How to Comprehend the Sun’s Inner Workings?

Our sun alone represents more than 99 % of the mass of the solar system. Without him, life on earth would be impossible. Beyond providing energy to earth, we can also consider it as a laboratory of fundamental physics. The study of its internal structure and its theoretical modeling makes it possible to highlight the limitations of our knowledge. For almost 4.6 billion years, our sun has been the only place of stable nuclear fusion reaction to the solar system. It is indeed by the fusion of hydrogen with helium that it produces its energy and that it will continue to do it for another 5 billion years.

For several decades, research groups around the world have endeavored to better reveal the interior of our star and to study the physical phenomena acting in these extreme conditions. The expertise is varied, ranging from European physicists and astrophysicists, of which I am a part, Russian and Japanese, passing by nuclear physics of the Los Alamos National Laboratory or the CEA of Paris-Saclay. Together, we try to unravel the mysteries of our star by revealing its hidden face, its internal structure. Our tools have astronomical observations made both from the soil and space, but also advanced digital simulations of the structure and the evolution of the sun, simply called “solar models” (in the sense of physical model, such as the models used in geophysics to describe the earth).

They constitute the theoretical basis on which the models used are developed to study all the other stars in the universe. Our sun serves as a calibrator for stellar physics. Consequently, changing solar model is changing a reference point for all stars.

Calculating a solar model is a balance exercise for an astrophysicist. You have to choose your constituent elements. It is immediately thinking of its chemical composition (mass: 73 % hydrogen, 25 % helium and 2 % heavier elements; in number of atoms: 92 % hydrogen, 7.8 % helium, 0.2 % heavier elements). However, other choices come into play. It is a question of modeling all the physical phenomena occurring within it. All these ingredients constitute elements of fundamental physics defining our vision of the sun, its “standard model”. The first definition of a standard model for our sun dates from around 1980 and is due to John Bahcall, an American astrophysicist.

The solar model crisis

The standard solar model had great successes, surviving the “crisis of solar neutrinos”. This crisis resulted from the detection of three times less neutrinos than theoretically expected. The divergence was explained by a review of Neutrino physics (rewarded by Nobel Prize winners in 2002 and 2015).

The standard solar model was reinforced, becoming a key element of the theory of stellar evolution. In the 2000s, the revision of solar chemical composition led to a new crisis. These measures were carried out using spectroscopic observations, making it possible to identify each chemical element present in the solar atmosphere. Improving spectroscopic measurements and analysis techniques was at the origin of this revision.

This work, confirmed later, led to a 30 % reduction in mass abundance of carbon and oxygen. This change destroyed the existing agreement of the standard model with neutrino measures and the constraints from the study of solar oscillations, called heliosemology. As in terrestrial seismology, which uses the waves crossing our planet to study the interior, heliosemology uses acoustic waves spreading in the sun to measure the internal conditions. Thanks to solar oscillations, we knew precisely certain properties such as the density in 95 % of the interior of our star.

The revision of the chemical composition of the sun was badly welcomed because it invalidated the standard model. Several groups wanted to maintain the values ​​of the XX^e century. The controversy inflated and recent independent measures by heliosemology confirmed the reduction in oxygen, while maintaining the differences observed in the central regions.

The explanation of the disagreements of theoretical models with the interior of the sun is to be found elsewhere … It is in this context of intense debate that my work began, ten years ago, during my doctoral thesis. I choose to adapt digital tools at my disposal to study the internal structure of the sun. What was to be a small detour during my thesis has become a major project, due to the renewed interest in heliosemology and for solar models.

The importance of fundamental physics in solar physics

A solar model is not limited to its chemical composition. It involves a series of elements that must follow the advances in theoretical and experimental physics. We know that the sun produces its energy by fusion of hydrogen with helium, the observations of solar neutrinos have confirmed it irrefutably. However, the speed of these reactions remains subject to small corrections. These revisions are minimal, but the almost surgical level of precision with which we study the sun makes them significant.

Another key ingredient in solar models is the opacity of solar material, linked to its ability to absorb energy from radiation. As said above, the sun generates its energy by nuclear fusion in its heart. This energy, before reaching us on earth, must be transported from inside the sun to its atmosphere. In 98 % of its mass, it is the high energy radiation (X -rays) that takes care of it. Thus, if we change the “transparency” of the solar environment, we completely change the internal structure of our star.

In the solar case, we are talking about extreme conditions, almost impossible to reproduce on earth (temperatures of several million degrees, high densities). Opacity has always played a key role in stellar physics, its successive revisions made it possible to resolve several crises in the past. Each time, the theoretical calculations had underestimated opacity. Quickly, it was considered that a new revision would “save” solar models. As early as 2009, astrophysicists were trying to estimate the required changes. However, one of the great difficulties resided in the knowledge of the chemical composition of the solar interior. Indeed, our star is not static. Over time, its chemical composition evolves under the effect of nuclear reactions at the heart and sedimentation. Thus, an oxygen atom on the surface of the sun, heavier than its environment, “will fall” towards the deep layers, changing the properties of the plasma.

The sun as a physics laboratory

These questions are linked to our knowledge of the internal physical conditions of the sun and therefore to our ability to measure them.

The precision reached on the density of the solar environment is phenomenal, less than the hundredth of percent. These very precise measures allowed me to develop methods of direct determination of the absorption of solar plasma, the much sought -after opacity.

They have shown that the opacity of current solar models is lower than heliosemics of approximately 10 %. These results independently confirmed the measures of the Sandia National Laboratories (United States), where physicists have reproduced almost solar conditions and measured the absorption capacity of plasma. In 2015, these measures had already shown significant differences between theory and experience. Ten years later, they are confirmed by new campaigns and independent measures. The ball is now in the theorists' camp, in order to explain worrying differences that reveal the limits of our understanding of physics under the extreme conditions of our star.

From sun to stars

However, the issue far exceeds our vision of the interior of the sun. Since the beginning of the XXI^e century, many missions are devoted to the study of stars and their exoplanets. Heliosemology techniques have naturally exported to other stars, leading to the exponential development of asterosismology.

No less than four major missions were devoted to this discipline: Corot, Kepler, Tess and soon Plato. All aim to determine precisely the masses, rays and ages of the stars of our galaxy, the stellar models being essential to map the evolution of the universe. However, all these considerations on the dating of cosmic objects bring us to the infinite and small. Giving the age of a star requires precisely understanding the physical conditions governing its evolution.

Thus, knowing how energy is transported within it is essential to understand how, from birth to its death, the star evolves. Opacity, governed by interactions on the scale of Angström (10^-10 m), is therefore essential to model the evolution of the stars, starting with our sun.