A group of researchers at Sandia National Laboratories has found that the astronomical model that predicts the Sun’s behavior (the standard solar model, or SSM) underestimates the amount of energy blocked by iron atoms, adding a layer of complexity to a long-standing mystery in solar science.

Stars like our Sun produce energy in their cores from nuclear fusion of hydrogen. The energy produced in the core must travel through several layers of ionized materials (plasma) to reach the surface, where it’s radiated to space.

Certain elements, such as iron, can resist the transmission of energy by absorbing and then reemitting light. This blockage effect is known as opacity and plays an important role in how astronomers model the interior of the Sun. Researchers were able to heat samples of iron and other metals to 2.1 million degrees for a fraction of a second, briefly reproducing the environment inside a star.

Although each element contributes differently to the total opacity within the Sun, the SSM relies on complex calculations to gauge their particular contributions since it’s extremely difficult to directly test their opacities in the lab.

Thanks to a facility called the Z machine that uses powerful X-ray pulses to produce very high temperatures, the Sandia researchers were able to heat samples of iron and other metals to 2.1 million degrees for a fraction of a second, briefly reproducing the environment inside a star. When the samples, each of them the size of a grain of sand, are heated, they turn into plasma, allowing researchers to measure their opacities.

The team found large discrepancies between their measurements and the modeled opacities, meaning that our understanding of the Sun and other stars isn’t as good as astronomers assumed. The results were published online in the 14 June issue of Physical Review Letters.

The new experiments tried to replicate the conditions inside the Sun at 0.7 solar radius, a boundary zone where convection becomes the main energy transport mechanism, replacing radiation.

“If calculated iron opacity is wrong only at 0.7 solar radii, the problem is not too worrisome,” said Taisuke Nagayama, a physicist at Sandia National Laboratories and first author of the new study. “However, if there are similar or bigger discrepancies at different conditions, concerns will be spread out to almost all applications that rely on solar or stellar models.”

Reimagining the Sun’s Interior

In the early 2000s, new observations led some astronomers to think that the amount of elements heavier than hydrogen and helium in the Sun was lower than previously thought. However, the new observations didn’t agree with existing numerical models of the Sun. Simply put, when astronomers put the revised values into their models, they didn’t work anymore.

Astronomers considered several possible explanations for this problem: The new heavy-element abundances could be inaccurate, the standard solar model could be wrong, or the opacity values used in the SSM could be wrong.

The last explanation was probably the easiest to assess from an astronomer’s viewpoint because it wouldn’t require substantive changes to the SSM, just finding out the right values for the opacities. But this path is turning out to be very challenging as well.

“The fact is we know opacities are uncertain, so the question is, how uncertain?” “Just changing the opacities won’t solve all the problems,” said Sarbani Basu, an astronomer at Yale University who authored a commentary about the new research in Physics magazine. “But the fact is we know opacities are uncertain, so the question is, how uncertain?”

To gauge this uncertainty, Nagayama and his colleagues tested not only iron but also chromium and nickel. These elements bracket iron in the periodic table, so researchers expected they would behave similarly. But they didn’t. Instead, the experimental opacity values for nickel and chromium largely agree with the model predictions.

“This experiment has created more problems, because we expected nickel to have the least disagreement, iron to have a bit more, and chromium to have the most disagreement,” Basu said. “Clearly, we are missing things, and we don’t know what.”

Implications for Astronomy

Opacities are key to determining if the new estimates of heavy-element abundances in the Sun are correct. This isn’t trivial because abundances of every other object in the universe are usually measured in relation to solar abundances. These values are then used to create models of other stars and even galaxies.

“You can’t find the age of a star without making a model, so if you want to find the age of an exoplanetary system, you need to make a model of the host star to figure out how old it is,” Basu said.

The lack of reliable metallicity values for the Sun introduces a lot of uncertainty when estimating the ages of other stars, an error that could amount to billions of years. “Anything that deals with stars would be affected,” Basu said.

Looking for an Explanation

To explain these intriguing experimental results, physicists are exploring new possibilities. One of them is considering that certain elements could absorb two photons at a time instead of one. This phenomenon is called two-photon opacity.

If this mechanism is able to solve the discrepancy for iron, researchers would be closer to a solution since nickel and chromium already agree with the models. Other elements should then be tested to see whether or not they agree with the models.

In the near future, the team plans to continue testing other elements, including oxygen, which is a key contributor to overall solar opacity.

—Javier Barbuzano (@javibarbuzano), Freelance Science Journalist