team found a solar metal content not of
2 percent, but of 1. 4 percent. The
researchers summarized their latest
work last year in the Annual Review of
Astronomy and Astrophysics.
Radiating puzzles
Lower metallicity doesn’t bode well
for other aspects of solar study — particularly models of the solar interior
based on helioseismology, the study of
how waves, such as sound waves, travel
through the sun.
Turbulence within the sun generates
sound waves that oscillate back and forth
within the star. Studies of this reverberation have yielded important insights into
the solar interior, such as how its layers
rotate at different speeds. The technique
can even be used to study sunspots on
the far side of the sun, since sunspots
absorb the sound waves and dampen
the reverberation.
Helioseismology studies have built
up a certain picture of the solar interior,
including such details as the depth of the
convection zone. These models of the
solar interior were reasonably successful, and physicists thought they understood the sun’s insides.
The work of Asplund’s team throws a
wrench in that picture. If models of the
solar interior are adjusted to fit the 30
to 40 percent lower oxygen abundance
in the photosphere, then they no longer
match up with helioseismologists’ obser-
vations. For instance, the models now cal-
culate incorrect values for the speed of
sound and the density within the sun, as
compared with those actually measured.
Disputes over selected solar abundances
Anders &
Grevesse
1989
M. Asplund
et al.
2009
E. Caffau
et al.
2010
Carbon 8.56 8.43 8.50
Nitrogen 8.05 7.83 7.86
Oxygen 8.93 8.69 8.76
Neon 8.09 7.93 n/a
sources: Geochimica et cosmochimica acta 1989; annual Review
of astRonomy and astRophysics 2009; solaR physics 2010
Debated values two teams have published
revised estimates for photospheric metal abundances (selected ones above) that differ from
1989 values. (the values are expressed on a
log scale that sets hydrogen at 12.)
Total solar composition Metal composition
What lies within although scientists are still debating the sun’s precise composition, one
new study suggests that hydrogen makes up just over 71 percent, helium 27 percent and metals
1.43 percent. of the metals, oxygen is by far the most abundant, followed by carbon, iron and neon.
Metals
1.4%
He
27.0%
H
71.5%
source: m. asplund
are correct. It’s possible to study metal
content in other galaxies without having
a solar reference to pin it to, she notes.
For instance, astronomers can compare
the abundance of oxygen with the abundance of hydrogen in a particular galaxy.
Using this same measure over many
galaxies allows researchers to build a
comparative picture of galaxy metallicity without ever having to worry about
what the sun’s true oxygen abundance is.
Still, arguments over the true solar
numbers are making a lot of researchers uncomfortable. Several years ago
at the Paris Observatory in France,
scientists led by Hans-Günter Ludwig
decided to use their own 3-D simulations of the solar atmosphere to see if
they could confirm Asplund’s work. And
they found that they couldn’t, at least not
exactly. Ludwig’s team, including then
graduate student Elisabetta Caffau, has
reported its own abundances of certain
elements — most of which were higher
than Asplund’s but not as high as the earlier, 1980s-era estimates.
This spring, the two teams converged
in Garching for a weeklong discussion
to try to hammer out their differences.
Both groups agree that their 3-D models
confirm lower metallicity. That finding
is also broadly supported by another line
of evidence: primordial meteorites. The
space rocks known as “CI carbonaceous
chondrites” are thought to be the most
chemically primitive rocks known, providing clues about what the chemistry
of the early solar system, including the
newborn sun, was like. Analyses of such
