One crucial reason astronomers study the Sun is to gain knowledge of stars other than the Sun. This is relevant to understanding the chemical composition of stars, the structure of stars, as well as the conditions of planets orbiting these stars. The Sun is a reference for studying other stars as it is the closest example of a star to the Earth and the one that astronomers have the tools to study and understand in the most depth. Thus, what astronomers know about the chemical abundance and structure of all other stars is based on how well astronomers understand the Sun.
For the last decade, there has been a “solar abundance crisis.” There was a conflict between the internal structure of the Sun, determined from solar oscillations – or helioseismology – and the internal structure of the Sun derived from the fundamental theory of stellar evolution, which relies on precise measurements of the chemical composition of the present-day Sun. The so-called “classical method” – otherwise known as standard solar model – for determining the internal structure of the Sun did not produce the same results as derived from helioseismology – the “new method.” The “new method” implied that the convective region of the Sun (where matter rises and sinks down again, like water in a boiling pot) was much larger than predicted by the “classical method.” Additionally, there were other discrepancies between the two models, such as the derived speed of sound waves that travel through the sun and the amount of helium in the Sun. The reason why these discrepancies existed puzzled astronomers for nearly a decade.
The ”classical method” – also known as the standard solar model – is a tried-and-true method that dates back to the 1800’s. This is a mathematical model of the Sun, which assumes that the Sun is a static – or unchanging – spherical ball of gas in various states of ionization (these different states represent the various layers of the Sun – the core, the convective zone, and the radiative zone). The model is constrained by the luminosity, radius, age, and composition of the Sun, which are well known. In this model there are two “free parameters” – or unknown variables – that are adjusted to model the observed Sun most closely. The “free parameters” in the model are the abundance of helium and the mixing length – which defines how material in the Sun is transferred in the convective zone.
Caption: This is an example of the spectrum of the Sun. Atoms in the Sun absorb specific colors, which leaves dark spectral lines – or absorption lines – in the observed spectrum. The location and strength of these lines indicates the which atom is present and how much of it exists. Credit: Wikimedia Commons
The “classical method” relies heavily on knowing the chemical composition of the Sun. The chemical composition of the Sun or any star can be determined by spectral analysis. A spectrum of a star is all the light being emitted by the star, split up into component wavelengths. Stellar spectra contain sharp lines – known as absorption lines – which are indications of the presence of specific chemical elements. Each elemental signature in a spectrum is unique, much like a human’s fingerprint, and will show up as a set of dark lines at specific, characteristic wavelengths in the spectrum of a star. How dark these lines appear can tell an astronomer how much of that material is present in the object – if the line is very strong, there is a large abundance of that material, if the line is very weak, there is a small abundance of that material. All of this information provides astronomers with a basis for the standard physical model for stars.
Caption: This plot shows the relative abundances of elements in the Sun, represented as blue dots, and plotted as a function of their atomic number – or number of protons. Credit: M. Asplund et al., Astronomy & Astrophysics 2009
The “new method” – also known as helioseismology – is the study of the structure and dynamics of the Sun through its oscillations. This method assumes that the Sun is constantly changing, unlike the previous method. The new method was pioneered in 2009 and is highly accurate. In this method, measurements are obtained that track the oscillations of the Sun. In other words, astronomers track the way the Sun expands and contracts. These oscillations occur in patterns and they repeat on timescales of seconds to hours. The oscillation modes are sensitive to the structure of the Sun, as the oscillations travel faster or slower depending on the material they are traveling through. Thus, given a reference model of the Sun and differences in oscillation modes, differences in internal structure can be determined. Helioseismology provides crucial information about the interior of the Sun, much like seismic waves on Earth provide crucial information about the interior of the Earth.
A new study published in Astronomy & Astrophysics provides a solution that resolves the “solar abundance crisis” that results from the discrepancies of the models. New calculations of the physics of the interior and atmosphere of the Sun by Ekaterina Magg, Maria Bergemann, and colleagues yields updated results for the chemical composition of the Sun.
The team, based out of the Max Planck Institute for Astronomy, revisited the models on which the spectral estimates of the chemical composition of the Sun are based. Early studied of how the spectra of stars are produced – the basis of the “classical model” – depend on local thermal equilibrium (LTE). LTE assumes that locally, the energy in each region of a star’s interior and atmosphere has time to spread out and reach equilibrium. This assumption allows astronomers to assign a specific temperature to each region of the star, which is an oversimplification of what is actually occurring in the star. In actuality, each region is not defined by one temperature, but a temperature range. In stellar atmospheres, densities are much too low to allow the system to reach equilibrium. Therefore, astronomers need to drop the assumption of local equilibrium and calculate the models of the Sun and stars based on non-LTE. Non-LTE includes a much more detailed description of how energy is exchanged in the system – atoms get excited by photons, photons are then emitted by atoms, then absorbed or scattered. These non-LTE calculations become very important in low density environments and lead to much different results than models that assume LTE.
The team threw out the assumption of LTE, and developed models of how stars evolve over time to include non-LTE physics. In their study, they tracked many different chemical elements in the Sun and employed multiple independent methods to describe the interactions between the Sun’s atoms and its radiation field to ensure that they obtained consistent results. They then compared their resulting chemical abundances to the most accurate solar spectrum available. They found that the chemical abundances they derived were consistent with the observations of the Sun, but inconsistent with the LTE models. When the new abundances they derived were used in solar structure models, the discrepancy between this method and the new method disappears!