At the heart of our solar system, the sun shines like a gigantic laboratory of fundamental physics. For 4.6 billion years, this star has maintained stable nuclear fusion reactions, transforming hydrogen into helium. How do scientists manage to unravel the mysteries of its internal structure, invisible to our eyes but crucial to understand the stellar evolution?
Why does the interior of the sun fascinate astrophysicists so much? © Alexandarlich, Istock
Our star, representing more than 99 % of the mass of the solar system, interests researchers from around the world who try to understand the internal mechanisms. Sun physics is not limited to explaining our star: it serves as a reference for the study of all the stars of theuniverseuniverse. Recent advances in heliosemologyheliosemology And in nuclear physics question our traditional models, opening up new perspectives on the extreme conditions prevailing at the heart of our star.
The composition and structure of the sun revealed
The sun has a chemical composition now well identified thanks to spectroscopic observations. Its constitution is distributed approximately as follows:
- 73 % d’hydrogenhydrogen in mass (92 % in number ofatomsatoms) ;
- 25 % d’héliumhélium in mass (7.8 % in number of atoms):
- 2 % heavier elements (0.2 % in number of atoms).
However, this composition is not frozen. A phenomenon of sedimentationsedimentation continuously takes place: heavier atoms like theoxygenoxygen “Fall” gradually towards the deep layers, thus modifying the properties of solar plasma over time.
Scientists have developed sophisticated mathematical models to understand this internal structure. These “standard solar models”, the first definition of which dates back to the 1980s thanks to the work of theastrophysicistastrophysicist American John Bahcall, integrate various fundamental physical data such as the rates of nuclear reactionsnuclear reactions and the opacity of mattermatter.
Heliosemology: probe the invisible
How to observe the inside of an inaccessible star? Heliosemology offers an ingenious solution. This technique, similar to terrestrial seismology, analyzes the Acoustic wavesAcoustic waves spreading in the sun to reveal its internal conditions.
Solar vibrations, comparable to those of a musical instrument, allow researchers to measure with remarkable precision (less than a hundredth of error percentage) parameters like the densitydensity in approximately 95 % of the solar interior. These precious data are used to confirm or deny the theoretical models.
Since the beginning of the XXIe A century, this approach extended to the study of other stars, giving birth to asterosismology. Several major space missions, such as CorotCorotKepler, TessTess And soon Plato, use these techniques to precisely determine the masses, rays and ages of the stars of our galaxy.
Crises of solar models and their implications
The standard modelstandard model Solar has crossed several crises. The first, known as “solar neutrino crisis”, resulted from the detection of three times less neutrinosneutrinos that theoretically expected. This enigma was resolved thanks to a revision of neutrino physics, works awarded by Nobel awards in 2002 and 2015.
A second crisis emerged in the 2000s with the revision of the solar chemical composition. Improved spectroscopic measures revealed a 30 % reduction in mass abundance of carbonecarbone and oxygen. This change destroyed the preexisting agreement between the standard model and the observations resulting from heliosemology.
More recently, direct measures of the opacity of solar plasma have shown that current models underestimate this property by around 10 %. These results, confirmed by laboratory experiences reproducing almost solar conditions to SandiaSandia National Laboratoriesreveal the limits of our understanding of physics under extreme conditions prevailing at the heart of the sun.
Beyond our star
The challenges of solar physics are far beyond our stellar systemstellar system. Understand how theenergyenergy is transported within the sun is essential to model the evolution of all stars. Opacity, governed by atomic interactions, directly influences how a star evolves from birth to his death.
Scientists continue to refine their models, integrating the latest advances into nuclear physics and astrophysicsastrophysics. Dialogue between observations, laboratory experiences and digital simulationsdigital simulations Intensifies itself, enriching our understanding not only of the sun but of the stellar universe as a whole.
The sun, a natural laboratory of fundamental physics, reminds us that even the most familiar star is still keeping many secrets to be revealed to the future generations of researchers.
