Astronomy – October 2019

10 ASTRONOMY • OCTOBER 2019

QUANTUM GRAVITY

As Sun-like stars mature, their

rotation tends to slow. But when

these stars are young, they rotate more

quickly and less predictably. Two stars

of the same mass may rotate at drasti-

cally different speeds. And stars that are

fast rotators tend to hurl more powerful

radiation and charged particles into

their systems, often to the detriment of

their planets.

Without other clues, learning how

fast the Sun once spun is difficult.

But researchers led by Prabal Saxena

of NASA’s Goddard Space Flight

Center published a study May 3 in The

Astrophysical Journal Letters that used

lunar samples to determine how fast our

Sun may have rotated in its early years, a

factor that helped to determine how the

inner planets, including Earth, evolved.

SOLAR BLAST

“After a billion years,” Saxena tells

Astronomy, “[stars with the same mass]

all converge to the same rotation rate.

There’s few clues as to what your state

was before,” based on the star itself. But

Saxena’s team found a workaround in

the amount of potassium and sodium

measured in lunar samples.

Sodium and potassium are volatiles,

meaning they’re easily dispersed from the

lunar surface by strong solar activity or

other violent phenomena. So, the amount

of these elements left on the Moon should

indicate just how active the Sun once was

— and thus how fast it rotated. Saxena’s

team tabulated the quantities of each in

lunar samples and used computer model-

ing to run the clock backward on three

imaginary Suns: a fast, medium, and

slow rotator. The faster the young Sun’s

rotation, the more coronal mass ejections

and solar f lares it gave off, stripping more

volatiles from the Moon.

SCIENTISTS READ THE SUN’S HISTORY

FROM MOON ROCKS

Lunar samples reveal our young star was a slower-than-average

rotator, but its outbursts still shaped the early solar system.

FLARING UP. NASA’s Solar Dynamics

Observatory spotted a solar flare and heated

material erupting from our star (lower right)

in October 2014. Although the Sun is active

today, it was even more so in the past. NASA/SDO