Sky & Telescope - November 2020 - 32

Galactic Timepieces

Period (days)

Sun
20

Age (billion years)
p SURPRISING SPINS Stars in young clusters (red) spin down as expected with age (black line). But stars age-dated with asteroseismology
(blue) spin too fast. The stars shown here have temperatures of 5600 to
5900 kelvin, similar to the Sun (about 5800K).

NOVEMBER 2 02 0 * SK Y & TELESCOPE

t STELLAR
SEISMIC WAVES
Acoustic waves take
many paths through
a star, depending on
the density and temperature of the plasma that the waves
travel through. These
waves change the
star's brightness
in subtle ways, enabling astronomers
to detect them.

150,000 main-sequence stars, it detected pulsations in only
about 500 of those targets. And, because those pulsations
become harder to detect in lower-mass stars, there are a mere
handful of main-sequence stars less massive than the Sun
with detected pulsations.
While stars with precise seismic ages are few in number,
we can use this whole new class of calibrators to tune our
rotation-based clocks. For the ﬁrst time, we could test our
period-age relations for stars older than the Sun.

Surprises and Puzzles
The picture that emerged from Kepler was puzzling. Measurements of periods in the open clusters aligned beautifully with
the period-age relations we already had, as did the seismic
stars that were younger than middle-aged. However, the old
seismic stars were a problem: They were spinning faster than
expected for their old ages.
Furthermore, something was wrong with the full,
30,000-some-star sample with measured periods. The
old, slowly rotating stars we thought should be there were
missing. It's very possible we simply overlooked their subtle
rotational signals: As stars age they become less spotty, and it
becomes harder to detect the brightness modulation created
by spots. However, it could also be that stars never actually
manage to spin down to those long periods, a fact we had no
way of knowing without having precise asteroseismic ages to
highlight the old stars.
My collaborators and I put forward an idea that might
explain what we saw: If stars stopped spinning down as
quickly midway through their main-sequence lives, we could
explain the behavior of both our young, trusted open clusters
and the old seismic stars. However, in order for that to happen, something drastic needed to change in the spin-down.
Either the strength or shape of the magnetic ﬁeld, or the
nature of the mass loss, must undergo a transition in order to
weaken the angular momentum loss.
We think the magnetic ﬁelds in Sun-like stars are driven
by a dynamo that results from the interaction between the
star's rotation and the circulation of material in the outer
convection zone. We argued that the spin-down might even-

HIG H SPINS: G REGG DINDER M A N / S&T, SOURCE: J. L. VA N
SA DERS E T A L. / NATUR E 2016; STELL A R SOUND: LE A H
TISCIONE / S&T, SOURCE: SOI / STA NFORD UNIV ERSIT Y

full of bright xenon lamps focused into a beam. Now imagine
you go stand in among these lights with a small LED ﬂashlight. Something with Kepler's sensitivity looking down would
be able to detect the difference in intensity as you turned your
ﬂashlight on and off against the background of those spotlights: a signal of only a few to tens of parts per million. This
was a requirement for Kepler's main mission to detect Earthlike planets passing in front of Sun-like stars, but it also made
it an incredibly powerful tool for studying stars overall.
Kepler's sensitivity gave us the ability to detect spots even
on quiet stars with relatively few of them, something very
difﬁcult to do from the ground. It enabled us to detect the
rotation of tens of thousands of stars, and of older cluster
stars than ever before.
Kepler also gave us access to another powerful technique:
asteroseismology (S&T: Jan. 2018, p. 22). Just as seismology
on Earth is the key to understanding the structure of Earth's
core, asteroseismology allows us to peer into stars' deep
interiors. In Sun-like stars, the churning convective regions
in their outer layers produce sound waves and set the star
ringing. These sound waves cause the star to pulsate, and the
surface becomes slightly brighter and dimmer over the pulsation, a signal we could detect with Kepler.
Some of those sound waves pass near the stellar core
where nuclear fusion is occurring. Sound passes at different
speeds through a core rich in hydrogen fuel and one where
most of that fuel has been converted to helium "ash." This
means that by studying these acoustic waves, we effectively
have a means of measuring how much of its fuel the star has
burned. Because the fraction of the fuel a star has burned is
closely connected to how long it has been burning, it gives us
a tight constraint on the stellar age, even in old stars.
Asteroseismology is incredibly powerful, but it's also
limited to the brightest stars. While Kepler stared at about

Sky & Telescope - November 2020

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