What do you do when a tried-and-true method for determining the sun’s chemical composition appears to be at odds with an innovative, precise technique for mapping the sun’s inner structure? That was the situation facing astronomers studying the sun—until new calculations that have now been published by Ekaterina Magg, Maria Bergemann, and colleagues, and that resolve the apparent contradiction.
The decade-long solar abundance crisis is the conflict between the sun’s internal structure as determined by solar oscillations (helioseismology) and the form derived from the fundamental theory of stellar evolution, which relies on the present-day sun’s measurements and chemical composition. The new calculations of the physics of the sun’s atmosphere yield updated results for the abundances of different chemical elements, which resolve the conflict. Notably, the sun contains more oxygen, silicon, and neon. The methods employed also promise considerably more accurate estimates of the chemical compositions of stars in general.
Astrochemistry using spectra
The tried-and-true method in question is spectral analysis. To determine our sun’s chemical composition or any other star, astronomers routinely turn to spectra: the rainbow-like decomposition of light into different wavelengths. Stellar spectra contain conspicuous, sharp dark lines, first noticed by William Wollaston in 1802, famously rediscovered by Joseph von Fraunhofer in 1814, and identified as tell-tale signs indicating the presence of specific chemical elements by Gustav Kirchhoff and Robert Bunsen in the 1860s.
Pioneering work by the Indian astrophysicist Meghnad Saha in 1920 related the strength of those “absorption lines” to stellar temperature and chemical composition, providing the basis for our physical models of stars. Cecilia Payne-Gaposchkin’s realization that stars like our sun consist mainly of hydrogen and helium, with no more than trace amounts of heavier chemical elements, is based on that work.
Solar oscillations that tell a different story
The underlying calculations relating spectral features to the chemical composition and physics of the stellar plasma have been of crucial importance to astrophysics ever since. They have been the foundation of century-long progress in our understanding of the chemical evolution of the universe and the physical structure and evolution of stars and exoplanets. That is why it came as something of a shock when, as new observational data became available and provided an insight into the inner workings of our sun, the different pieces of the puzzle did not fit together.
The modern standard model of solar evolution is calibrated using a famous (in solar physics circles) set of measurements of the solar atmosphere’s chemical composition, published in 2009. But in several important details, a reconstruction of our favorite star’s inner structure based on that standard model contradicts another set of measurements: helioseismic data, that is, measurements that track the minute oscillations of the sun as a whole very precisely—the way that the sun rhythmically expands and contracts in characteristic patterns, on time scales between seconds and hours.
Like seismic waves provide geologists with crucial information about the Earth’s interior, or like the sound of a bell encodes information about its shape and material properties, helioseismology includes information about the sun’s interior.