The Chemistry of the Sun: Resolving a Decade-Long Controversy About the Composition of Our Star

The ten-year-old debate about the composition of our star is been settled by new estimates of the solar spectrum.

Our sun still retains secrets, despite being far closer than any other star in the cosmos. Since we only have a limited view, it is still 93 million miles (150 million kilometers) away from Earth. Its atmosphere and surface are both very hot, and it continually ejects particles at a speed of around 1 million miles per hour. It makes sense that we're still finding discoveries now.

In fact, the conflict between the internal structure of the Sun as determined by solar oscillations (helioseismology) and the structure derived from the basic theory of stellar evolution, which in turn depends on measurements of the current Sun's chemical composition, has just been resolved by astronomers. Updated figures for the abundances of various chemical elements are produced by new simulations of the physics of the Sun's atmosphere, which put an end to the disagreement. Notably, more amounts of oxygen, silicon, and neon are found in the Sun than previously believed. The techniques used also offer far more precise calculations of the chemical makes-up of stars in general.

What do you do when a new, accurate approach for mapping the Sun's inner structure appears to be at conflict with a tried-and-true method for identifying the Sun's chemical composition? Up until the latest calculations, which have now been published by Ekaterina Magg, Maria Bergemann, and colleagues and which reconcile the seeming contradiction, astronomers studying the Sun were faced with that dilemma.

Spectral analysis is the time-tested approach in issue. Astronomers frequently use spectra, the rainbow-like breakdown of light into its many wavelengths, to ascertain the chemical makeup of our Sun or any other star in the universe. William Wollaston initially saw prominent, sharp black lines in stellar spectra in 1802, and Joseph von Fraunhofer notably rediscovered them in 1814. In the 1860s, Gustav Kirchhoff and Robert Bunsen recognized these lines as telltale indications indicating the existence of particular chemical elements.

Our physical models of stars are based on Meghnad Saha's groundbreaking work, which connected the intensity of those "absorption lines" to stellar temperature and chemical composition in 1920. Based on her studies, Cecilia Payne-Gaposchkin came to the conclusion that stars like our Sun are mostly composed of hydrogen and helium, with just tiny quantities of heavier chemical elements.

Since then, astrophysics has placed a premium on the underlying computations linking spectrum characteristics to the chemistry and physics of the star plasma. They have served as the cornerstone of a century-long advancement in our knowledge of the universe's chemical development, as well as the physical makeup and evolution of stars and exoplanets. That is why it was a bit of a surprise when it appeared that the various parts of the puzzle did not fit together when fresh observational data became available and revealed information about the inner workings of our Sun.

A well-known collection of observations of the chemical make-up of the solar atmosphere, released in 2009, is used to calibrate the current standard model of solar development. However, helioseismic data, which are measurements that track very precisely the minute oscillations of the Sun as a whole and the way that the Sun rhythmically expands and contracts in characteristic patterns, on time scales between seconds and hours, contradict a reconstruction of the inner structure of our favorite star based on that standard model in a number of key details.

Helioseismology offers information about the inside of the Sun, just as seismic waves provide geologists important information about the core of the Earth, or way the sound of a bell encodes information about its structure and material qualities.

The Sun's internal structure was shown by very precise helioseismic observations to be different from what was predicted by solar standard models. Helioseismology discovered that our Sun's 'convective area,' where matter rises and falls back down like water in a boiling pot, was far greater than what the traditional model anticipated. The amount of helium in the Sun overall and the speed of sound waves towards the bottom of that area both departed from predictions made by the mainstream model. Additionally, several measurements of solar neutrinos, which are transient, difficult to detect, and directly emitted from the Sun's core regions, were slightly wrong compared to experimental evidence.

In quest of a solution, astronomers experienced what they quickly began to refer to as a "solar abundances crisis," and some of their suggestions varied from the uncommon to the downright bizarre. Did the Sun possibly accrete any gas lacking in metal during the planet-forming process? Are the infamously non-interacting dark matter particles carrying energy?

By reviewing the models upon which the spectral estimations of the Sun's chemical composition are based, the recently published work by Ekaterina Magg, Maria Bergemann, and colleagues was able to end that dilemma. Early investigations on how stars' spectra are created had depended on a concept called local thermal equilibrium. They had thought that energy had enough time to disperse locally and attain a state of equilibrium in each area of a star's atmosphere. Assigning a temperature to each of these regions would be achievable as a result, greatly simplifying the computations.

But as early as the 1950s, astronomers knew how simple this image was. Since then, an increasing number of studies have abandoned the notion of local equilibrium by including so-called Non-LTE computations. The energy exchange processes inside the system—atoms being stimulated by photons, or colliding, releasing, absorbing, or scattering photons—are thoroughly described in the Non-LTE calculations. That type of attention to detail pays off in stellar atmospheres when densities are much too low to allow the system to establish thermal equilibrium. There, calculations using Non-LTE provide outcomes that differ noticeably from those produced by calculations using local equilibrium.

When it comes to using Non-LTE calculations to study stellar atmospheres, Maria Bergemann's team at the Max Planck Institute for Astronomy is among the top in the world. Ekaterina Magg set out to more precisely compute the interaction of radiation and matter in the solar photosphere as part of the research for her PhD in that group. Most of the Sun's light is produced in the photosphere, which is also where the absorption lines are recorded on the solar spectrum.

In order to ensure that their findings were consistent, they utilized multiple independent methodologies to explain the interactions between the Sun's atoms and its radiation field while keeping track of all chemical elements that are pertinent to the existing theories of how stars developed through time. They employed pre-existing models ("STAGGER" and "CO5BOLD") that account for both the mobility of the plasma and the physics of radiation to describe the convective areas of our Sun. They selected the data set with the greatest quality readily accessible for the comparison with spectral measurements: the solar spectrum released by the Institute for Astro- and Geophysics, University of Göttingen. According to Magg, "We also heavily concentrated on the investigation of statistical and systematic factors that may restrict the precision of our conclusions."

The new computations revealed a fundamentally different connection between the abundances of these essential chemical elements and the intensity of the accompanying spectral lines from what had previously been asserted. The chemical abundances that result from the measured solar spectrum are thus somewhat different from what was predicted by earlier study.

"We found that the Sun contains 26 percent more elements heavier than helium, according to our study, than prior studies had assumed," says Magg. Such elements heavier than helium are referred to as "metals" in astronomy. Only a tiny fraction of the Sun's atomic nuclei—about one thousandth of one percent—are metals, and this extremely small amount has now altered by 26 percent of its prior value. "The figure for the oxygen abundance was roughly 15% greater than in earlier research," continues Magg. However, the new numbers are in good accord with the chemical makeup of ancient meteorites ("CI chondrites") that are supposed to represent the composition of the very early solar system.

The perplexing disparity between the outcomes of those models and helioseismic data is eliminated when those new values are applied as the input for current models of solar structure and evolution. With its dependence on noticeably more detailed models of the underlying physics, Magg, Bergemann, and their colleagues' in-depth examination of how spectral lines are generated successfully resolves the solar abundance dilemma.

According to Maria Bergemann, "the new solar models based on our new chemical composition are more realistic than ever before. They produce a model of the Sun that is consistent with all of the data we currently have about the Sun's current structure, including sound waves, neutrinos, luminosity, and the Sun's radius, without the need for unusual or exotic physics in the solar interior."

The new models also make it simple to use them on stars other than the Sun. This kind of advancement is extremely valuable at a time when massive surveys like SDSS-V and 4MOST are supplying high-quality spectra for an increasing number of stars. It gives future analyses of stellar chemistry a stronger foundation than ever before, with implications for reconstructions of the chemical evolution of our universe.