New York/Paris – Iron inside the Sun is significantly more opaque than previously estimated, U.S. and French scientists reported on Monday, challenging long-standing assumptions at the core of solar and stellar models.
In a study published in Physical Review Letters, researchers at Sandia National Laboratories in the U.S., in collaboration with French scientists, confirmed that the discrepancy between observed solar data and model predictions stems from inaccurate theoretical opacity values of iron — not from errors in experimental data.
The team exposed a thin iron sample to intense X-rays under solar-like conditions and used ultrafast X-ray imaging to track changes in temperature and particle density more than a billion times per second. Their results showed that iron absorbs radiation far more efficiently than models had accounted for — casting a “darker shadow” in spectrometer readings.
“Our new measurements contradict the hypothesis that temporal evolution in plasma explains the model-data mismatch,” the team wrote. “Instead, it is the theoretical models themselves that need to be revised.”
Opacity — the ability of a material to absorb and scatter radiation — plays a critical role in how energy travels from a star’s core to its surface. Earlier models underestimated iron’s opacity by up to 400% under solar conditions, especially near the Sun’s radiation-convection zone boundary, around 30% from its surface.
The new findings support seismic studies published earlier this year that suggested opacity levels were 10% higher than theoretical values near 2 million degrees Celsius. These independent studies converge on a central message: existing models of the Sun’s interior likely undervalue iron’s contribution to energy transport.
Researchers say these insights could have far-reaching implications for how scientists understand the evolution and structure of stars, including those far beyond our solar system.
However, the authors noted that further work is needed. While the current research used a metric called “line optical depth” to measure opacity, future studies will need to produce “absolute transmission” data — a more direct measure — and include formal uncertainty assessments.
“Such an absolute opacity approach is presently under investigation,” the study noted.
The findings represent a rare breakthrough in stellar physics, where experimental validation of deep-solar conditions has remained elusive due to extreme temperature and pressure requirements. The experiments at Sandia required energizing plasma electrons to 180 eV and reaching densities of 30,000 billion billion particles per millilitre — conditions now achievable only with cutting-edge technology.
The study comes a decade after models began to struggle with discrepancies in measured abundances of solar elements like carbon, oxygen, and nitrogen — sparking renewed focus on opacity as a possible source of error.
Iron, it seems, may hold the key.
