Science News - July 31, 2010

Scientists are still trying to pin down the
abundances of elements that swirl and
churn beneath the sun’s busy surface.

elements out into the universe.

“People always compare stars of the
same type to the sun, and now the sun
has changed,” says astronomer Nicolas
Grevesse of the University of Liège in Bel-
gium. “Now we’re rechecking everything,
restarting all the analyses from A to Z.”
Slowly, however, researchers are edg-
ing toward an answer. New, more sophis-
ticated computer models have improved
understanding of the sun’s atmosphere,
permitting better estimates of chemical
abundances. Deeper discussions of which
data to include, and which to leave out,
are helping smooth battle lines between
research teams arguing over what the
final numbers should be. Soon, stories
about what Earth’s superstar is made of
could read more like trusted newspaper
copy than celebrity gossip.

Reading between the lines Researchers’ preferred way to study the sun’s chemical makeup, from nearly 150 million kilometers away, is to analyze the light flowing from it. Spread sunlight out into its spectrum of wave-

Emission lines

656

lengths, as a prism creates a rainbow, and the light appears riddled with dark lines. Since the late 1850s scientists have known that these dark lines correspond to chemical elements in the sun’s outer layers; the elements absorb radiation coursing outward from the core and blot out the light. The more light is absorbed, the more of that element is assumed to be present.

In 1929, astronomer Henry Norris Russell used this spectroscopic technique to publish solar abundances for

56 elements. Since then, astronomers have refined the numbers further, and generally believe that hydrogen, the lightest element, makes up around 70 percent of the sun by mass. Over millions of years, nuclear fusion in the solar core slowly converts the hydrogen to helium and subsequently to heavier elements, collectively known to astronomers as “metals” (though they include gases such as oxygen). The question has been exactly how much of the sun was made of metals.

An influential 1989 review paper,
coauthored by Grevesse, reported that
metals make up 2 percent of the solar
photosphere, the lower level of the
sun’s atmosphere where spectral lines
are formed. But during the 1990s, new
and more sophisticated analyses began
to throw that estimate into doubt.
Though spectral lines alone offer
basic clues to the sun’s
innards, researchers
need models to inter-
pret those lines. Such
models try to reproduce
the churning and flow-
ing of gases in the sun’s
photosphere. Previous
modeling had used one-
dimensional computer
simulations that divided
the solar atmosphere into simple strati-
fied layers. In contrast, the new models
look at a small chunk of the atmosphere
but simulate it in three dimensions and
in much more detail—including, for
instance, how mass and energy churn
out from the convection zone, the out-
ermost layer of the sun’s interior, into
lower layers of the atmosphere.

The 3-D simulations had a profound implication: Astronomers’ earlier interpretations of spectral lines had to be changed. In some cases, the strength of an absorption line could signal an abundance that was different from what scientists had thought.

These recalculations seemed to improve things, by tightening estimates for the abundances of elements like iron and silicon. “It was only when we started applying it to more important elements like oxygen and carbon, the most abundant metals in the sun, that we quite quickly realized our results were going to be very different,” says Martin Asplund, an astronomer at the Max Planck Institute for Astrophysics in Garching, Germany.

Asplund’s team published estimates of solar photospheric oxygen abundance that were 30 to 40 percent lower than commonly accepted values, with similar changes for carbon, nitrogen and neon. The researchers also analyzed the other elements in the sun, most of which required only minor revisions. In all, the

Though spectral
lines alone offer
basic clues to the
sun’s innards,
researchers
need models to
interpret those
lines.

Wavelength

400 nm 500 nm 600 nm 700 nm

Peering within scientists take advantage of the fact that all elements absorb and emit light at particular wavelengths to study the composition of the sun and other celestial bodies.

Continuous spectrum
White
light Prism
When a narrow beam of hot white light is viewed through an instrument that acts as a prism,
the light is separated into its component parts, creating a continuous spectrum.

Gas similarly, gases emit light of different wavelengths depending on the elements present. When viewed through the instrument, a gas appears as a set of bright lines (hydrogen shown).