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  Current Research Highlight :: May 2009 all highlights

The shining make-up of our star

An international team lead by Prof. Martin Asplund, director at the Max Planck Institute for Astrophysics in Garching, has completed a decade-long research program to determine a fundamental astronomical yardstick: the chemical composition of the Sun. Surprisingly, the new analysis reveal that the Sun has a substantially smaller content of elements heavier than H and He than previously thought, a change which has far-reaching implications for our understanding of the solar system, stars in general and evolution of galaxies like the Milky Way.

Fig. 1: The chemical composition of the Sun is a fundamental astronomical yardstick against which other cosmic objects are measured.
Photo credit: ESA/NASA

Fig. 2: The new solar elemental abundances determined by the MPA-lead team as a function of atomic number. The given abundances are on the customary astronomical logarithmic scale where hydrogen is defined to be 12. The high abundances of hydrogen and helium is consequence of them largely being produced within the first few minutes after the Big Bang while all other elements have been forged in the fiery interiors of stars or during the cataclysmic supernova events ending their lives. The particular shape of the solar abundance pattern reveals a great deal not only about nuclear physics and the interior structure of stars but also how a galaxy like the Milky Way has evolved with time.

Fig. 3: A comparison between the newly measured logarithmic solar abundances with those in the most pristine meteorites reveal a very good agreement with the differences consistent with the measurement uncertainties. Such meteorites have remained unaltered since the formation of the solar system 4.5 billion years ago except for a depletion of some crucial elements like H, He, C, N, O and Ne, which therefore have to be gauged through solar spectroscopy.

The solar chemical composition is an important ingredient in our understanding of the formation, structure and evolution of the Sun and our solar system. Furthermore, it is an essential reference standard against which the elemental contents of other astronomical objects are compared, be it other stars, planets, gas clouds in the interstellar medium or whole galaxies. The chemical composition of a star like the Sun is inferred from its radiation spectrum, which provides the elemental fingerprints in the form of absorption lines. To convert the strength of a spectral line to an elemental abundance requires detailed modelling of the stellar atmosphere and the processes between atoms and radiation that shape the spectrum. For the Sun, a major complication arises because of convection — the bubbling motion akin to boiling water — which modifies the structure of the solar atmosphere where the spectrum is formed.

The theoretical foundation for the new study is a more realistic model of the solar atmosphere based on 3-dimensional hydrodynamical simulations compared with previous 1-dimensional modelling. This challenging endeavour has been pioneered and systematically refined by the MPA-group together with their colleagues in Denmark, USA and Australia. In addition, the interactions between the radiation and the gas have been followed in detail. Finally new and carefully selected input data for the considered spectral lines have been employed throughout.

The refined analysis tools and improved atomic data have enabled the team to reach an unprecedented accuracy in the measured solar chemical composition. Furthermore, rather than tackling just one or a handful of elements at a time as customary done in the field, the new study reports elemental abundances for all 71 elements that can be determined using solar spectroscopy. The ambitious nature of this undertaking is reflected in the ten years it has taken to complete the project. The most surprising and radical finding is that the contents of carbon, nitrogen, oxygen and neon -- the four most abundant elements after hydrogen and helium -- are only about two-thirds of what they previously were thought to be. The new 3D modelling, non-equilibrium spectrum calculations, improved atomic data and more robust selection of spectral lines all play a significant role for these elements and conspire to act in the same direction. The new results are supported by the excellent agreement between the predictions of the 3D solar model and various observational diagnostics. The study will be published in the prestigious journal Annual Reviews of Astronomy and Astrophysics later this year, and should become the de facto standard in astronomy for many years to come.

The revised solar chemical composition can be validated through a comparison with the abundances in the most pristine meteorites that has remained largely unaltered since the formation of the solar system. Although the meteorites can not be used to measure meaningful abundances of H, He, C, N, O and Ne as they have been partly evaporated in meteorites, the agreement for the remaining elements is excellent. Furthermore, the new solar abundances have resolved a long-standing conundrum why the Sun, that was born 4.5 billion years ago contained more heavy elements than the present-day interstellar medium and young, massive stars in the Galactic neighborhood. The overall content of elements heavier than He in the Milky Way should steadily increase with time as stars die and spew out their nuclear-processed ashes from which subsequent generations of stars and formed. The new solar values make the Sun normal in this respect.

Everything is, however, not rosy with this changed astronomical yardstick. The lower content of heavy elements imply changes in the deep interior of solar and stellar structure models. The predicted sound speed variations as a function of depth are now in stark conflict with those inferred observationally from the oscillations of the Sun. Because sound waves of different frequencies penetrate to different depths the sound speed variations can be mapped, a technique dubbed helioseismology. The revised solar chemical composition has already spurred a flurry of studies devoted to finding a resolution to the discrepancy. Several possible explanations have been put forward but unfortunately most can already be ruled out. Whatever the final solution will be, studies of both the Sun and other stars will then be on firmer footing. With stars as widely used probes of the cosmos, this will also mean a better understanding of the universe as a whole.

Martin Asplund


Martin Asplund, Nicolas Grevesse, Jacques Sauval and Pat Scott, "The chemical composition of the Sun", 2009, to appear in Annual Reviews in Astronomy and Astrophysics

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