by NASA's Solar Dynamics Observatory.
If and when physicists are able
to pin down the metal content of the sun,
that number could upend much of what
we thought we knew about the
evolution and life span of stars.
But it's the sun's tiny concentration of heavier elements, which astronomers call metals, that controls its fate.
The more metallic a star, the more opaque it is (since metals absorb radiation), and how opaque it is in turn relates to its size, temperature, brightness, life span and other key properties.
But the sun's metallicity, beyond revealing its own story, also serves as a kind of yardstick for calibrating measurements of the metallicity of all other stars, and thus the ages, temperatures and other properties of stars, galaxies and everything else.
Yet, ever more precise measurements of the sun's metallicity have raised more questions than they've answered.
Astronomers' inability to solve the mystery known variously as the solar metallicity, solar abundance, solar composition or solar modeling problem suggests there could be,
Twenty years ago, astronomers thought they had the sun sorted.
Direct and indirect ways of inferring its metallicity both gauged the sun as approximately 1.8 percent metal - a happy convergence that led them to believe they understood not only the length of their solar yardstick but also how the sun works.
However, throughout the 2000s, increasingly precise spectroscopic measurements of sunlight - a direct probe of the sun's composition, since each element creates telltale absorption lines in the spectrum - indicated a far lower metallicity of just 1.3 percent.
Meanwhile, helio-seismology, the competing, indirect approach for inferring metallicity based on the way sound waves of different frequencies propagate through the sun's interior, still said 1.8 percent.
But if astronomers' theory of the sun, called the "standard solar model," is correct, spectroscopy and helio-seismology should agree.
That is, astronomers should be able to use the helio-seismological measurements to calculate the depth of an important boundary layer in the sun where radiation gives way to convection.
And this depth relates, according to the equations, to the sun's opacity, and therefore to its metallicity.
This sequence of calculations should predict the same value for the metallicity as spectroscopers measure directly from sunlight. It does not.
After years of talking about what might be going wrong - including speculations about dark matter in the sun - the debate has reached,
But there's hope.
Recently, a weak hint about the solar metallicity has come from fleeting particles emanating from the sun called solar neutrinos.
Different nuclear fusion reactions produce solar neutrinos of different energies, and so the particles carry information about the sun's composition.
At a conference last month in Heidelberg, Germany, the Borexino experiment based at Italy's Gran Sasso National Laboratory reported detections of solar neutrinos that marginally favor the higher, 1.8 percent estimate of the sun's metallicity.
If this high-metallicity estimate is indeed correct, this raises questions about what, exactly, went wrong with Martin Asplund and collaborators' spectroscopic measurements.
But Asplund stands by his 1.3 percent spectroscopic estimate.
He points to a 2015 study (A Higher-than-predicted Measurement of Iron Opacity at Solar Interior Temperatures) in Nature indicating that metals might increase opacity even more than previously thought in the high-pressure conditions of the sun's core.
Correcting for this difference in the standard solar model could bring the helio-seismological and neutrino estimates of metallicity down to 1.3 percent, he said.
In the coming years, the Borexino team expects to detect rare solar neutrinos produced in the CNO cycle, a fusion reaction in the sun in which carbon, nitrogen and oxygen atoms serve as catalysts for fusing hydrogen into helium.
If it turns out that the sun is, in fact, only 1.3 percent metal, this would mean the standard solar model really does have opacity wrong.
Estimated ages of stars and galaxies would have to be revised by as much as 10 to 15 percent.
Unfortunately for the sun itself (and future life on Earth), low-metallicity stars burn fuel faster than high-metallicity stars, so our sun would die about a billion years sooner than we thought...