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 “something fundamentally wrong” with their understanding of the sun, and therefore of all stars, said Vagnozzi. “That would be huge.”
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, helioseismology, 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 helioseismology should agree. That is, astronomers should be able to use the helioseismological 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.
“This is a problem not only for solar physics, but by extension for astronomy as a whole,” said Asplund, who led the team behind the precise spectroscopic measurements. “Either astronomers do not understand how to measure elemental abundances of stars using spectroscopy, or our understanding of stars’ interiors and how they oscillate is incomplete,” he said. “Either way, it has major ramifications, since stars are the fundamental probes of the cosmos, with stellar astrophysics providing much of the foundation for modern astronomy and cosmology.”
After years of talking about what might be going wrong—including speculations about dark matter in the sun—the debate has reached “a bit of a stalemate,” said Sarbani Basu, a solar astrophysicist at Yale. 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.