Sky & Telescope - November 2020 - 33

interest in gyrochronology to begin with. Kepler and its successor mission K2 added young, low-mass cluster stars to our
sample, and already we're seeing that the picture we've constructed for the Sun's rotational evolution doesn't describe
the behavior of these low-mass stars all that well. Calibrator
stars are even harder to come by for these low masses, and the
hunt is on to ﬁnd them. We're very motivated in our search
- these are the most numerous stars in the galaxy, the most
difﬁcult to age-date, and the most likely host stars for future
interesting exoplanets discovered by the ongoing Transiting
Exoplanet Survey Satellite (TESS) mission.
For researchers working on this problem, these puzzles are
just as exciting as the prospect of creating a galactic timepiece. When we are eventually able to describe the spin-down
well, we will suddenly have an incredibly powerful tool for
measuring ages in our hands. However, every time a new
observation breaks our models, we are equally excited: It's a
chance to learn about deeply uncertain aspects of stellar evolution that stellar astronomers have struggled with for more
than 100 years.
It turns out when you ask a star its age, you get its whole
(magnetic) life story.

tually change the nature of the dynamo, which would then
weaken the angular momentum loss. If that transition also
meant that stars were less spotty, it could explain the full
sample as well: Stars never make it to long periods (they stop
spinning down), and we don't see a pileup of stars that have
stalled their slowdowns because they have very few spots and
it's hard to see them rotating.
If this scenario is true, it means that all solar mass stars
halfway through the main sequence - in other words, the
same age as the Sun - are undergoing this transition, and
that our Sun is at a somewhat "special" point in its life. This
is an uncomfortable claim for any scientist who has had the
Copernican Principle drilled into them for decades. But it's
also intriguing. What if the Sun is undergoing a magnetic
transition? What does that mean for space weather on Earthlike planets? Will the Sun continue to have an 11-year magnetic cycle? Will the number of spots on its surface decline?

Moving Forward
So far, our attempts to understand how rotation changes
with age have raised more questions than they've answered.
And it's very unlikely that the surprises are over: We've only
looked at the tiny fraction of stars similar to the Sun, and as
we expand that view we'll need to expand our physical understanding as well.
For example, we've still had little chance to study the
rotational evolution of those low-mass stars that ignited our

¢ JENNIFER VAN SADERS is a professor at the University of
Hawai'i at Manoa. She spent the first few years of her career
convinced she didn't want to study stars, but then she saw the
light and never looked back.

q MISSING STARS? When astronomers plotted some 30,000 stars' rotations, they found that far fewer of them spun slowly than expected. The
lefthand plot shows the observed stars. The righthand plot is a comparison of the data to what's expected if stars continued to spin down with age.
In both plots, 95% of stars lie below the orange line. The redder the region on the righthand plot, the more stars are "missing" compared to what
astronomers expected to see. If there's a point at which the rotation rate is slow enough that it doesn't affect motions in the star's convective zone, then
it could affect the magnetic field and explain why magnetic braking hasn't continued to spin stars down. Note that stars cooler than 5100K (red line)
aren't old enough to have spun down yet. The tall purple "fin" on the righthand plot is from older, swollen stars, which the calculations don't handle well.

Number of stars expected, divided by
number of stars observed
More stars than expected

Fewer stars than expected

Number of stars

0.00
70

PLOTS: THE AU THOR

Period (days)

)

0
6500

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

0
6000

5500

5000

4500

Temperature (kelvin)

4000

3500

6000

5500

5000

4500

4000

3500

Temperature (kelvin)
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Sky & Telescope - November 2020

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