Chemical composition of the sun clarified

New Spektrum bills end decades of controversy

Astronomers have ended a decades-long conflict. So far, reconstructions of the internal structure of the Sun from analysis of solar oscillations (helioseismology) have not led to the structure that emerges from the basic theory of stellar evolution, which in turn is based on measurements of the chemical composition of the present-day Sun. New calculations by the Max Planck Institute for Astronomy in Heidelberg on the physics of the solar atmosphere correct the abundance values ​​for several chemical elements. In particular, the sun contains more oxygen, silicon and neon than previously thought. The structural discrepancy thus disappears. The methods used also promise more accurate estimates of the chemical composition of stars in general. On May 20th, 2022 open access in Astronomy & Astrophysics released.

Sonne – Photo © Gerhard Hofmann, agency future, for Solarify

What to do when a proven method for determining the chemical composition of the sun seems at odds with an innovative, precise technique for mapping the structure of the sun? This was the situation that astronomers have been confronted with researching the sun in recent years – until new calculations that Ekaterina Magg, Maria Bergemann and their colleagues have now published resolved the apparent contradiction.

Astrochemistry with spectra

The proven method in question is spectral analysis. In order to determine the chemical composition of our sun or other stars, astronomers routinely use spectra: rainbow-like decompositions of light into its different wavelengths. Stellar spectra contain prominent, sharp dark lines, first discovered by William Wollaston in 1802, rediscovered by Joseph von Fraunhofer in 1814, and recognized by Gustav Kirchhoff and Robert Bunsen in the 1860s as evidence of the presence of certain chemical elements.

The pioneering work of Indian astrophysicist Meghnad Saha in 1920 showed the quantitative relationship between the strength of these “absorption lines” and stellar temperature and chemical composition. This provided the basis for our physical models of stars. Cecilia Payne-Gaposchkin’s finding that stars like our sun are composed primarily of hydrogen and helium, with only trace amounts of heavier chemical elements, is based on this work.

Solar vibrations tell a different story

The underlying calculations, which relate spectral properties on the one hand and chemical composition and physics of the stellar plasma on the other, have been of crucial importance for astrophysics since Saha’s time. They formed the basis of centuries of advances in understanding the chemical evolution of the Universe, as well as reconstructions of the physical structure and temporal evolution of stars and exoplanets. It was therefore quite a shock when new observational data became available that provided insights into the inner workings of our Sun – but the results absolutely did not match what had been reconstructed on the basis of the spectra.

The modern standard model of solar evolution is calibrated against a famous (in solar physics) series of measurements of the chemical composition of the solar atmosphere, published in 2009. The novel data are so-called helioseismic data, measurements that very accurately capture the tiny oscillations of the Sun as a whole – the way the Sun’s surface rhythmically expands and contracts in characteristic patterns, on timescales ranging from seconds to hours .

Just as seismic waves provide geologists with important information about the interior of the Earth, or the sound of a bell provides information about its shape and material properties, helioseismology provides information about the interior of the Sun.

A Crisis in Solar Chemistry

High-precision helioseismic measurements allowed conclusions to be drawn about the internal structure of the sun that contradicted the standard models of the structure of our star based on solar chemistry. Specifically, according to helioseismology, the so-called convective region inside our Sun, where matter rises and falls like water in a saucepan, was much larger than the Standard Model predicted. The speed of sound waves in the lower regions of the convection zone also differed from what the Standard Model predicted, as did the total amount of helium in the Sun. To make matters worse, certain measurements of solar neutrinos — ephemeral, elusive elementary particles that reach us directly from the Sun’s core regions — didn’t quite fit the Standard Model.

The astronomers soon spoke of a “solar abundance crisis”, analogous to a solar chemistry crisis. The proposed solutions ranged from the unusual to the exotic: had the sun perhaps accumulated metal-poor gas during its planet formation phase? Is the energy in the sun’s interior transported by the actually non-interacting particles of dark matter?

Calculations beyond local thermal equilibrium

The study by Ekaterina Magg, Maria Bergemann and colleagues that has now been published presents a solution that does not require any exotic physics. Instead, it offers a fundamental revision of the models, on the basis of which the chemical composition is inferred from the solar spectrum. Early studies of this type relied on the assumption of what is known as local thermal equilibrium: they assumed that the energy in each part of a star’s atmosphere has time to dissipate and reach some kind of equilibrium at each of the phases of its evolution. This means that a temperature can be assigned to each such region. This leads to a considerable simplification of the calculations.

But as early as the 1950s, some astronomers realized that this picture was oversimplified. Since then, so-called non-LTE calculations have been carried out in more and more studies, in which the assumption of a local equilibrium (English local thermal equilibrium, LTe) omitted. The non-LTE calculations provide a detailed description of the energy exchange within the system – atoms are excited by photons (particles of light) or collide, photons are emitted, absorbed or scattered. In stellar atmospheres where the density is far too low for the system to reach thermal equilibrium, this level of detail pays dividends. There, non-LTE calculations deliver results that differ significantly from calculations that postulate local thermal equilibrium.

Application of non-LTE to the photosphere of our sun

Maria Bergemann’s group at the Max Planck Institute for Astronomy is a world leader in applying non-LTE computations to stellar atmospheres. As part of her doctoral work in this group, Ekaterina Magg set out to more precisely calculate the interaction of radiation with matter in the solar photosphere – the photosphere is the outer layer of the solar atmosphere from which most of the sun’s outwardly radiated light originates and in which the absorption lines in the solar spectrum are also stamped.

In the study in question, the scientists looked at all chemical elements relevant to the current models of stellar evolution. To ensure that they obtained consistent results, the researchers applied several independent methods to describe the interactions between the atoms and the sun’s radiation field. To describe the convective regions of our sun, they used existing simulations that take into account both the movement of the plasma and the physics of radiation (“STAGGER” and “CO5BOLD”). For the comparison with spectral data, they chose the data set with the highest quality available: the solar spectrum published by the Institute for Astrophysics and Geophysics at the University of Göttingen. “We worked intensively on the analysis of statistical and systematic effects that limit the accuracy of our results,” explains Magg.

A sun with more oxygen and more heavier elements

For some elements, the new calculations revealed a significantly different relationship between the element abundance and the strength of the corresponding spectral lines than in previous work. Accordingly, significantly different chemical abundances emerge when analyzing the observed solar spectrum compared to previous analyses.

Magg says, “We found that the proportion of elements heavier than helium in the Sun is 26% higher than previous studies had suggested.” Astronomers call these heavier elements metals. Altogether, metals make up only a few thousandths of a percent of all atomic nuclei in the sun; the best estimate for this value is now 26% higher than in previous studies. Magg adds, “The oxygen abundance value was almost 15% higher than in previous studies.” chemical composition of the early solar system.

Crisis overcome!

If the new values ​​are used as input to models of the structure and evolution of the Sun, then the puzzling discrepancy between the results of those models and the helioseismic measurements disappears. The thorough analysis of the origin of the spectral lines by Magg, Bergemann and their colleagues, based on much more complete models of the underlying physics than previous work, shows how the crisis can be overcome. Maria Bergemann says: “The new solar models, based on the new chemical composition values ​​we have determined, are more realistic than ever before: they result in a model of the sun that is compatible with all the information that we have about the structure of the sun today – sound waves, neutrinos, luminosity and solar radius – without having to consult exotic inner-solar physics.”

An additional advantage is that the new models can easily be applied to stars other than the Sun. At a time when large-scale surveys such as SDSS-V and 4MOST are yielding high-quality spectra for an ever-increasing number of stars, this kind of advance is valuable indeed – and provides future analysis of stellar chemistry with its broader implications for reconstructions of the chemical development of our cosmos on a more solid basis than ever before.

-> Source and further information:

  • Original release: Ekaterina Magg, Maria Bergemann, Aldo Serenelli, Manuel Bautista4, Bertrand Plez, Ulrike Heiter, Jeffrey M. Gerber, Hans-Günter Ludwig, Sarbani Basu, Jason W. Ferguson, Helena Carvajal Gallego, Sébastien Gamrath, Patrick Palmeri and Pascal Quinet: Observational constraints on the origin of the elements. IV: The standard composition of the Sunin Astronomy & Astrophysics, – open access