’s magnetic activity
waxes and wanes in an 11-year cycle, our star has remained fairly calm while
humans have been watching. Inferences from certain radioactive elements found
in tree rings and ice cores suggest that same overall cycle of magnetic
activity has held steady for the last 9,000 years.

Because other stars are so
far away, tiny changes in brightness that reveal magnetic
changes were hard to detect
until
2009, when the Kepler space telescope launched (SN: 9/18/19). The now-defunct
telescope found exoplanets by picking up on slight dips in
starlight
as planets orbited in front
of stars (SN: 10/30/18), but the spacecraft’s data include a wealth of
information on other changes in stars’ brightness.

To see how the sun’s
brightness compared with its stellar kin from 2009 to 2013, Reinhold and his
colleagues studied stars whose age, surface gravity, chemical makeup
and temperature are similar to the sun’s (SN: 8/3/18). The team also
sought stars that rotate at nearly the same rate as the sun, roughly once every
24 days.

Not every star’s rotation
period was measurable, so Reinhold’s team split the stars into two groups: 369
“solarlike” stars, with rotation periods between 20 and 30 days, and 2,898
“pseudo-solar” stars, whose period could not be detected.

Surprisingly, although the
stars with no detectable rotation periods looked as magnetically calm as the
sun, the stars with sunlike rotations were up to five times as active.

Either something is
different about those stars, Reinhold says, or the sun may go through periods
of greater variability in its brightness — and thus, magnetic activity — that
scientists just haven’t seen. Perhaps “the sun did not reveal its full range of
activity over the last 9,000 years,” he says. “The sun is 4.5 billion years old;
9,000 years is nothing.”

Still another explanation
for the finding is related to the idea that stars might stop slowing their
rotation because of a midlife change in their magnetic field (SN: 8/2/19), says astronomer Travis Metcalfe
of the Space Science Institute in Boulder, Colo.

Many stellar physicists think that stars continually lose momentum and slow their spins as they get old. But in 2016, Metcalfe and colleagues reported that Kepler was seeing stars that rotate too fast for their advanced ages. The team suggested that stars might stop their slowdowns at middle age, and that the sun is currently going through this transition.

The new result “could be the
best evidence yet that the sun is in the midst of a magnetic midlife crisis,”
Metcalfe says. The hyperactive stars in Reinhold’s sample appear to be slightly
younger than the sun, and so may not have gone through their magnetic
transition yet. The sun and the other calmer stars could already be on the
other side.

“It’s super interesting either way it turns out,” Metcalfe says.