In his models, each of the universe’s four most abundant heavy elements took a major hit. Compared with numbers published 20 years earlier, the 2009 article by Asplund and colleagues recommended sharply lower values. The new models slashed the estimated oxygen level in the Sun and thus in the universe by a whopping 42 percent. Carbon, another prerequisite for life, fell 26 percent, while the neon and nitrogen levels plummeted 31 percent and 40 percent, respectively.
By all calculations, these four elements account for the vast majority (88 percent in Asplund’s work, a bit more in other numbers) of all heavy atoms in the universe. If Asplund was right, the universe had far fewer of them than anyone had thought. And that meant huge trouble for models of the Sun’s interior.
Inside the Sun
Heavy elements such as oxygen alter the Sun’s interior, because they absorb radiation as it wends its way outward from the solar core to the surface. Using the old solar abundances, astronomers thought they had the Sun’s interior figured out, thanks to a technique known as helioseismology. Just as our world has earthquakes, so the Sun’s interior vibrates with sound waves. And just as seismologists use quakes to deduce the structure of the Earth’s interior, so the vibrations rippling through the Sun have revealed its inner structure.
For example, in most of the Sun’s interior, radiation bounces from atom to atom, slowly carrying heat from the core outward. In the outermost parts of the Sun, however, material is cooler and more opaque, largely because heavy elements, such as oxygen, absorb photons. This opacity means photons can’t ferry heat there. Instead, a process called convection sets in: Hot gas rises to the solar surface, radiates heat, then cools and sinks back down. You see something similar when you boil a pot of water.
Helioseismology pinpoints the position of the boundary between the Sun’s radiative interior and its convective envelope. “That shows up as a glitch in the sound waves,” Pinsonneault says. As a result, we know that this boundary occurs at precisely 71.3 percent of the solar radius. But if the Sun actually has less oxygen, carbon, neon and nitrogen, then the Sun’s interior is less opaque, allowing radiation to carry heat farther from the Sun’s center, contradicting the helioseismological observations. “Either we don’t understand the Sun or the [new solar abundances] are wrong,” Pinsonneault said at a 2011 talk where he favored a higher oxygen abundance.