SOHO’s EIT (Extreme ultraviolet Imaging Telescope) images the solar atmosphere at several wavelengths and, therefore, shows solar material at different temperatures. In the images taken at 304 Angstroms, the bright material is at 60,000 to 80,000°K. In those taken at 171, at 1 million Kelvin. 195 Angstrom images correspond to about 1.5 million Kelvin. The hotter the temperature, the higher you look in the solar atmosphere. Credit: SOHO instrument consortium. Public Domain.

Researchers have developed an innovative method using helioseismology to measure solar radiative opacity under extreme conditions.

This groundbreaking work not only reveals gaps in our understanding of atomic physics but also confirms recent experimental results, thereby opening new perspectives in astrophysics and nuclear physics.

Helioseismology is a discipline dedicated to studying the Sun’s acoustic oscillations, enabling us to probe the interior of our star with remarkable precision.

By analysing these waves, it is possible to reconstruct fundamental parameters such as the density, temperature, and chemical composition of the Sun’s plasma – essential elements for understanding how our star works and evolves.

This method transforms the Sun into a true astrophysical laboratory, providing crucial data for refining stellar models and better understanding the evolution of stars in the Universe.

ESA’s mission Plato, PLAnetary Transits and Oscillations of stars, will use its 26 cameras to study terrestrial exoplanets in orbits up to the habitable zone of Sun-like stars. The mission will discover the sizes of exoplanets and discover exomoons and rings around them. Plato will also characterise their host stars by studying tiny light variations in the starlight it receives. Credit: ESA. Public Domain.

Independent measurement

A new international study, led by Gaël Buldgen, a researcher at the University of Liège, has used helioseismic techniques to provide an independent measurement of the absorption of high-energy radiation by the solar plasma in the deep layers of its structure.

This collaborative work sheds new light on solar radiative opacity, a crucial physical quantity for understanding the interaction between matter and radiation in the extreme conditions of the Sun’s interior.

The results confirm observations made in renowned American laboratories such as the Sandia National Laboratories and ongoing efforts at the Livermore National Laboratory, while revealing persistent gaps in our understanding of atomic physics and differences between the predictions of research groups at the Los Alamos National Laboratory, the Ohio State University and the research centre of the CEA Paris-Saclay in France.

Unprecedented precision in stellar modelling

The scientific team used advanced numerical tools developed at ULiège, drawing on the university’s expertise in helioseismology and stellar modelling. 

Buldgen explained: “By detecting the Sun’s acoustic waves with unparalleled precision, we can reconstruct our star’s internal properties, in much the same way as we would deduce the characteristics of a musical instrument from the sounds it produces.”

The precision of helioseismic measurements is exceptional: they allow us to estimate the mass of a cubic centimetre of matter inside the Sun with an accuracy surpassing that of a high-precision kitchen scale without ever seeing or touching the matter.

Helioseismology, developed at the end of the twentieth century, has played a major role in advancing fundamental physics. In particular, it has contributed to major discoveries, such as neutrino oscillations, which the 2015 Nobel Prize recognised.

These advances demonstrated that solar models were not to blame for the origin of this phenomenon. Still, adjustments were needed with the revision of the solar chemical composition in 2009, confirmed in 2021.

Crisis

This revision caused a crisis in solar models, which no longer agreed with the helioseismic observations.

To meet this challenge, advanced tools have been developed at the University of Liège, initially as part of doctoral work, and then enriched through international collaborations in Birmingham and Geneva.

These tools have made it possible to revisit the internal thermodynamic conditions of the Sun and to reopen an issue that the scientific community had somewhat neglected.

At the same time, the work carried out in 2015 by James Bailey at Sandia National Laboratory highlighted the crucial role of radiative opacity.

The first experimental measurements were first met with some skepticism, as they revealed significant differences with theoretical predictions.

Today’s helioseismic measure provides valuable confirmation and makes it possible to specify the temperature, density and energy regimes in which these experiments should be concentrated in order to better reproduce solar conditions.

The Z machine, the largest X-ray generator in the world, is located in Albuquerque, New Mexico. As part of the Pulsed Power Program, which started at Sandia National Laboratories in the 1960s, the Z machine concentrates electrical energy and turns it into short pulses of enormous power, which can then be used to generate X-rays and gamma rays. Credit: Randy Montoya/ Sandia National Laboratories.

Z Machine

In addition, the Z Machine experiments, although extremely valuable, have prohibitive energy and financial costs.

Helioseismic measurements, on the other hand, offer an economical and complementary alternative while guiding experimentalists towards optimal windows for their laboratory measurements.

The implications of this research extend far beyond stellar modelling; it improves the accuracy of the theoretical models used to estimate the age and mass of stars and exoplanets, thereby contributing to our understanding of galactic evolution and stellar populations. 

Buldgen added: “The Sun is our great calibrator of stellar evolution, our preferred laboratory for finding out whether we are on the right track, or not.

“These results are even more important as we prepare to launch the PLATO satellite in 2026, one of the objectives of which is to accurately characterize solar-type stars to find habitable terrestrial planets. 

“What’s more, these results have resonances in nuclear fusion, as the Sun remains the only stable nuclear fusion reactor in our solar system. 

“Improving our understanding of the Sun’s internal conditions directly impacts fusion energy research, a key issue in the development of clean energy solutions.”

A call for refined theoretical models

The results highlight the need to improve existing atomic models to resolve the discrepancies between experimental observations and theoretical calculations.

These advances should redefine our understanding of stellar evolution and the physical processes that govern the structure and evolution of stars.

This research confirms the University of Liège’s position at the cutting edge of astrophysical science, demonstrating the key role of helioseismology in unlocking the mysteries of the cosmos.

The study is published in Nature Communications.